# Why are accreting objects portrayed with a white flash at the point where the gas stream from the star enters the accretion disk?

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When I see images of accreting objects, the gas from the donor has a white flash where it meets the accretion disk, like in the image below. Why does this happen if it is true, and why is it there?

When the stream of gas falls towards the disk it gains a significant fraction of the orbital kinetic energy (after all, it is falling from the top of the Roche lobe) which means that it is moving fast and then slows down sharply when it interacts with the disk. This produces a hot spot that in theory could reach $$10^8$$ K but in practice "merely" is very hot.

## Swift: Black Holes

Most galaxies, including our own, possess a central supersized black hole weighing millions of times the sun's mass. According to the new studies, the black hole in the galaxy hosting Swift J1644+57 may be twice the mass of the four-million-solar-mass black hole lurking at the center of our own Milky Way galaxy. As a star falls toward a black hole, it is ripped apart by intense tides. The gas is corralled into a disk that swirls around the black hole and becomes rapidly heated to temperatures of millions of degrees.

The innermost gas in the disk spirals toward the black hole, where rapid motion and magnetism creates dual, oppositely directed "funnels" through which some particles may escape. Particle jets driving matter at velocities greater than 80-90 percent the speed of light form along the black hole's spin axis. In the case of Swift J1644+57, one of these jets happened to point straight at Earth.

Theoretical studies of tidally disrupted stars suggested that they would appear as flares at optical and ultraviolet energies. The brightness and energy of a black hole's jet is greatly enhanced when viewed head-on. The phenomenon, called relativistic beaming, explains why Swift J1644+57 was seen at X-ray energies and appeared so strikingly luminous.

When first detected on March 28, the flares were initially assumed to signal a gamma-ray burst, one of the nearly daily short blasts of high-energy radiation often associated with the death of a massive star and the birth of a black hole in the distant universe. But as the emission continued to brighten and flare, astronomers realized that the most plausible explanation was the tidal disruption of a sun-like star seen as beamed emission.

Credit: NASA's Goddard Space Flight Center/CI Lab

An X-ray nova is a short-lived X-ray source that appears suddenly, reaches its emission peak in a few days and then fades out over a period of months. The outburst arises when a torrent of stored gas suddenly rushes toward one of the most compact objects known, either a neutron star or a black hole.

Named Swift J1745-26 after the coordinates of its sky position, the nova is located a few degrees from the center of our galaxy toward the constellation Sagittarius. While astronomers do not know its precise distance, they think the object resides about 20,000 to 30,000 light-years away in the galaxy's inner region. The pattern of X-rays from the nova signals that the central object is a black hole.

Ground-based observatories detected infrared and radio emissions, but thick clouds of obscuring dust have prevented astronomers from catching Swift J1745-26 in visible light.

The black hole must be a member of a low-mass X-ray binary (LMXB) system, which includes a normal, sun-like star. A stream of gas flows from the normal star and enters into a storage disk around the black hole. In most LMXBs, the gas in the disk spirals inward, heats up as it heads toward the black hole, and produces a steady stream of X-rays.

But under certain conditions, stable flow within the disk depends on the rate of matter flowing into it from the companion star. At certain rates, the disk fails to maintain a steady internal flow and instead flips between two dramatically different conditions — a cooler, less ionized state where gas simply collects in the outer portion of the disk like water behind a dam, and a hotter, more ionized state that sends a tidal wave of gas surging toward the center.

This phenomenon, called the thermal-viscous limit cycle, helps astronomers explain transient outbursts across a wide range of systems, from protoplanetary disks around young stars, to dwarf novae - where the central object is a white dwarf star - and even bright emission from supermassive black holes in the hearts of distant galaxies.

A ULX is thought to be a binary system containing a black hole that is rapidly accreting gas from its stellar companion. However, to account for the brilliant high-energy output, gas must be flowing into the black hole at a rate very near a theoretical maximum, a feeding frenzy that astronomers do not yet fully understand.

As gas spirals toward a black hole, it becomes compressed and heated, eventually reaching temperatures where it emits X-rays. As the rate of matter ingested by the black hole increases, so does the X-ray brightness of the gas. At some point, the X-ray emission becomes so intense that it pushes back on the inflowing gas, theoretically capping any further increase in the black hole's accretion rate. Astronomers refer to this as the Eddington limit, after Sir Arthur Eddington, the British astrophysicist who first recognized a similar cutoff to the maximum luminosity of a star.

Black-hole binaries in our galaxy that show accretion at the Eddington limit also exhibit powerful radio-emitting jets that move near the speed of light. Although astronomers know little about the physical nature of these jets, detecting them at all would confirm that the ULX is accreting at the limit and identify it as a stellar mass black hole.

The European Space Agency's XMM-Newton observatory first detected the ULX, dubbed XMMU J004243.6+412519 after its astronomical coordinates, on Jan. 15. Middleton and a large international team then began monitoring it at X-ray energies using XMM-Newton and NASA's Swift satellite and Chandra X-ray Observatory. The scientists conducted radio observations using the Karl G. Jansky Very Large Array (VLA) and the continent-spanning Very Long Baseline Array, both operated by the National Science Foundation in Socorro, N.M., and the Arcminute Microkelvin Imager Large Array located at the Mullard Radio Astronomy Observatory near Cambridge, England.

In a paper published online by the journal Nature on Wednesday, Dec. 12, 2012, the scientists reveal their successful detection of intense radio emission associated with a jet moving at more than 85 percent the speed of light. VLA data reveal that the radio emission was quite variable, in one instance decreasing by a factor of two in just half an hour.

This tells astronomers that the region producing radio waves is extremely small in size — no farther across than the distance between Jupiter and the sun.

Black holes have been conclusively detected in two varieties: "lightweight" ones created by stars and containing up to a few dozen times the sun's mass, and supermassive "heavyweights" of millions to billions of solar masses found at the centers of most big galaxies. Astronomers have debated whether many ULXs represent hard-to-find "middleweight" versions, containing hundreds to thousands of solar masses.

When a massive star runs out of fuel, it no longer has the energy to support its mass. The core collapses and forms a black hole. Shockwaves bounce out and obliterate the outer shells of the star. Previously scientists thought that a single explosion is followed by a graceful afterglow of the dying embers. Now, according to Swift observations, it appears that a newborn black hole in the core somehow re-energizes the explosion again and again, creating multiple bursts all within a few minutes.

The innermost region of our galaxy lies 26,000 light-years away in the direction of the constellation Sagittarius. At the center of it all lurks Sgr A* (pronounced "saj a-star"), a behemoth black hole containing 4 million times the sun's mass.

Sgr A* regularly produces bright X-ray flares today, but astronomers know it was much more active in the past.

To better understand its long-term behavior, the Swift team began regular observations of the galactic center in February 2006. Every few days, the spacecraft turns toward the inmost galaxy and takes a 17-minute-long "snapshot" with its X-Ray Telescope (XRT).

Swift's XRT has now detected six bright flares, during which the black hole's X-ray emission brightened by up to 150 times for a couple of hours. These new detections, in addition to four found by other spacecraft, enabled astronomers to estimate that similar flares occur every five to 10 days.

The Swift XRT team is on the lookout for the first sign that a small cold gas cloud named G2, which is swinging near Sgr A*, has begun emitting X-rays. This is expected to start sometime in spring 2014. The event will unfold for years and may fuel strong activity from the monster black hole.

The monitoring campaign has already yielded one important discovery: SGR J1745-29, an object called a magnetar. This subclass of neutron star has a magnetic field thousands of times stronger than normal so far, only 26 magnetars are known. A magnetar orbiting Sgr A* may allow scientists to explore important properties of the black hole and test predictions of Einstein’s theory of general relativity.

The new findings confirm that the black holes "light up" when galaxies collide — and may offer insight into the future behavior of the black hole in our own galaxy.

The intense emission from galaxy centers, or nuclei, arises near a supermassive black hole containing between a million and a billion times the sun's mass. Giving off as much as 10 billion times the sun's energy, some of these active galactic nuclei (AGN) — a class that includes quasars and blazars — are the most luminous objects in the universe.

The galaxy, which is known as Markarian 739 or NGC 3758, lies 425 million light-years away toward the constellation Leo. Only about 11,000 light-years separate the two cores, each of which contains a black hole gorging on infalling gas.

Astronomers refer to galaxy centers exhibiting such intense emission as active galactic nuclei (AGN). Yet as common as monster black holes are, only about one percent of them are currently powerful AGN. Binary AGN are rarer still: Markarian 739 is only the second identified within half a billion light-years.

Many scientists think that disruptive events like galaxy collisions trigger AGN to switch on by sending large amounts of gas toward the black hole. As the gas spirals inward, it becomes extremely hot and radiates huge amounts of energy.

## Chandra Frontiers in Time-Domain Science

I present an overview of observational efforts across the electromagnetic spectrum to identify and study tidal disruption events (TDEs), when a star wanders too close to a super-massive black hole and is torn apart by tidal forces. I describe insights from growing samples of these sources, as well as a number of open puzzles: the origin of the luminous UV/optical emission, the ubiquity of outflows ranging from speeds of 100 km/s to near the speed of light, and the unique host galaxy population. Finally I discuss prospects in the near-future for addressing some of these issues with discoveries from eROSITA and the Vera Rubin Observatory.

The accretion-powered high-mass X-ray binary GX 301-2 is composed of a pulsar accreting from a B1-type stellar companion, and is one of the brightest X-ray sources in the sky. Sources like GX 301-2 are important to our understanding of the physics underlying accretion and mass-transfer mechanisms. Especially useful are simultaneous measurements of the flux and the change in spin frequency, which can be tested against models of angular momentum transfer under different accretion scenarios. In the case of GX 301-2, NICER and Swift-XRT observations across two orbital periods between MJD 58469 and 58552 during a spin-up episode measured by Fermi GBM allow for testing for the formation of a transient accretion disk. Using this data, we examine the correlation between the spin frequency and flux based on accretion torque models and calculate how much of the observed luminosity results from the mass transfer. We determine that even though the contribution to the luminosity from the temporary accretion disk is negligible compared to the luminosity produced by wind accretion, the presence of an accretion disk is indicated by changes in the pulsed fraction of the emission.

We present the HECATE, a catalog of 205K galaxies with D Lightning Talk in Tidal Disruption Events and Transients I on Wednesday, Oct 7th 11:00&ndash12:30 EDT

Recently, two faint X-ray transients have been discovered in the Chandra Deep Field-South. Both lasted a few hours and are extragalactic with z = 0.74 (spectroscopic) and z

2.1 (photometric), implying large total energy release. The first has been proposed to be a magnetar-powered X-ray transient resulting from a binary neutron-star merger, while the nature of the second is less clear. These findings demonstrate that a population of similar transients should exist in archival X-ray observations. We have thus recently set systematic rate constraints on such transients based on 19 Ms of Chandra surveys data. Rapid searching of incoming Chandra and XMM-Newton observations, using our methodology, should allow discovery of additional such transients for prompt follow-up. Future large-grasp X-ray missions such as Athena and Einstein Probe are needed to open the faint-fast X-ray transient discovery space fully.

Fast Blue Optical Transients (FBOTs) are a recently identified new class of transients. To date, only two of the few dozen known FBOTs have been observed at X-ray wavelengths. In one FBOT the X-ray emission indicated the presence of a central engine. I will outline what X-ray emission has taught us about FBOTs and why X-ray observations are crucial to determine their nature.

The art of modeling the tidal disruption of stars by massive black holes forms the main theme of my talk. Detailed simulations should tell us what happen when stars of diﬀerent types get tidally disrupted, and what radiation a distant observer might detect as the observational signature of such events.

Tidal disruption events (TDEs) offer a unique opportunity to study a single super-massive black hole (SMBH) under feeding conditions that change over timescales of days or months. Despite the increasing number of observed TDEs, it is unclear whether most of the energy in the initial flare comes from accretion near the gravitational radius or from circularizing debris at larger distances from the SMBH. We use the MOSFiT transient fitting code to calculate the conversion efficiency from mass to radiated energy, and find that, for many events, it is similar to efficiencies inferred for active galactic nuclei. However, for some events, it is also similar to stream collision efficiencies. The systematic uncertainties in the measured efficiency must be reduced before we can definitively resolve the emission mechanism of individual TDEs with this method. While the early light curve is generally dominated by optical and UV emission from a large photosphere, observations of late time emission at x-ray frequencies suggest the possibility of directly observing the accretion disk, motivating the need for late-time Chandra observations to further constrain the emission mechanism of TDEs.

The structures through which long-term persistent accretion occurs in supermassive black holes evolve secularly over hundreds to thousands of years. Different accretion geometries lead to changes in emission properties, observed as different accretion states. Stellar disruptions by supermassive black holes experience a wide range of accretion rates, which could provide opportunities to study the formation/evolution of accretion structures over timescales of just months to years. However, such structural changes have not been observed hitherto. I will describe our recent discovery where we identify three distinct accretion states following a tidal disruption event by a supermassive black hole, with properties strikingly similar to stellar-mass black holes as they evolve through their outbursts. Our findings demonstrate the scale invariance of accretion processes over seven orders of magnitude in black hole mass. This result demonstrates that tidal disruption events can be used to study accretion states in individual supermassive black holes, removing limitations inherent to commonly used ensemble studies.

Be/X-ray binaries can go into periodic outburst at periastron, when mass is transferred from an O or B star’s disk of ejected material onto a compact object. Characterizing these outbursts informs our understanding of magnetospheric accretion onto objects with extreme magnetic fields, as well as the resulting feedback between the high-mass star and the orbiting compact object, which is most commonly a pulsar. In June 2020, eROSITA detected an outburst from LXP 69.5, a BeXRB system in the Large Magellanic Cloud. We triggered ToO observations from NICER and Swift. We found a spin period of 68.68 ± .01 seconds. The resulting spectral and temporal X-ray properties of the system call into question the correlation between pulse profile morphology and luminosity. The OGLE V and I band light curves, which stopped in March 2020, exhibit a changing flare periodicity, best modeled by three epochs with period values of 149, 171, and 200 days, respectively. Archival Swift data and our ToO observations suggest that the optical and X-ray flares of the system do not coincide. Thus, the behavior of this system challenges established understanding of accretion in BeXRBs.

Here I present our long-term multiwavelength follow-up campaign of two very special X-ray tidal disruption events. One is a decade-long tidal disruption event that showed the spectral state transition from super-Eddington accretion state of quasisoft X-ray spectra to the thermal state of supersoft X-ray spectra. Recently we also observed the cooling of the thermal state spectra. The other event is associated with an off-center intermediate-mass black hole of few 10^4 solar mass. Recently we confirmed that it resides in a massive star cluster through the HST imaging and that the X-ray spectra cool as expected for a black hole of such a mass.

Swift J164449.3+573451 (Sw J1644+57) is a tidal disruption event (TDE) where a star became unbound after getting too close to a supermassive black hole and got torn apart by tidal forces. Sw J1644+57 was the first TDE discovered in 2011, and to date is the only TDE where the launch and subsequent turnoff of a relativistic jet has been observed in detail. In this talk, I will give an overview of almost a decade of Sw 1644+57 observations, from its initial discovery by Swift to its transition to a sub-relativistic phase. I will also provide an update of the TDE from recent Chandra and Very Large Array observations in the X-ray and radio as the shockwave continues to expand and interact with the black hole's circumnuclear environment. Finally, I will explain how Sw J1644+57 fits into the broader picture of TDE studies, and how it will continue to provide a benchmark for these transient phenomena for years to come.

The Atacama Large Millimeter/submillimeter Array (ALMA), located at an altitude of 5000 m in the Atacama desert, is the first of the large facilities of the 2020s landscape to be operational. Inaugurated in 2013, it provides unprecedented performance capabilities in the millimeter/submillimeter range for exploration of the Universe. Since the start of its early operations in 2011, it has revolutionised our view on the process of planet formation or how the first stars and galaxies in the Universe were born. In this talk I will present some of the scientific topics that can only be investigated by exploiting the synergies between Chandra and ALMA. I will also discuss some millimeter/X-ray synergy topics that will need of future X-ray facilities such as Athena to match the ALMA capabilities.

In this presentation I will address possible synergies between Chandra and eROSITA with an outlook to Athena. The eROSITA instrument on-board the Russian/German SRG mission is currently performing its second of eight consecutive X-ray imaging all-sky surveys. Over the course of four years, eROSITA will probe transient and variable X-ray phenomena on timescales of seconds to years uncovering a rich sample of extra-galactic (e.g., Tidal Disruption Events, Gamma-ray Burst afterglows, AGN releated phenomena) and Galactic (e.g., X-ray binaries, novae, cataclysmic variables, stellar phenomena) transients and variables. Athena will be next large X-ray observatory in the ESA large mission program to be launched in the early 2030s. Equipped with a large effective area optics, thanks to the novel Silicon Pore Optics technology, and two technology development pushing instruments, a very high-resolution micro-calorimeter (X-IFU) and a wide-field camera (WFI).

Active galactic nuclei (AGN) rank among the most powerful sources in the universe and are capable of dramatically altering their environments through energetic feedback. In the prevailing view, feedback associated with large-scale jets with extents of 10’s or 100’s of kpc is the primary means by which AGN regulate galaxy evolution. However, the role of feedback by compact, sub-galactic jets on the interstellar medium, particularly at “cosmic noon” (z = 1-3), remains poorly understood. New multi-epoch radio surveys offer a promising means of identifying compact jets based on radio variability. I present a sample of currently radio-loud AGN identified in the on-going Very Large Array Sky Survey (VLASS) that were radio-quiet just 1-2 decades ago based on previous radio surveys. These newly radio-loud AGN may be associated with young jets, thus providing new insights into jet triggering and the link between feedback driven by compact jets and galaxy evolution. I discuss the unique and important role of Chandra observations in conjunction with new multi-epoch radio surveys for measuring the accretion states and host galaxy environments of powerful AGN with young jets.

In working to determine the astrophysical source of high-energy gamma rays, a prime impediment is the lack of sub-arcminute localizations. Multi-wavelength observations, primarily in the X-ray and radio regimes, are important to unlocking the secrets of the gamma-ray sky. For X-ray follow-up, a key decision is the choice of which observatory should be used, and must take into account the likely nature of the source. I will describe a few of the more common follow-up methods for Fermi-LAT sources, and discuss how Chandra and other current facilities fit best into this process.

Dust particles scatter X-ray light, producing a diffuse halo image around X-ray point sources. In our own Galaxy, spectacular dust scattering echoes from X-ray variable sources can be used to map the 3D spatial distribution of interstellar dust. I will review the legacy of dust scattering echoes imaged by Chandra, and discuss how future X-ray telescopes will soon become dust echo imaging factories. Finally, I will show how dust scattering halos around X-ray bright quasars could be used to probe the abundance and grain sizes of dust inhabiting the intergalactic medium. Future telescope missions with Chandra-like resolution – such as Lynx – provide an opportunity to image extragalactic scattering echoes, directly probing the abundance of dust in galaxy halos and absorption systems.

Pre main-sequence stars are variable sources. In stars with protoplanetary disks, this variability is mainly due to disk-related phenomena. Accreting gas heats the stellar atmosphere and create hot spots whose energetic emission is modulated by stellar rotation. Accretion itself is a non-stationary process. Disked stars can also experience variable extinction due to the material in the inner disk and accretion funnels. The main source of variability in stars without disks is due to the magnetic activity, which is orders of magnitude more intense than in main sequence stars. Flares, spots, faculae, and coronal active regions are intrinsically variable, and their emission is modulated by stellar rotation. These phenomena can be studied with simultaneous multi-band observations, which are rare and technically challenging. I will present the results of the CSI2264 project, based on simultaneous multi-wavelength observations of young stars (in this case, the members of the cluster NGC2264). In particular, I will show how simultaneous optical (CoRoT) and X-rays (Chandra) observations deeply probed the phenomena associated with the inner disks, accretion, and magnetic activity.

We present X-ray and optical monitoring observations and simulations showing how gravitational microlensing is used to infer the structure near the event horizons of supermassive black holes, constrain the spin of the black holes and test general relativity in the strong-gravity regime.

During microlensing events, magnification caustics cross the accretion disk revealing the gradual change in the energy of Mg, Si, S and Fe fluorescence lines arising form the accretion disk. The change in the energy of these lines is the result of special and general relativistic effects. The distribution of the energy shifts of the lines provide constraints on the innermost stable circular orbit and spin of the black hole. The change in the relative strengths of the disk lines as the caustic moves across the disk provides insight into the ionization structure of the disk.

We also show how these microlensing observations can be expanded to a statistically large sample of z = 0.5-5 lensed quasars with the predicted discovery by Vera C. Rubin Observatory of > 10,000 additional gravitationally lensed systems and with a next generation X-ray telescope.

I will discuss a new 2D deconvolution algorithm to analyze x-ray dust echoes. The technique allows joint analysis of multiple temporally distinct observations of dust scattering echoes and will enable more flexible future observing strategies.

Multi-wavelength, time domain observations of the Galactic Center have opened a completely new view of the dynamic environment around our closest supermassive black hole. I will discuss Sgr A*’s unique variability alongside other time domain phenomena in the Galactic Center, traced out over more than 20 years of observations from Chandra and coordinated multi-wavelength campaigns. I will also briefly explore how we can continue to push this frontier with existing and next-generation observatories.

We present multi-wavelength observations of the first calcium-rich transient, SN 2019ehk, with a luminous X-ray detection. Our panchromatic observations of supernova (SN) 2019ehk begin 10 hours after explosion and continue for

400 days. Short-lived X-ray emission observed by Swift-XRT is coincident with both an optical "flare" at

3 days after explosion and "flash-ionized" Hydrogen and Helium emission lines in the SN spectrum. Combining these observations provides, for the first time, direct evidence of a dense, compact shell of circumstellar material surrounding a calcium-rich SN progenitor. We will discuss the implications of these observations with respect to the phase space of X-ray transients and explore how X-ray production in calcium-rich transients can be applied as a novel probe of progenitor mass-loss history.

The Gregory-Loredo period searching algorithm, which employs Bayes’s theorem to the phase-folded light curve and is well-suited for irregularly sampled X-ray data, is applied to deep Chandra observations of the inner Galactic bulge (the Limiting Window field) and the Nuclear Star Cluster (NSC). This leads to dozens of newly detected periodic sources, most of which are classified as magnetic CVs including polars and IPs. Under reasonable assumptions about the geometry of CVs and a large set of simulated X-ray light curves, we estimate the fraction of magnetic CVs in the inner bulge to be 3.3 hours) CVs contrasted with the field CVs, which may be understood as an age effect.

We report the detection of 8 candidate extragalactic fast X-ray transients (FXRTs) from a parent sample of 214,701 sources in the Chandra Source Catalog Release 2.0 (160.96 Ms over 592.4 deg2). We characterize their X-ray light curves and spectra. Two candidates have visible counterparts in archival imaging, allowing us to assess weak photometric redshift probability distributions (z

0.3-5.2) and host properties. Four FXRTs show a plateau in their light curves and a softening trend in their hardness ratio (HR), implying a possible relation with CDF-S XT2 and GRBs. We compute the local event rates and investigate a possible relation with a central engine scenario, driven by a proto-magnetar emission.

Flares are a fact of life for stars on or near the main sequence, and they are the most energetic releases that occur over pretty much all of the star's main sequence lifetime. The launch of the Chandra X-ray Observatory occurred only a few years after the discovery of the first extrasolar planets, and there is a link between important stellar astrophysics questions and the associated implications for habitability. For instance, the factors that control the coronal emission of stars and their brightenings are important not just for understanding energy balance in the corona, but also for understanding planetary atmosphere irradiation. In this context, it is important to understand not just the details of how flares work but also how these extrasolar space weather or habitability impacts might change with time. In this talk I'll detail what we've learned with Chandra about the types of flaring stars, the details of flares, and how stars can impact their environment, both for forming planets and for ones already formed. I'll then turn to what the future might hold as far as Chandra's continued contribution to understanding flares and planetary habitability.

We investigate the variability properties of X-ray sources located within several intermediate age (20-300 Myrs) clusters using different variability metrics. We use multi-wavelength properties from various catalogs to classify these variable Chandra X-ray sources and explore the variability for different classes of sources. We further explore trends between spectral and variability properties.

Chandra has been charting the magnetic heartbeats of late-type stars via high-contrast coronal X-rays. Goal is to characterize the stellar Dynamo, whose internal workings remain elusive. The Sun's high-energy modulations play an important "Space Weather" role in our heliosphere, as do stellar counterparts for their exoplanets. The nearby pair Alp Cen A (G2 V) and B (K1 V) has been followed by Chandra semiannually since 2005 adding to 1990’s ROSAT/HRI and, since 2003, XMM-Newton. AB show clear coronal cycles, 19 yr and 8 yr, respectively bracketing the solar 11-year cycle by significant amounts, despite the only small differences in stellar masses. Several years ago, nearby bright Procyon (F5 IV-V) was added to the program. Despite Procyon’s high coronal intensity, the slightly evolved F star has displayed a very flat X-ray light curve. More recently, two new promising visual binaries, Xi Boo (G8 V + K4 V) and 70 Oph (K0 V + K5 V), were added to "Cycles." A key question involves the origin of diverging branches in a diagram pitting rotational period versus cycle duration, where the Sun's iconic 11-yr example sits in the middle, possibly in a transitional state.

Observations have shown that young stars are highly X-ray variable. What does that mean for the properties and chemistry of their circumstellar planet forming disks? We present a theoretical study connecting disk chemical evolution to stellar X-ray flaring events. We find that X-ray flares drive chemistry on observationally relevant time scales and have a long-term (decades) cumulative impact.

The chemistry of protoplanetary disks sets the initial composition of newly formed planets and may also regulate the efficiency by which planets form. Disk chemical abundances typically evolve over timescales spanning thousands if not millions of years. Consequently, it was a surprise when ALMA observations taken over the course of a single year showed significantly variable molecular emission in H13CO+ relative to the otherwise constant thermal dust emission in the IM Lup protoplanetary disk. HCO+ is a known X-ray sensitive molecule, and one possible explanation is that stellar activity is perturbing the chemical "steady state" of the disk. If confirmed, simultaneous observations may provide a new tool to measure (and potentially map) fundamental disk parameters, such as electron density, as the light from X-ray flares propagates across the disk.

While X-ray variability and flaring of Young Stellar Objects (YSOs) has been known for some time, suitable and systematic complementary (nonthermal) centimetric radio observations are now becoming available. I will illustrate the complexity emerging from simultaneous radio and X-ray observations of hundreds of YSOs in the Orion Nebula Cluster on timescales of minutes and outline the next steps.

Pre-main sequence stars (PMS) are young stars with bright and frequent magnetic activity. In some cases, the flare loops are so long that they can connect the star and its surrounding protoplanetary disk. PMS also show X-ray variability that is unique to this evolutionary stage: An important component of their soft X-ray emission originates in an accretion shock which might change over a rotation period or when blobs of material fall into the star. As the accretion stream or disk warps rotate into and out of the line-of-sight that emission is more or less absorbed and spectral changes allow us to probe the dust content of the absorber. As this dust, pebbles, or even proto-planets fall into the star, we can watch the elemental composition the hot plasma change in the X-ray spectrum over a few years. Chandra was instrumental in many of these discoveries due to the high spatial resolution that allows us to separate close sources in crowded star forming regions. I close this review with a list of open questions on variability from PMS that future X-ray observatories can address.

Many objects in our solar system emit X-rays through a variety of physical processes. Their emission is often highly variable in time as they respond to local space weather conditions, on solar flares, or on inherent properties of the observed object. Solar system observations are often TOO or rapid follow up observations as some of the most interesting events are unpredictable.

We present the results from contemporary Swift and Chandra observation of Jupiter family Comet 46P/Wirtanen during its 2018 apparition. Water production rate and charge exchange emission were measured during three different epochs over 1.5 months allowing for a comparison with the variability of the solar wind.

Since the first Chandra observations of Jupiter in 2000, the planet has been observed to produce mysterious quasi-periodic X-ray flares on timescales of 10s of minutes. The unprecedented X-ray campaigns that are accompanying the Jupiter-orbiting Juno spacecraft have enabled us to decipher the entire chain of physical processes that produce these clockwork-like pulsations. These campaigns reveal that Jupiter’s X-ray aurora pulses in time with periodic magnetic field fluctuations called compressional mode waves. These waves interact with the plasma population through cyclotron resonances causing periodic flows of keV-MeV particles towards the planet. When these particles collide with Jupiter’s atmosphere they cause X-ray bursts. Further observations reveal that this is not unique to the X-rays: the X-rays pulse in time with radio and UV flares, which are all synchronised through compressional mode waves. We close by showing videos of Jupiter’s aurora demonstrating that the term X-ray ‘hot spot’ (Gladstone+2002 Dunn+2017) is misleading for Jupiter’s since it is not produced by a single coherent process but several distinct processes, of which the pulsations are only one component. * W.R.D and ZHY contributed equally to this work

Time domain astronomy can require that observers use multiple facilities simultaneously. I will discuss the Simultaneous Multiwavelength Astronomy Research in Transients NETwork (SmartNET), an open networking tool designed to help astronomers organize such observations. I will present some of the recent SmartNET campaigns and focus on how SmartNET and Chandra can combine to tackle new frontiers.

The emergence of time-domain multi-messenger (astro)physics asks for new and more efficient ways of interchanging information, as well as collaboration. Many space- and ground-based observatories have web pages dedicated to showing information about the complete observations and planned observation schedule. The aim is to standardise the exchange of information about observational schedules and set-ups between facilities and in addition, to standardise the automation of visibility checking for multiple facilities. To reach this, we propose to use the VO protocols (ObsTAP-like) to write services to expose these data to potential client applications and to develop cross facilities visibility servers.

I plan to start with some examples of scientific rewards of effective communication in time-domain astronomy. Next, I plan to highlight tools that facilitate effective co-ordination. I will conclude with some challenges in this context before opening the floor for discussion.

Multi-wavelength observations offer astronomers a great deal of information that can help to interpret the astrophysics of a source. For time-variable sources, coordinating these multi-wavelength observations in time can be crucial. Over the first 21 years of the mission, Chandra has offered proposers the opportunity to coordinate their Chandra observations with observations at other observatories. We continue to do so as the mission enters its third decade of service and have opened this opportunity up to coordinations with any observatory, space- or ground-based. We discuss how Chandra has accomplished this in the past and how we will do so moving forward.

This talk gives a short summary on X-ray observations of supernovae and young supernova remnants (SNRs) carried out in the last decades, with particular focus on the results obtained with Chandra. X-ray emission of supernovae helps to constrain the supernova type, explosion mechanism, and the properties of the circumstellar medium. X-ray studies of SNRs allow us to understand the distribution of the ejecta, expansion of the shocks, and the interaction of a SNR with its environment. I will report on the recent results on SN 1987A, Kepler's SNR, Tycho's SNR, and others, which have provided us with new interesting facts about the evolution of supernovae and SNRs.

We present an analysis of five years of coordinated CXO and NuSTAR observations showing a strongly interacting and peculiar supernova, SN 2014C. This is the first broad-band X-ray monitoring of an extragalactic SN over six years of evolution in both the hard and soft X-rays. Our analysis of the bright thermal bremsstrahlung radiation reveals that SN 2014C, initially a Type Ib SN, metamorphosed into a Type IIn as a result of interaction with a hydrogen-rich circumstellar medium (CSM) consisting of 2-3 M_☉, located 5.5 x 10^ <16>cm from the explosion site. This H-rich CSM shell of material has a density profile that goes as between R^ <-2.5>and R^ <-4.3>, which clearly deviates from wind-like density profiles (R^<-2>) that are expected around massive stars. These findings require updates to our understanding of mass loss in massive stars that are approaching core-collapse.

We present X-ray spectra spanning 18 years of evolution for SN1996cr, one of the five nearest (

4 Mpc) SNe detected in the modern era. Chandra-HETG observations allow us to resolve spectrally the velocity profiles of Ne, Mg, Si, S, and Fe emission lines and monitor their evolution as tracers of the ejecta-circumstellar medium (CSM) interaction. To explain the diversity of X-ray line profiles, we explore several possible geometrical models. Based on the highest signal-to-noise 2009 epoch, we find that a polar geometry with two distinct opening angle configurations and internal obscuration can successfully reproduce all of the observed line profiles. We extend this model to seven further epochs with lower S/N ratio and/or lower spectral-resolution between 2000-2018, yielding several interesting evolutionary trends.

Supergiant Fast X-ray Transients (SFXTs) are a class of supergiant high mass X-ray binary systems, characterised by extreme variability in the X-ray domain. Current models attribute flares to the clumpy nature of the stellar wind coupled with gating mechanisms involving the spin and magnetic field of the neutron star. eROSITA is the primary instrument on-board the Russian-German "Spectrum-Roentgen-Gamma" mission, with a mission is to perform an imaging all-sky survey within two years. The location of the Large Magellanic Cloud makes it an ideal laboratory to detect transient systems with eROSITA and follow them with multi-wavelength campaigns. We performed a detailed temporal and spectral analysis of the eROSITA and XMM-Newton data of XMMU J053108.3−690923. We confirm the putative pulsations previously reported from the source certifying its nature as a neutron star in orbit with a supergiant companion. We identified fast flares in the eROSITA light curves, while the long term flux exhibits variability with a dynamic range of >1000 confirming its nature as an SFXT. Further, an estimate of the clumpiness of the medium and the magnetic field of the neutron star.

Supergiant high mass X-ray binaries (HMXBs) offer a unique chance to directly probe the highly structured, clumpy winds of O/B stars. In these systems, a compact object (black hole or neutron star) accretes matter from the stellar wind of the companion. The wind's variability drives changes in the accretion and thus the system’s X-ray emission. The interaction of this emission with the wind material is used to study the wind itself and thus to constrain the mass loss processes in the most massive stars. The systems are highly dynamic, with time scales ranging from day-long orbital cycles to minutes and below when the passage of individual wind clumps is observed.
Chandra, with its superb spectral resolution, has opened a new window onto the the wind structure in HMXBs. It has, for example, revealed the onion-like structure of the wind clumps in Cygnus X-1, and the complex multi-phase wind accretion flow in Vela X-1, similar to what has been hinted onto in simulations. In this talk, I will discuss these and other Chandra results and argue that Chandra is ideally posed to continue its crucial contribution to our understanding of HMXBs and massive stars.

Over the past decade, the discovery that many, if not most, novae produce GeV gamma-rays has revolutionized our understanding of these common major stellar eruptions. Powerful shocks are now thought to play a key role in numerous aspects of nova eruptions, from helping eject the white dwarf's envelope, to powering the optical emission, to possibly triggering catastrophic cooling and dust formation. And X-rays from shock-heated gas are vital for diagnosing these shocks. In this talk, I will briefly review Chandra's past contributions to nova research as well as some current investigations of gamma-ray producing shocks. Looking forward, I'll argue that working ever more closely with other observatories will be crucial for Chandra to continue to make ground-breaking contributions to the study of novae and other related stellar transients.

WR 25 is a colliding-wind binary star system comprised of a very massive O2.5If*/WN6 primary and an O4 secondary in a 208-day period eccentric orbit. These hot stars have strong, highly-supersonic winds which interact to form a bright X-ray source from wind collision- shocks whose conditions change with stellar separation. Different views through the WR and O star winds are afforded with orbital phase as the stars move about each, allowing for exploration of wind structure in ways not easy or even possible for single stars. We have analyzed an on-axis Chandra/HETGS spectrum of WR 25 obtained shortly before periastron at maximum light. From the on-axis observations, we constrain the line fluxes, centroids, and widths of various emission lines, including He-triplets of Si XIII and Mg XI. We have also been able to include several serendipitous off-axis HETG spectra from the archive and study their flux variation with phase. This is the first report on high-resolution spectral studies of WR 25 in X-rays.

WZ Sge is one of the best-known dwarf novae due to its spectacular super-outbursts every

30 years. These eruptions are characterised by a primary burst, followed by a decline phase that includes a sharp “dip” and multiple “echo outbursts”. As these decline features are not yet fully understood, we investigate one recently proposed interpretation that the switches between these states represent transitions into and out of a magnetic propeller state. If this is the case, the distinctive UV spectrum observed in the prototypical magnetic propeller system AE Aqr may provide us with a definitive observational signature of the process in WZ Sge. In this talk, I present time-resolved, high-resolution UV spectroscopic observations taken with STIS/HST just before, during and after the dip in WZ Sge’s 2001 super-outburst. We construct time-averaged and RMS spectra for all states during the decline phase and we test whether the spectroscopic signature of a propeller is present and limited to the faint states during which the propeller is thought to operate. Finally, I discuss the broader implications of our findings for our understanding of the disk instability model and dwarf nova eruptions.

Several approachs to timing analysis with ACIS-S HETG spectra of stellar sources are discussed, including light curves, fourier period determination, and statistical studies. Issues and benefits of HETG data in pursuing timing studies are explored. A specific example of the early O star zeta Pup is presented, with coordinated observations at other wavelengths.

Ultra-Luminous X-ray Sources (ULXs) are ideal laboratories to study the effects of super-Eddington accretion, in particular in those powered by accreting neutron stars. These systems were identified only recently through the discovery of coherent pulsations. This discovery lead to a paradigm change in the field, i.e., it completely changed our picture of the make-up of the ULX population, their evolution, and their impact on the environment. While Chandra did not yet see pulsations from ULXs, its superb angular resolution has helped to disentangle sources and monitor their long-term variability. It is likely that strong variability, including off-states, is a tell-tale sign of ULX pulsars, driven the by the strong magnetic field of the neutron star. This strong magnetic field might be measured directly through cyclotron lines, as Chandra as done for M51 ULX8. Chandra has also discovered the first X-ray bubble around a ULX, highlighting the profound impact these sources have on their environment. I will try to summarize the current state-of-the-art on ULX pulsars and their variability and outline how Chandra has and will continue to contribute to this exciting field.

Currently, 17 ultraluminous X-ray sources (ULXs) with globular cluster (GC) counterparts have been identified. ULXs in the old GC environment represent a new population of ULXs, and ones likely to be black holes. These sources show a diverse behaviour with regards to temporal variability, both on long (16 years) and short (

hours) timescales, in both the X-ray and optical wavelengths. These sources can switch on or off over the course of many years, or remain at a constant luminosity. Some sources exhibit a long-term change in their luminosity with no discernible variability within the other observations, Other sources show a stunning long-term variability while also demonstrating variability on the timescale of around four hours. I will undertake a comprehensive comparison of the temporal variability of the zoo of currently known GC ULXs and discuss the possible origins of some of the extreme variability observed.

Some ultraluminous X-ray sources, particularly those identified as neutron star accretors, demonstrate extreme long-term variation in flux of over an order of magnitude. Multiple mechanisms can cause such variability, such as the 'propeller effect' in which accretion is halted at the magnetospheric radius, and super-orbital periods caused by precession in the system. Monitoring ULX-rich galaxies with Chandra can begin to provide us with a population-level understanding of long-term ULX variability and its rarity.

Spectral variability is among the many properties recorded in the Chandra Source Catalog 2.0 (CSC2), and also one of the most relevant astrophysical properties for X-ray sources. TDEs, ULXs, X-ray binaries, all show specific spectral variability features that facilitate their identification. We show results of using an anomaly detection algorithm to identify potentially interesting transient sources in CSC 2.0 that are also spectrally variable. We report on the identification of several luminous transients detected in CSC2 using an anomaly detection algorithm, and on the characterization of their spectral properties. These transients have been hiding for several years as serendipitous sources in Chandra data without being previously spotted or fully characterized. Mining CSC2 has made their discovery and characterization possible. Among the transients reported are soft luminous transients that are compatible with being accreting white dwarfs, X-ray binaries with evolved companions, and flaring young stars. We provide a summary of our early findings.

In this talk, I present the late-time afterglow monitoring and host galaxy discovery of the intermediate-duration GRB180418A, using X-ray facilities and large ground-based telescopes. I present deep Chandra observations till 39 days after the burst, which constrain the GRB outflow to have a wide opening angle >10 deg. I also present a comparison of the afterglow behavior of GRB 180418A to the short and long GRB populations.

The association of a host galaxy with a short gamma-ray burst (SGRB) depends on an accurate localization of the SGRB. 20-30% of well-localized SGRBs lack a coincident host to deep optical and NIR limits. These SGRBs have been identified as observationally hostless due to their lack of strong host associations. Considering early time extit observations of short GRB afterglows we derive lower limits on their circumburst densities. We calculate the gas density at the virial redshift of an average SGRB host galaxy, and by adopting this threshold identify that Contributed Talk in Gamma-ray Bursts on Tuesday, Oct 20th 11:00&ndash12:30 EDT

The discovery of the first binary neutron star merger directly associated with a short gamma-ray burst (GW170817/GRB170817A) reveal the presence of a local population of off-axis events. The onset of the X-ray ray emission from these events is expected after several days from the GRB discovery and is significantly fainter than the on-axis afterglow as revealed by Chandra observation of GRB170817A. In our work we investigate whether similar nearby ( Lightning Talk in Gamma-ray Bursts on Tuesday, Oct 20th 11:00&ndash12:30 EDT

The non-thermal emission from GW170817 came from a structured relativistic outflow from a binary neutron star merger. Understanding how the structure is imprinted on the outflow holds the keys to understanding the nature of the remnant and the jet launching process and composition. I will combine numerical simulations and theoretical considerations to gain insight into such an important process.

Chandra’s fantastic combination of high spatial resolution and sensitivity has paved the path to careful studies of accretion in some of the most fascinating subclasses of X-ray binaries, especially in crowded regions. From the elusive black hole (candidate) X-ray binaries in Galactic and extragalactic globular clusters to the rare transitional millisecond pulsars, such studies have provided invaluable insights about accretion mechanisms in these systems, and subsequently about their population and evolution. Specifically, characterizing variability on short (minutes to hours) and long (days to months/years) timescales in these systems, and possible links with emission in other bands (e.g., radio), have been key diagnostic tools and furthered our understanding of accretion mechanisms. In this talk, I will review some of these findings from the last few years, made possible in part through the eyes of Chandra. I will discuss their impact on our understanding of the evolution of these classes of X-ray binaries.

We report new, strictly simultaneous radio and X-ray observations of the nearby stellar-mass black hole X-ray binary GS 2000+25 in its quiescent state. In deep Chandra observations we detect the system at a faint X-ray luminosity of Lx = 1.1x10^30(d/2 kpc)^2 erg/s (1–10 keV). This is the lowest X-ray luminosity yet observed for a quiescent black hole X-ray binary, corresponding to an Eddington ratio Lx/LEdd

10^−9. In 15 hours of observations with the Karl G. Jansky Very Large Array, no radio continuum emission is detected to a 3σ limit of 1032 erg s^−1. Observations of these sources tax the limits of our current X-ray and radio facilities, and new routes to black hole discovery are needed to study the lowest-luminosity black holes.

Low-mass X-ray binaries (LMXBs) can lay dormant in quiescence, remaining undetected for decades, accreting at very low rates. Despite quiescent LMXBs being notoriously difficult to detect at X-ray energies, they can be routinely detected using relatively small optical telescopes. We have been monitoring

40-50 LMXBs for 15 years using the Faulkes Telescopes/Las Cumbres Observatory (LCO). We are now detecting the early stages of these outbursts with these optical telescopes, before they become bright enough for X-ray detection. Our new real-time optical monitoring pipeline, the "X-ray Binary New Early Warning System (XB-NEWS)" aims to detect and announce new XRB outbursts within a day of first optical detection. This allows us to trigger X-ray and multi-wavelength campaigns during the very early stages of outbursts, to constrain the outburst triggering mechanism. We have an active very fast TOO trigger with Chandra to achieve very early detections coming out of quiescence. Using the results from the XB-NEWS pipeline, we present long term optical monitoring of some LMXBs, and show how Chandra is the key to solving the outburst triggering mechanism.

If X-ray binaries are an ideal laboratory for studying the physics of black hole accretion, then Chandra is a microscope, offering a close-up view of these transients and the intricate life cycles of stellar mass black holes. With a unique combination of high-resolution spectroscopy and high-resolution imaging, Chandra uncovers the microphysics of black hole feedback, enabling us to draw connections between ionized outflows, relativistic jets, and the variable underlying accretion flow. In this talk, I will highlight some of Chandra's insights into the structure, evolution, and inflow-outflow dynamics of disks around black holes. Along the way, I'll discuss some lessons learned from the last 21 years and some hopes and challenges for the future.

We present an analysis of the first observation of the iconic High Mass X-ray Binary 4U 1700-37 with Chandra High Energy Transmission Gratings during X-ray eclipse. This allow us to study in depth the back illuminated stellar wind of the O6Ia star HD153919 =V884 Sco, the earliest donor in any Galactic HMXB, with unprecedented detail. We analyse the behaviour of the emission line spectrum as a function of the continuum emission and present physical properties of the irradiated stellar wind.

Short timescale variability in the lightcurves of X-ray binaries provides an interesting insight into the accretion dynamics. We analyse the "shots" observed in Cygnus X-1 in 0.1-80 keV energy band using simultaneous observation with AstroSat and NICER. We detect simultaneous shots in the soft X-ray band with NICER and in the hard X-rays band with AstroSat-LAXPC. We observe the shot profile for the first time in soft X-rays (0.1-3 keV) and determine the features of the profile in different spectral bands. The relative shot profile peaks in 1.5-3 keV energy range. Using the shot-phase resolved spectroscopy, we break the degeneracy regarding the origin of the sudden surge of the thermal photons. We find that during a shot, the accretion rate remains constant and the inner edge of the accretion disk moves inwards as the shot rises and outwards as the shot decays.This event produces a surge of photons which then get upscattered by the comptonising cloud, thus producing the asymmetric shot profile. We discuss the possible mechanisms causing the swing in the inner radius

For relatively bright X-ray binaries the Chandra HETG allows one to do time resolved X-ray spectroscopy on timescales of hours. It is possible to examine how various spectral features vary as a function of activity and orbital phase. As a case study we will review some of the result from a recent study of Cygnus X-3 (Kallman, et al. 2019).

I will be summarizing the spectral and timing evolution of black hole low-mass X-ray binaries during state transition focusing on the outburst decay stage. I will compare the luminosity distribution of these sources in different states and discuss the impact of observables on the distribution.

Time-domain observations now offer a promising new way to study accretion and jet physics in X-ray binaries. Through detecting and characterizing rapid flux variability in these systems across a wide range of wavelengths/energy bands (probing emission from different regions of the accretion flow and jet), we can measure properties that are difficult, if not impossible, to measure by traditional spectral and imaging methods (e.g., size scales, geometry, jet speeds, the sequence of events leading to jet launching). While variability studies in the X-ray bands are a staple in the X-ray binary community, there are many challenges that accompany such studies at longer wavelengths. However, with recent advances to observing techniques/instrumentation, the availability of new computational tools, and today's improved coordination capabilities, we are no longer limited by these challenges. In this talk, I will discuss new results from fast timing observations of MAXI J1820+070 from the radio through X-ray bands, highlighting how we can directly connect variability properties to internal jet physics. Additionally, I will discuss the role that Chandra can play in this science.

NGC 300 ULX-1 is a pulsar-high mass X-ray binary that has undergone extreme flux variations (by nearly four order of magnitude) since its discovery ten years ago. The outbursting, ultraluminous X-ray behavior has been followed by prolonged decreases in flux - observations with Chandra, Swift, XMM-Newton, and NuSTAR suggest that this variation in flux may be the result of an accretion disk that is precessing due to the Lense-Thirring effect. We present preliminary results of recent efforts with Chandra and Swift to monitor the flux of NGC 300 ULX-1, and discuss Chandra's important contributions to ultraluminous X-ray source monitoring campaigns.

Black hole X-ray binaries in the quiescent state display softer X-ray spectra compared to higher-luminosity black hole X-ray binaries in the hard state. However, the cause of this softening, and its implications for the underlying accretion flow, are still uncertain. Here, we present quasi-simultaneous X-ray and radio spectral monitoring of the black hole X-ray binary MAXI J1820+070 during the decay of its 2018 outburst and of a subsequent re-flare in 2019, providing an opportunity to monitor a black hole X-ray binary as it actively transitions into quiescence. We use our dense coverage of MAXI J1820+070 over four decades in X-ray luminosity to show how rapid multi-wavelength follow-up of fading X-ray binaries can help us understand accretion at low luminosities.

The spin of the black hole in the X-ray binary GRS 1915+105 has long been debated, largely due to the variability of GRS 1915+105 and the need for constraining the mass, distance, and inclination of the black hole. We present a re-analysis of both Middleton et al. 2006 and McClintock et al. 2006, accounting for new constraints on the mass, distance, and inclination and report our findings for the spin of GRS 1915+105.

Knowing the dust content in interstellar matter is necessary to understand composition and evolution of the interstellar medium (ISM). The metal composition of the ISM enables us to study the cooling and heating processes that dominate the star formation rates in our Galaxy. The Chandra High Energy Transmission Grating (HETG) Spectrometer provides a unique opportunity to measure element dust compositions through X-ray edge absorption structure. We measure gas to dust optical depth ratios towards several bright Low-Mass X-ray Binaries (LMXBs) in the Galactic Bulge with the highest precision so far. We also explore edge variability due to ionized Si as was proposed in a previous study. Well calibrated and pile-up free optical depths are measured for a large range broadband hydrogen equivalent absorptions (log N_H [cm^-2] = 21.6 - 22.8). From the optical depths we deduce gas to dust ratios for various silicates in the ISM. The final goal is to model neutral Si gas, Si dust and contributions of ionized Si for different lines of sight towards the Galactic Bulge.

Accretion is a very important physical process that occurs on many different scales throughout our universe. Binary systems that are composed of a neutron star and a low-mass companion, are exciting laboratories to study accretion and associated outflows: Apart from devouring gas, these collapsed stellar remnants also spit matter and energy back into space via collimated radio jets and dense disk winds. These outflows can have a significant impact on the accretion process, the evolution of the binary, and the environment. In this talk, I will highlight Chandra’s past, present, and future contribution to time-domain studies of accretion and outflows in neutron star low-mass X-ray binaries.

It is now established that hard-state accreting neutron stars in low-mass X-ray binaries show outflows — and sometimes jets — in the general manner of accreting black holes. However, the quantitative link between the accretion flow (traced by X-rays) and the outflow/jet (traced by radio emission) is much less well-understood for neutron stars than for black holes. Here we use the deep MAVERIC radio continuum survey of 50 Galactic globular clusters to do a systematic study of the radio and X-ray properties of all the luminous (L_X > 10^34 erg/s) persistent neutron star X-ray binaries in our survey, as well as two other transients also captured in our data. We find that these neutron star X-ray binaries show a much larger range in radio luminosity than previously observed, and some have outflows as luminous as those of black holes. These results show that neutron stars do not evince a single relation between inflow and outflow and that the accretion dynamics are more complex than for black holes.

We present a Bayesian analysis, using the fact that Chandra is a "single photon detector", showing eclipse timing of the quiescent neutron star binary 4U 2129+47. Variations in this eclipse time reveal the presence of a third body in this likely hierarchical triple, and allow us to determine the most likely triple orbital parameters. Furthermore, we discuss long term cooling of the NS, as revealed by both Chandra and XMM-Newton observations conducted over a time span of a decade.

ML algorithms provide an efficient way of identifying the astrophysical nature of many thousands of unclassified X-rays sources in large catalogs, such as CSCv2. The addition of multi-wavelength (MW) and temporal features provides a wealth of information about the sources that need to be analyzed. Variability, which can be quantified in many ways (significance, magnitude, timescale, etc), is one of the most important features that sheds light on the physical processes occurring in different kinds of astrophysical objects. In this talk, I will outline our approach to supervised ML classification of X-ray sources based on CSCv2 and MW catalogs, focusing on the variability and temporal features used in our ML pipeline. The limitations and possible extensions of the variability features in CSCv2 will be briefly discussed. I will also talk about the correlations between the long-term (inter-observation on year long time scales) and the short term (intra-observation on time scales of days or less) variability for different astrophysical classes of sources in CSCv2. Preliminary results regarding the classifications of some variable sources in Galactic fields will be presented.

I will summarise all what we have learned in the past two decades about strongly magnetised neutron stars, and the unexpected Chandra discoveries in the field. From their steady X-ray emission, their bright bursts, their possible cyclotron lines, their powerful outburst events, to the physics involved in their extreme emission and their connection with the rest of the pulsar population.

The "Chandra ACIS Timing Survey at Brera And Roma observatories" project ([email protected]), aimed at searching for new pulsators in the X-rays, is entirely based on 20 years of Chandra ACIS TE archival data. This timing survey, with 14000 analyzed ACIS pointing and almost 500000 time series extracted, more than 200000 of which has been searched for coherent signals, is one of the largest ever carried out in the 0.5-10keV band. So far we discovered about 50 new X-ray pulsators of different nature, such as cataclysmic variables, accreting neutron stars, and black hole candidates both in the Milky Way and in nearby galaxies. To test the signal goodness, we started a parallel project aimed at identifying and studying their optical counterparts and by requesting follow-up X-ray observations with Chandra and XMM. As a by-product of this project, we studied in details the spurious signals which are present in the ACIS time series. Additionally, this is a living project and the detection algorithm will continue to be routinely applied to the new Chandra data as they become public. Based on the results obtained so far, we expect to discover about three new pulsators every year.

In this talk, I summarize the current status in the field of the crust cooling emission for strongly magnetized neutron stars in Be/X-ray transients under an observational point of view. Additionally, I highlight the role of Chandra in the long-term monitoring of these systems that allow to study the potential effects of different crustal magnetic-field configuration on the crust cooling behavior.

Since their discovery in 2007, much effort has been devoted to uncovering the sources of the extragalactic, millisecond-duration fast radio bursts (FRBs). The short durations and energetics of FRBs favored magnetar progenitors. In this talk, we present the discovery of a millisecond-duration radio burst from the Galactic magnetar SGR J1935+2154, with a fluence of 1.5±0.3 Mega-Jansky milliseconds. The isotropic-equivalent energy released in this event, termed ST 200428A (=FRB 200428) is 4000 times greater than in any Galactic radio burst previously observed on similar timescales. ST 200428A is just 40 times less energetic than the weakest extragalactic FRB observed to date, and is arguably drawn from the same population as the observed FRB sample. This event is the first FRB with an X-ray counterpart, which was typical in energy and duration for a magnetar burst, but has a significantly harder spectrum than a typical magnetar burst. The discovery of ST 200428A implies that active magnetars like SGR 1935+2154 can produce FRBs at extragalactic distances.

Over the last two decades, Chandra Deep Surveys allowed to monitor AGN variability on a large range of timescales, redshifts and luminosities. I will discuss the progresses made on the use of X-ray variability of AGNs in order to constrain the physics and the main properties of accreting supermassive BH through cosmic time, as well as a tool for cosmological studies.

I will report on Chandra observations of the unusual Narrow-Line Seyfert 1 Galaxy WPVS 007, and AGN that exhibits Broad Absorption lines which are only seen in Quasars. Models explaining the strong UV variability observed by HST while the AGN remains in an extreme X-ray low state will be discussed.

Chandra and uGMRT have imaged NGC4869 and its X-ray environment. We detect a steep-spectrum sheath layer enveloping a flat-spectrum spine, hinting at transverse velocity structure with fast-moving spine surrounded by a slow-moving sheath. Also seen is a ridge of radio emission, i.e, flaring of a collimated jet as it crosses a surface brightness edge, which is due to Kelvin-Helmholtz instabilities.

Active Galactic Nuclei show X-ray variability on timescales down to minutes, indicating a compact emission region close to the central supermassive black hole. In the past decade Chandra and XMM-Newton have made pivotal and complementary observations to probe these compact environments through variability studies. In this review talk, I will discuss how Chandra's exquisite spatial resolution enables studies of microlensing of distant quasars to probe the inner accretion flow geometry, and how XMM-Newton's complementary large effective area has enabled X-ray reverberation mapping of these same compact regions. Chandra and XMM-Newton’s independent measurements show that the X-ray corona in luminous AGN is much more compact than the accretion disc, which is not highly truncated and extends down to the scale of the innermost stable circular orbit.

We use Fermi-LAT data to establish a blazar sequence based solely on the flux variability properties of blazars at timescales of days to years. We find that variability patterns are correlated with blazar spectral properties, and that BL Lac-type blazars and flat spectrum radio quasars show distinguishable flaring features. Our results align with predictions from leptonic emission scenarios with the differences in flux variability properties being explained by varying rates of radiative cooling. BL Lac-type blazars display higher levels of gamma-ray flux variability as their luminosity increases and their broadband spectral energy distribution shifts to redder frequencies. The variability observed in flat spectrum radio quasars also displays a range of behaviors, but a correlation between physical properties of the AGN and the measures of variability was not found.

I will present analysis of Chandra X-ray observations of seven quasars that were identified as candidate subparsec binary supermassive black hole (SMBH) systems in the Catalina Real-Time Transient Survey based on apparent periodicity in their optical light curves. Simulations predict that close-separation accreting SMBH binaries will have different X-ray spectra than single accreting SMBHs, including harder or softer X-ray spectra, ripple-like profiles in the Fe K-α line, and distinct peaks in the spectrum due to the separation of the accretion disk into a circumbinary disk and mini disks around each SMBH. I found these seven quasar spectra were all well fit by simple absorbed power-law models, with the rest-frame 2–10 keV photon indices, Γ, and the X-ray-to-optical power slopes, α(OX), indistinguishable from those of the larger quasar population. This could mean that these seven quasars are not truly SMBH binaries, and/or that differences between single and binary SMBH spectra lie outside the Chandra band, and/or that models of binary SMBHs need to be adjusted.

We present the class of Locally Stationary ARMA models with parameters that are smooth functions of time and have potential to statistically describe the X-ray PSD evolution of AGN and X-ray binaries. Chandra’s 20 years archive, in synergy with other X-ray observatories, provides means to test the LSARMA description and study physical processes governing the X-ray variability of accreting objects.

I will present a new unified model for time-dependent relativistic X-ray reflection in accreting compact objects. We self-consistently merge the best spectral and timing reflection models to make accurate predictions for the flux-energy, lag-frequency, lag-energy spectra simultaneously. I will show the application of this model to X-ray data from accreting black holes.

As optical time-domain surveys uncover new classes of astrophysical transients, it has become clear that classical mechanisms for powering their light curves (e.g. nuclear fusion, radioactive decay, residual heat of explosion) are insufficient to explain many of their luminosities and timescales. These classes potentially include: superluminous supernovae, neutron star mergers, white dwarf-neutron star mergers, classical novae, binary star mergers, "fast blue optical transients" (FBOT), and tidal disruption events. In many cases, the light from these events may be powered by a long-lived "central engine" enshrouded in the ejecta, whether that engine is a bonafide compact object (magnetar or black hole) or internal shocks generated as the ejecta collides with an external medium. I will describe a unifying picture for engine-powered optical transients, focusing on the crucial role X-ray observations play in directly probing the "engine" (particularly in cases where X-rays can escape the ejecta to be observed). I will highlight a few illustrative examples: long-lived magnetars in binary neutron star mergers, superluminous supernovae, and the FBOT AT2018cow.

The origin of very-high energy cosmic rays remains a mystery, decades after the initial discovery. Neutrinos provide a unique tool in the search for the sources of cosmic rays. The blazar TXS 0506+056 was one of the first AGN connected to a neutrino event, IceCube-170922A. X-ray observations have played a crucial role in identifying the origin of astrophysical neutrinos, in particular related to the flux constraints expected from secondary cascades. Follow-up observations of IceCube alerts at all wavelengths have not revealed obvious source counterparts, with blazars not being able to explain all of the IceCube neutrinos. We present results from the IceCube-190331A neutrino event and have identified a possible type as the dominant IceCube neutrino emitter. We show that radio-quiet, gamma-ray quiet AGN are in agreement with observed neutrinos, particularly for the case of the neutrino IceCube-190331A.

In this talk, I will describe ongoing work using the Zwicky Transient Facility (ZTF) to identify ultracompact binaries (P 50) LISA gravitational wave sources, and many of the others should be detectable at lower SNR.

The recent discovery of high-energy astrophysical neutrinos has opened a new window to the Universe. In September 2017, the detection of a high-energy neutrino in coincidence with a flaring gamma-ray blazar revealed the first compelling high-energy neutrino source candidate. At the same time, gamma-ray blazars are disfavored as the dominant neutrino source class. Other plausible source candidates are tidal disruption events, low-luminosity gamma-ray bursts and supernovae. Combining neutrino data with electromagnetic measurements in a multi-messenger approach will increase the sensitivity to identify neutrino sources and help to solve long-standing problems in astrophysics such as the origin of cosmic rays. I will review the recent progress in neutrino multi-messenger astronomy.

In 2017 our understanding of compact binary mergers was transformed by the spectacular discovery of GW170817, the first neutron star merger observed through gravitational waves and light. Like all revolutionary discoveries, GW170817 posed as many questions as it answered. What is the fate of the merger remnant? Do all NS mergers launch successful relativistic jets? Is radioactive decay the only power-source of kilonovae? How does a neutron star - black hole merger look like? Stemming from the experience of GW170817, I'll discuss the crucial role of X-ray observations of gravitational wave sources.

As advanced LIGO-Virgo approach “design sensitivity” these gravitational wave experiments anticipate one or more high-significance events per day. Even if only a small fraction of these are likely to boast an electromagnetic counterpart, rapid target selection, characterization, and vetting will be essential for prioritizing candidates for other elite ground- and space-based facilities. Are scientists prepared to design and launch campaigns that maximize discovery from these and other time domain events? How prepared are Chandra and other observatories to meet the demand for these time domain observations? I’ll share a few ideas and likely ask more questions than I answer.

I will briefly highlight the predicted diversity in the outcomes of binary neutron star mergers and their associated electromagnetic counterparts (and, time permitting, of neutron star-black hole mergers). An empirical map between gravitational wave and electromagnetic observables could become a powerful probe for constraining the equation of state of neutron stars.

I will present a critical review of the observational constraints on the neutron star merger GW170817

## Ocean energy conversion

### 21.1 Introduction

Oceans can be considered as large accumulators of solar and gravitational energy which are converted into mechanical energy in the form of ocean streams, waves, tides, and other patterns of water movement. The mechanism of this energy conversion, as broadly discussed in Chapter 2 , relies on fundamental energy sources, which are infinite on the humankind scale and therefore can be considered as purely renewable.

While ocean energy has been known and somehow utilized for many years, its commercial implementation has not yet been put on a global and considerable scale. With less than 1% of the total electricity generation, ocean energy of all sorts is still a future, yet promising source of green and sustainable energy. Very few commercial ocean energy facilities have been built to date. Of the approximately 536 MW of operating capacities at the end of 2016, more than 90% was represented by two tidal barrage facilities (a 254 MW plant in the Republic of Korea and a 240 MW in France) [1] .

In general, ocean energy resources are contained in ocean waves, tidal range, tidal currents, ocean currents, ocean thermal energy, and salinity gradients [2,3] . These resources are vast but not evenly distributed however, there is always some source of ocean energy available at every coast. These various forms of energy can potentially be harnessed and converted into electricity.

Given the relative immaturity of ocean energy technologies versus most other energy technologies, much of the focus in terms of barriers to deployment has been on the level and type of energy innovation support for ocean energy [4] .

It is important to mention that marine energy is probably among the least visible and least supported among emerging energy sources. Of course, most countries in which ocean energy technologies are currently being developed have renewable energy targets, although very few of them have specific policies to promote ocean energy. They mostly implement various mechanisms like capital grants or financial incentives to create early-stage opportunities in renewables generally, but almost never use market mechanisms or initiatives specifically for ocean energy development [3] .

## Contents

For most of history, humanity did not recognize or understand the concept of the Solar System. Most people up to the Late Middle Ages–Renaissance believed Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system. [11] [12]

In the 17th century, Galileo discovered that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it. [13] Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn. [14] Edmond Halley realised in 1705 that repeated sightings of a comet were recording the same object, returning regularly once every 75–76 years. This was the first evidence that anything other than the planets orbited the Sun. [15] Around this time (1704), the term "Solar System" first appeared in English. [16] In 1838, Friedrich Bessel successfully measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism. [17] Improvements in observational astronomy and the use of uncrewed spacecraft have since enabled the detailed investigation of other bodies orbiting the Sun.

The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally. [18] The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the Solar System's total mass. [g]

Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. The planets are very close to the ecliptic, whereas comets and Kuiper belt objects are frequently at significantly greater angles to it. [22] [23] As a result of the formation of the Solar System, planets (and most other objects) orbit the Sun in the same direction that the Sun is rotating (counter-clockwise, as viewed from above Earth's north pole). [24] There are exceptions, such as Halley's Comet. Most of the larger moons orbit their planets in this prograde direction (with Triton being the largest retrograde exception) and most larger objects rotate themselves in the same direction (with Venus being a notable retrograde exception).

The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of mostly rocky asteroids, and four giant planets surrounded by the Kuiper belt of mostly icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets. [25] Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune. [26]

Most of the planets in the Solar System have secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which, Titan and Ganymede, are larger than the planet Mercury). The four giant planets have planetary rings, thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent. [27]

Kepler's laws of planetary motion describe the orbits of objects about the Sun. Following Kepler's laws, each object travels along an ellipse with the Sun at one focus. Objects closer to the Sun (with smaller semi-major axes) travel more quickly because they are more affected by the Sun's gravity. On an elliptical orbit, a body's distance from the Sun varies over the course of its year. A body's closest approach to the Sun is called its perihelion, whereas its most distant point from the Sun is called its aphelion. The orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. The positions of the bodies in the Solar System can be predicted using numerical models.

Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum. [28] [29] The planets, dominated by Jupiter, account for most of the rest of the angular momentum due to the combination of their mass, orbit, and distance from the Sun, with a possibly significant contribution from comets. [28]

The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium. [30] Jupiter and Saturn, which comprise nearly all the remaining matter, are also primarily composed of hydrogen and helium. [31] [32] A composition gradient exists in the Solar System, created by heat and light pressure from the Sun those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points. [33] The boundary in the Solar System beyond which those volatile substances could condense is known as the frost line, and it lies at roughly 5 AU from the Sun. [4]

The objects of the inner Solar System are composed mostly of rock, [34] the collective name for compounds with high melting points, such as silicates, iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula. [35] Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapour pressure, such as hydrogen, helium, and neon, which were always in the gaseous phase in the nebula. [35] Ices, like water, methane, ammonia, hydrogen sulfide, and carbon dioxide, [34] have melting points up to a few hundred kelvins. [35] They can be found as ices, liquids, or gases in various places in the Solar System, whereas in the nebula they were either in the solid or gaseous phase. [35] Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune's orbit. [34] [36] Together, gases and ices are referred to as volatiles. [37]

### Distances and scales

The distance from Earth to the Sun is 1 astronomical unit [AU] (150,000,000 km 93,000,000 mi). For comparison, the radius of the Sun is 0.0047 AU (700,000 km). Thus, the Sun occupies 0.00001% (10 −5 %) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly one millionth (10 −6 ) that of the Sun. Jupiter, the largest planet, is 5.2 astronomical units (780,000,000 km) from the Sun and has a radius of 71,000 km (0.00047 AU), whereas the most distant planet, Neptune, is 30 AU (4.5 × 10 9 km) from the Sun.

With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearer object to the Sun. For example, Venus is approximately 0.33 AU farther out from the Sun than Mercury, whereas Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances (for example, the Titius–Bode law), [38] but no such theory has been accepted.

Some Solar System models attempt to convey the relative scales involved in the Solar System on human terms. Some are small in scale (and may be mechanical—called orreries)—whereas others extend across cities or regional areas. [39] The largest such scale model, the Sweden Solar System, uses the 110-metre (361 ft) Ericsson Globe in Stockholm as its substitute Sun, and, following the scale, Jupiter is a 7.5-metre (25-foot) sphere at Stockholm Arlanda Airport, 40 km (25 mi) away, whereas the farthest current object, Sedna, is a 10 cm (4 in) sphere in Luleå, 912 km (567 mi) away. [40] [41]

If the Sun–Neptune distance is scaled to 100 metres, then the Sun would be about 3 cm in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about 3 mm, and Earth's diameter along with that of the other terrestrial planets would be smaller than a flea (0.3 mm) at this scale. [42]

Distances of selected bodies of the Solar System from the Sun. The left and right edges of each bar correspond to the perihelion and aphelion of the body, respectively, hence long bars denote high orbital eccentricity. The radius of the Sun is 0.7 million km, and the radius of Jupiter (the largest planet) is 0.07 million km, both too small to resolve on this image.

The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud. [h] This initial cloud was likely several light-years across and probably birthed several stars. [44] As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars. As the region that would become the Solar System, known as the pre-solar nebula, [45] collapsed, conservation of angular momentum caused it to rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc. [44] As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU [44] and a hot, dense protostar at the centre. [46] [47] The planets formed by accretion from this disc, [48] in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed, leaving the planets, dwarf planets, and leftover minor bodies. [49]

Due to their higher boiling points, only metals and silicates could exist in solid form in the warm inner Solar System close to the Sun, and these would eventually form the rocky planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large. The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid. The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium, the lightest and most abundant elements. Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud. [49] The Nice model is an explanation for the creation of these regions and how the outer planets could have formed in different positions and migrated to their current orbits through various gravitational interactions. [51]

Within 50 million years, the pressure and density of hydrogen in the centre of the protostar became great enough for it to begin thermonuclear fusion. [52] The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved: the thermal pressure equalled the force of gravity. At this point, the Sun became a main-sequence star. [53] The main-sequence phase, from beginning to end, will last about 10 billion years for the Sun compared to around two billion years for all other phases of the Sun's pre-remnant life combined. [54] Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process. The Sun is growing brighter early in its main-sequence life its brightness was 70% that of what it is today. [55]

The Solar System will remain roughly as we know it today until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5 billion years from now. This will mark the end of the Sun's main-sequence life. At this time, the core of the Sun will contract with hydrogen fusion occurring along a shell surrounding the inert helium, and the energy output will be much greater than at present. The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler (2,600 K at its coolest) than it is on the main sequence. [54] The expanding Sun is expected to vaporize Mercury and render Earth uninhabitable. Eventually, the core will be hot enough for helium fusion the Sun will burn helium for a fraction of the time it burned hydrogen in the core. The Sun is not massive enough to commence the fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will move away into space, leaving a white dwarf, an extraordinarily dense object, half the original mass of the Sun but only the size of Earth. [56] The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun—but now enriched with heavier elements like carbon—to the interstellar medium.

The Sun is the Solar System's star and by far its most massive component. Its large mass (332,900 Earth masses), [57] which comprises 99.86% of all the mass in the Solar System, [58] produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium, making it a main-sequence star. [59] This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light. [60]

The Sun is a G2-type main-sequence star. Hotter main-sequence stars are more luminous. The Sun's temperature is intermediate between that of the hottest stars and that of the coolest stars. Stars brighter and hotter than the Sun are rare, whereas substantially dimmer and cooler stars, known as red dwarfs, make up 85% of the stars in the Milky Way. [61] [62]

The Sun is a population I star it has a higher abundance of elements heavier than hydrogen and helium ("metals" in astronomical parlance) than the older population II stars. [63] Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, whereas stars born later have more. This high metallicity is thought to have been crucial to the Sun's development of a planetary system because the planets form from the accretion of "metals". [64]

The vast majority of the Solar System consists of a near-vacuum known as the interplanetary medium. Along with light, the Sun radiates a continuous stream of charged particles (a plasma) known as the solar wind. This stream of particles spreads outwards at roughly 1.5 million kilometres per hour, [65] creating a tenuous atmosphere that permeates the interplanetary medium out to at least 100 AU (see § Heliosphere). [66] Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturbs the heliosphere, creating space weather and causing geomagnetic storms. [67] The largest structure within the heliosphere is the heliospheric current sheet, a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium. [68] [69]

Earth's magnetic field stops its atmosphere from being stripped away by the solar wind. [70] Venus and Mars do not have magnetic fields, and as a result the solar wind is causing their atmospheres to gradually bleed away into space. [71] Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into Earth's upper atmosphere, where its interactions create aurorae seen near the magnetic poles.

The heliosphere and planetary magnetic fields (for those planets that have them) partially shield the Solar System from high-energy interstellar particles called cosmic rays. The density of cosmic rays in the interstellar medium and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic-ray penetration in the Solar System varies, though by how much is unknown. [72]

The interplanetary medium is home to at least two disc-like regions of cosmic dust. The first, the zodiacal dust cloud, lies in the inner Solar System and causes the zodiacal light. It was likely formed by collisions within the asteroid belt brought on by gravitational interactions with the planets. [73] The second dust cloud extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the Kuiper belt. [74] [75]

The inner Solar System is the region comprising the terrestrial planets and the asteroid belt. [76] Composed mainly of silicates and metals, the objects of the inner Solar System are relatively close to the Sun the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn. This region is also within the frost line, which is a little less than 5 AU (about 700 million km) from the Sun. [77]

### Inner planets

The four terrestrial or inner planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of refractory minerals, such as the silicates—which form their crusts and mantles—and metals, such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather all have impact craters and tectonic surface features, such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets that are closer to the Sun than Earth is (i.e. Mercury and Venus).

#### Mercury

Mercury ( 0.4 AU from the Sun) is the closest planet to the Sun and on average, all seven other planets. [78] [79] The smallest planet in the Solar System (0.055 M ), Mercury has no natural satellites. Besides impact craters, its only known geological features are lobed ridges or rupes that were probably produced by a period of contraction early in its history. [80] Mercury's very tenuous atmosphere consists of atoms blasted off its surface by the solar wind. [81] Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, or that it was prevented from fully accreting by the young Sun's energy. [82] [83]

#### Venus

Venus (0.7 AU from the Sun) is close in size to Earth (0.815 M ) and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere, and evidence of internal geological activity. It is much drier than Earth, and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C (752 °F), most likely due to the amount of greenhouse gases in the atmosphere. [84] No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is being replenished by volcanic eruptions. [85]

#### Earth

Earth (1 AU from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only place where life is known to exist. [86] Its liquid hydrosphere is unique among the terrestrial planets, and it is the only planet where plate tectonics has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen. [87] It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System.

Mars (1.5 AU from the Sun) is smaller than Earth and Venus (0.107 M ). It has an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (roughly 0.6% of that of Earth). [88] Its surface, peppered with vast volcanoes, such as Olympus Mons, and rift valleys, such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago. [89] Its red colour comes from iron oxide (rust) in its soil. [90] Mars has two tiny natural satellites (Deimos and Phobos) thought to be either captured asteroids, [91] or ejected debris from a massive impact early in Mars's history. [92]

### Asteroid belt

• Sun
• Jupiter trojans
• Planetary orbit
• Asteroid belt
• Hilda asteroids
• NEOs(selection)

Asteroids except for the largest, Ceres, are classified as small Solar System bodies [f] and are composed mainly of refractory rocky and metallic minerals, with some ice. [93] [94] They range from a few metres to hundreds of kilometres in size. Asteroids smaller than one meter are usually called meteoroids and micrometeoroids (grain-sized), depending on different, somewhat arbitrary definitions.

The asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter. [95] The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. [96] Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth. [21] The asteroid belt is very sparsely populated spacecraft routinely pass through without incident. [97]

#### Ceres

Ceres (2.77 AU) is the largest asteroid, a protoplanet, and a dwarf planet. [f] It has a diameter of slightly under 1000 km , and a mass large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in 1801 and was reclassified to asteroid in the 1850s as further observations revealed additional asteroids. [98] It was classified as a dwarf planet in 2006 when the definition of a planet was created.

#### Asteroid groups

Asteroids in the asteroid belt are divided into asteroid groups and families based on their orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets, which may have been the source of Earth's water. [99]

Jupiter trojans are located in either of Jupiter's L4 or L5 points (gravitationally stable regions leading and trailing a planet in its orbit) the term trojan is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids are in a 2:3 resonance with Jupiter that is, they go around the Sun three times for every two Jupiter orbits. [100]

The inner Solar System also contains near-Earth asteroids, many of which cross the orbits of the inner planets. [101] Some of them are potentially hazardous objects.

The outer region of the Solar System is home to the giant planets and their large moons. The centaurs and many short-period comets also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid. [49]

### Outer planets

The four outer planets, or giant planets (sometimes called Jovian planets), collectively make up 99% of the mass known to orbit the Sun. [g] Jupiter and Saturn are together more than 400 times the mass of Earth and consist overwhelmingly of the gases hydrogen and helium, hence their designation as gas giants. [102] Uranus and Neptune are far less massive—less than 20 Earth masses ( M ) each—and are composed primarily of ices. For these reasons, some astronomers suggest they belong in their own category, ice giants. [103] All four giant planets have rings, although only Saturn's ring system is easily observed from Earth. The term superior planet designates planets outside Earth's orbit and thus includes both the outer planets and Mars.

#### Jupiter

Jupiter (5.2 AU), at 318 M , is 2.5 times the mass of all the other planets put together. It is composed largely of hydrogen and helium. Jupiter's strong internal heat creates semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Jupiter has 79 known satellites. The four largest, Ganymede, Callisto, Io, and Europa, show similarities to the terrestrial planets, such as volcanism and internal heating. [104] Ganymede, the largest satellite in the Solar System, is larger than Mercury.

#### Saturn

Saturn (9.5 AU), distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at 95 M . Saturn is the only planet of the Solar System that is less dense than water. [105] The rings of Saturn are made up of small ice and rock particles. Saturn has 82 confirmed satellites composed largely of ice. Two of these, Titan and Enceladus, show signs of geological activity. [106] Titan, the second-largest moon in the Solar System, is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.

#### Uranus

Uranus (19.2 AU), at 14 M , is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the other giant planets and radiates very little heat into space. [107] Uranus has 27 known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel, and Miranda. [108]

#### Neptune

Neptune ( 30.1 AU ), though slightly smaller than Uranus, is more massive (17 M ) and hence more dense. It radiates more internal heat, but not as much as Jupiter or Saturn. [109] Neptune has 14 known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen. [110] Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by several minor planets, termed Neptune trojans, that are in 1:1 resonance with it.

### Centaurs

The centaurs are icy comet-like bodies whose orbits have semi-major axes greater than Jupiter's (5.5 AU) and less than Neptune's (30 AU). The largest known centaur, 10199 Chariklo, has a diameter of about 250 km. [111] The first centaur discovered, 2060 Chiron, has also been classified as a comet (95P) because it develops a coma just as comets do when they approach the Sun. [112]

Comets are small Solar System bodies, [f] typically only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye.

Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are thought to originate in the Kuiper belt, whereas long-period comets, such as Hale–Bopp, are thought to originate in the Oort cloud. Many comet groups, such as the Kreutz Sungrazers, formed from the breakup of a single parent. [113] Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult. [114] Old comets whose volatiles have mostly been driven out by solar warming are often categorised as asteroids. [115]

Beyond the orbit of Neptune lies the area of the "trans-Neptunian region", with the doughnut-shaped Kuiper belt, home of Pluto and several other dwarf planets, and an overlapping disc of scattered objects, which is tilted toward the plane of the Solar System and reaches much further out than the Kuiper belt. The entire region is still largely unexplored. It appears to consist overwhelmingly of many thousands of small worlds—the largest having a diameter only a fifth that of Earth and a mass far smaller than that of the Moon—composed mainly of rock and ice. This region is sometimes described as the "third zone of the Solar System", enclosing the inner and the outer Solar System. [116]

### Kuiper belt

• Sun
• Jupiter trojans
• Giant planets
• Kuiper belt
• Scattered disc
• Neptune trojans

The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice. [117] It extends between 30 and 50 AU from the Sun. Though it is estimated to contain anything from dozens to thousands of dwarf planets, it is composed mainly of small Solar System bodies. Many of the larger Kuiper belt objects, such as Quaoar, Varuna, and Orcus, may prove to be dwarf planets with further data. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of Earth. [20] Many Kuiper belt objects have multiple satellites, [118] and most have orbits that take them outside the plane of the ecliptic. [119]

The Kuiper belt can be roughly divided into the "classical" belt and the resonances. [117] Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two). The first resonance begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 AU to 47.7 AU. [120] Members of the classical Kuiper belt are classified as cubewanos, after the first of their kind to be discovered, 15760 Albion (which previously had the provisional designation 1992 QB1), and are still in near primordial, low-eccentricity orbits. [121]

#### Pluto and Charon

The dwarf planet Pluto (with an average orbit of 39 AU) is the largest known object in the Kuiper belt. When discovered in 1930, it was considered to be the ninth planet this changed in 2006 with the adoption of a formal definition of planet. Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion. Pluto has a 3:2 resonance with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinos. [122]

Charon, the largest of Pluto's moons, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a barycentre of gravity above their surfaces (i.e. they appear to "orbit each other"). Beyond Charon, four much smaller moons, Styx, Nix, Kerberos, and Hydra, orbit within the system.

#### Makemake and Haumea

Makemake (45.79 AU average), although smaller than Pluto, is the largest known object in the classical Kuiper belt (that is, a Kuiper belt object not in a confirmed resonance with Neptune). Makemake is the brightest object in the Kuiper belt after Pluto. It was assigned a naming committee under the expectation that it would prove to be a dwarf planet in 2008. [6] Its orbit is far more inclined than Pluto's, at 29°. [123]

Haumea (43.13 AU average) is in an orbit similar to Makemake, except that it is in a temporary 7:12 orbital resonance with Neptune. [124] It was named under the same expectation that it would prove to be a dwarf planet, though subsequent observations have indicated that it may not be a dwarf planet after all. [125]

### Scattered disc

The scattered disc, which overlaps the Kuiper belt but extends out to about 200 AU, is thought to be the source of short-period comets. Scattered-disc objects are thought to have been ejected into erratic orbits by the gravitational influence of Neptune's early outward migration. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia far beyond it (some more than 150 AU from the Sun). SDOs' orbits are also highly inclined to the ecliptic plane and are often almost perpendicular to it. Some astronomers consider the scattered disc to be merely another region of the Kuiper belt and describe scattered disc objects as "scattered Kuiper belt objects". [126] Some astronomers also classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc. [127]

Eris (with an average orbit of 68 AU) is the largest known scattered disc object, and caused a debate about what constitutes a planet, because it is 25% more massive than Pluto [128] and about the same diameter. It is the most massive of the known dwarf planets. It has one known moon, Dysnomia. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane.

The point at which the Solar System ends and interstellar space begins is not precisely defined because its outer boundaries are shaped by two forces, the solar wind and the Sun's gravity. The limit of the solar wind's influence is roughly four times Pluto's distance from the Sun this heliopause, the outer boundary of the heliosphere, is considered the beginning of the interstellar medium. [66] The Sun's Hill sphere, the effective range of its gravitational dominance, is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud. [129]

### Heliosphere

The heliosphere is a stellar-wind bubble, a region of space dominated by the Sun, in which it radiates its solar wind at approximately 400 km/s, a stream of charged particles, until it collides with the wind of the interstellar medium.

The collision occurs at the termination shock, which is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind. [130] Here the wind slows dramatically, condenses and becomes more turbulent, [130] forming a great oval structure known as the heliosheath. This structure is thought to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind evidence from the Cassini and Interstellar Boundary Explorer spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field. [131]

The outer boundary of the heliosphere, the heliopause, is the point at which the solar wind finally terminates and is the beginning of interstellar space. [66] Voyager 1 and Voyager 2 are reported to have passed the termination shock and entered the heliosheath, at 94 and 84 AU from the Sun, respectively. [132] [133] Voyager 1 is reported to have crossed the heliopause in August 2012. [134]

The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere. [130] Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way. [135]

• inner Solar System and Jupiter
• outer Solar System and Pluto
• orbit of Sedna (detached object)
• inner part of the Oort Cloud

Due to a lack of data, conditions in local interstellar space are not known for certain. It is expected that NASA's Voyager spacecraft, as they pass the heliopause, will transmit valuable data on radiation levels and solar wind to Earth. [136] How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A NASA-funded team has developed a concept of a "Vision Mission" dedicated to sending a probe to the heliosphere. [137] [138]

### Detached objects

90377 Sedna (with an average orbit of 520 AU) is a large, reddish object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 940 AU at aphelion and takes 11,400 years to complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper belt because its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, sometimes termed "distant detached objects" (DDOs), which also may include the object 2000 CR105 , which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3,420 years. [139] Brown terms this population the "inner Oort cloud" because it may have formed through a similar process, although it is far closer to the Sun. [140] Sedna is very likely a dwarf planet, though its shape has yet to be determined. The second unequivocally detached object, with a perihelion farther than Sedna's at roughly 81 AU, is 2012 VP 113 , discovered in 2012. Its aphelion is only half that of Sedna's, at 400–500 AU. [141] [142]

### Oort cloud

The Oort cloud is a hypothetical spherical cloud of up to a trillion icy objects that is thought to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU (around 1 light-year (ly)), and possibly to as far as 100,000 AU (1.87 ly). It is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events, such as collisions, the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way. [143] [144]

### Boundaries

Much of the Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light-years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU. [145] Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury and the Sun. [146] Objects may yet be discovered in the Solar System's uncharted regions.

Currently, the furthest known objects, such as Comet West, have aphelia around 70,000 AU from the Sun, but as the Oort cloud becomes better known, this may change.

The Solar System is located in the Milky Way, a barred spiral galaxy with a diameter of about 100,000 light-years containing more than 100 billion stars. [147] The Sun resides in one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm or Local Spur. [148] The Sun lies about 26,660 light-years from the Galactic Centre, [149] and its speed around the center of the Milky Way is about 247 km/s, so that it completes one revolution every 210 million years. This revolution is known as the Solar System's galactic year. [150] The solar apex, the direction of the Sun's path through interstellar space, is near the constellation Hercules in the direction of the current location of the bright star Vega. [151] The plane of the ecliptic lies at an angle of about 60° to the galactic plane. [i]

The Solar System's location in the Milky Way is a factor in the evolutionary history of life on Earth. Its orbit is close to circular, and orbits near the Sun are at roughly the same speed as that of the spiral arms. [153] [154] Therefore, the Sun passes through arms only rarely. Because spiral arms are home to a far larger concentration of supernovae, gravitational instabilities, and radiation that could disrupt the Solar System, this has given Earth long periods of stability for life to evolve. [153] However, the changing position of the Solar System relative to other parts of the Milky Way could explain periodic extinction events on Earth, according to the Shiva hypothesis or related theories. The Solar System lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life. [153] Even at the Solar System's current location, some scientists have speculated that recent supernovae may have adversely affected life in the last 35,000 years, by flinging pieces of expelled stellar core towards the Sun, as radioactive dust grains and larger, comet-like bodies. [155]

### Neighbourhood

The Solar System is in the Local Interstellar Cloud or Local Fluff. It is thought to be near the neighbouring G-Cloud but it is not known if the Solar System is embedded in the Local Interstellar Cloud, or if it is in the region where the Local Interstellar Cloud and G-Cloud are interacting. [156] [157] The Local Interstellar Cloud is an area of denser cloud in an otherwise sparse region known as the Local Bubble, an hourglass-shaped cavity in the interstellar medium roughly 300 light-years (ly) across. The bubble is suffused with high-temperature plasma, that suggests it is the product of several recent supernovae. [158]

There are relatively few stars within ten light-years of the Sun. The closest is the triple star system Alpha Centauri, which is about 4.4 light-years away. Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the small red dwarf, Proxima Centauri, orbits the pair at a distance of 0.2 light-year. In 2016, a potentially habitable exoplanet was confirmed to be orbiting Proxima Centauri, called Proxima Centauri b, the closest confirmed exoplanet to the Sun. [159] The stars next closest to the Sun are the red dwarfs Barnard's Star (at 5.9 ly), Wolf 359 (7.8 ly), and Lalande 21185 (8.3 ly).

The largest nearby star is Sirius, a bright main-sequence star roughly 8.6 light-years away and roughly twice the Sun's mass and that is orbited by a white dwarf, Sirius B. The nearest brown dwarfs are the binary Luhman 16 system at 6.6 light-years. Other systems within ten light-years are the binary red-dwarf system Luyten 726-8 (8.7 ly) and the solitary red dwarf Ross 154 (9.7 ly). [160] The closest solitary Sun-like star to the Solar System is Tau Ceti at 11.9 light-years. It has roughly 80% of the Sun's mass but only 60% of its luminosity. [161] The closest known free-floating planetary-mass object to the Sun is WISE 0855−0714, [162] an object with a mass less than 10 Jupiter masses roughly 7 light-years away.

### Comparison with extrasolar systems

Compared to many other planetary systems, the Solar System stands out in lacking planets interior to the orbit of Mercury. [163] [164] The known Solar System also lacks super-Earths (Planet Nine could be a super-Earth beyond the known Solar System). [163] Uncommonly, it has only small rocky planets and large gas giants elsewhere planets of intermediate size are typical—both rocky and gas—so there is no "gap" as seen between the size of Earth and of Neptune (with a radius 3.8 times as large). Also, these super-Earths have closer orbits than Mercury. [163] This led to the hypothesis that all planetary systems start with many close-in planets, and that typically a sequence of their collisions causes consolidation of mass into few larger planets, but in case of the Solar System the collisions caused their destruction and ejection. [165] [166]

The orbits of Solar System planets are nearly circular. Compared to other systems, they have smaller orbital eccentricity. [163] Although there are attempts to explain it partly with a bias in the radial-velocity detection method and partly with long interactions of a quite high number of planets, the exact causes remain undetermined. [163] [167]

This section is a sampling of Solar System bodies, selected for size and quality of imagery, and sorted by volume. Some large objects are omitted here (notably Eris, Haumea, Makemake, and Nereid) because they have not been imaged in high quality.

1. ^ ab As of August 27, 2019.
2. ^Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar System" and "solar system" structures in their naming guidelines document. The name is commonly rendered in lower case ("solar system"), as, for example, in the Oxford English Dictionary and Merriam-Webster's 11th Collegiate Dictionary.
3. ^ The natural satellites (moons) orbiting the Solar System's planets are an example of the latter.
4. ^ Historically, several other bodies were once considered planets, including, from its discovery in 1930 until 2006, Pluto. See Former planets.
5. ^ The two moons larger than Mercury are Ganymede, which orbits Jupiter, and Titan, which orbits Saturn. Although bigger than Mercury, both moons have less than half its mass. In addition, the radius of Jupiter's moon Callisto is over 98% that of Mercury.
6. ^ abcde According to IAU definitions, objects orbiting the Sun are classified dynamically and physically into three categories: planets, dwarf planets, and small Solar System bodies.
• A planet is any body orbiting the Sun whose mass is sufficient for gravity to have pulled it into a (near-)spherical shape and that has cleared its immediate neighbourhood of all smaller objects. By this definition, the Solar System has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Because it has not cleared its neighbourhood of other Kuiper belt objects, Pluto does not fit this definition. [5]
• A dwarf planet is a body orbiting the Sun that is massive enough to be made near-spherical by its own gravity but that has not cleared planetesimals from its neighbourhood and is also not a satellite. [5] Pluto is a dwarf planet and the IAU has recognized or named four other bodies in the Solar System under the expectation that they will turn out to be dwarf planets: Ceres, Haumea, Makemake, and Eris. [6] Other objects commonly expected to be dwarf planets include Gonggong, Sedna, Orcus, and Quaoar. [7] In a reference to Pluto, other dwarf planets orbiting in the trans-Neptunian region are sometimes called "plutoids", [8] though this term is seldom used.
• The remaining objects orbiting the Sun are known as small Solar System bodies. [5]
7. ^ ab The mass of the Solar System excluding the Sun, Jupiter and Saturn can be determined by adding together all the calculated masses for its largest objects and using rough calculations for the masses of the Oort cloud (estimated at roughly 3 Earth masses), [19] the Kuiper belt (estimated at roughly 0.1 Earth mass) [20] and the asteroid belt (estimated to be 0.0005 Earth mass) [21] for a total, rounded upwards, of

37 Earth masses, or 8.1% of the mass in orbit around the Sun. With the combined masses of Uranus and Neptune (

## Formation and evolution

Stars form within extended regions of higher density in the interstellar medium, although the density is still lower than the inside of a vacuum chamber. These regions - known as molecular clouds - consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula. [55] Most stars form in groups of dozens to hundreds of thousands of stars. [56] Massive stars in these groups may powerfully illuminate those clouds, ionizing the hydrogen, and creating H II regions. Such feedback effects from star formation may ultimately disrupt the cloud and prevent further star formation.

All stars spend the majority of their existence as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosity and the impact they have on their environment. Accordingly, astronomers often group stars by their mass: [57]

• Very low mass stars with masses below 0.5 M are fully convective and distribute helium evenly throughout the whole star while on the main sequence. Therefore, they never undergo shell burning, never become red giants, and are theorized to become helium white dwarfs which simply cool off after exhausting their hydrogen. [58] However, as the lifetime of 0.5 M stars is longer than the age of the universe, no such star has yet reached the white dwarf stage.
• Low mass stars (including the Sun), with a main sequence mass above about 0.5 M and below 1.8–2.5 M depending on composition, do become red giants when their core hydrogen is depleted, then ignite a degenerate helium core in a helium flash, develop a degenerate carbon-oxygen core on the asymptotic giant branch, and finally produce a planetary nebula to become a white dwarf.
• Intermediate-mass stars, between 1.8–2.5 M and 5–10 M, pass through similar evolutionary stages to the low mass stars, but after a relatively short period on the RGB they ignite helium without a flash and spend an extended period in the red clump before forming a degenerate carbon-oxygen core.
• Massive stars generally have a minimum mass of 7–10 M, but this may be as low as 5–6 M. After exhausting the hydrogen at the core these stars become supergiants and go on to fuse elements heavier than helium. They end their lives when their cores collapse and they explode as supernovae.

### Star formation

The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density - often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy). [59] [60] Once a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force. [61]

As the cloud collapses, individual conglomerations of dense dust and gas form "Bok globules". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core. [62] These pre–main sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 to 15 million years.

Early stars of less than 2 M are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects. [63] [64] These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed. [65]

Early in their development, T Tauri stars follow the Hayashi track—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.

Most stars are observed to be members of binary star systems, and the properties of these binaries are the result of the conditions in which they formed. [66] A gas cloud must lose its angular momentum in order to collapse and form a star, and fragmentation of the cloud into multiple stars uses up some of the angular momentum. The primordial binaries will get processed by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart wider (soft) binaries while causing closer (hard) binaries to become more tightly bound, producing the distribution of binary separations seen in the field.

### Main sequence

Stars spend about 90% of their existence fusing hydrogen into helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence, and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity. [67] The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 10 9 ) years ago. [68]

Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10 −14 M every year, [69] or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10 −7 to 10 −5 M each year, significantly affecting their evolution. [70] Stars that begin with more than 50 M can lose over half their total mass while on the main sequence. [71]

The duration that a star spends on the main sequence depends primarily on the amount of fuel it has to fuse and the rate at which it fuses that fuel, i.e. its initial mass and its luminosity. For the Sun, its life is estimated to be about 10 billion (10 10 ) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 M, called red dwarfs, are able to fuse nearly all of their mass as fuel while stars of about 1 M can only use about 10% of their mass as fuel. The combination of their slow fuel-consumption and relatively large usable fuel supply allows about 0.25 M stars to last for about one trillion (10 12 ) years according to stellar-evolution calculations, while the least-massive hydrogen-fusing stars (0.08 M) will last for about 12 trillion years. Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and start to cool. [58] However, since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 M [72] are expected to have moved off the main sequence.

Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers consider all elements heavier than helium "metals", and call the chemical concentration of these elements the metallicity. The metallicity can influence the duration that a star will burn its fuel, control the formation of magnetic fields, [73] and modify the strength of the stellar wind. [74] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time these clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.

### Post–main sequence

As stars of at least 0.4 M [2] exhaust their supply of hydrogen at their core, they start to fuse hydrogen in a shell outside the helium core. Their outer layers expand and cool greatly to form a red giant. In about 5 billion years, when the Sun enters this phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size. As a giant, the Sun will lose roughly 30% of its current mass. [68] [75]

As hydrogen shell burning produces more helium, the core increases in mass and temperature. In a red giant of up to 2.25 M, the helium core becomes degenerate before it is compressed enough to start helium fusion. When the temperature increases sufficiently helium fusion begins explosively in the helium flash, and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch. For more massive stars, the helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump before the outer convective envelope collapses and the star moves to the horizontal branch. [4]

After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. The star then follows an evolutionary path (the asymptotic giant branch or AGB) that parallels the original red giant phase at a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate.

#### Massive stars

During their helium-burning phase, very high-mass stars with more than nine solar masses expand to form red supergiants. Once this fuel is exhausted at the core, they continue to fuse elements heavier than helium.

The core contracts until the temperature and pressure suffice to fuse carbon (see Carbon burning process). This process continues, with the successive stages being fueled by neon (see neon burning process), oxygen (see oxygen burning process), and silicon (see silicon burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within the star. Each shell fuses a different element, with the outermost shell fusing hydrogen the next shell fusing helium, and so forth. [76]

The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy—the process would, on the contrary, consume energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission. [77] In relatively old, very massive stars, a large core of inert iron will accumulate in the center of the star. The heavier elements in these stars can work their way to the surface, forming evolved objects known as Wolf-Rayet stars that have a dense stellar wind which sheds the outer atmosphere.

#### Collapse

As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than 1.4 M, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf. White dwarfs lack the mass for further gravitational compression to take place. [78] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. Eventually, white dwarfs fade into black dwarfs over a very long period of time.

In larger stars, fusion continues until the iron core has grown so large (more than 1.4 M) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before. [79]

Supernova explosions blow away the star's outer layers, leaving remnants such as the Crab Nebula. [79] There remains a neutron star (which sometimes manifests itself as a pulsar or X-ray burster) or, in the case of the largest stars (large enough to leave a remnant greater than roughly 4 M), a black hole. [80] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole the matter is in a state that is not currently understood.

The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium. [79]

#### Binary stars

The post–main-sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass. If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other star. A variety of phenomena can result from these systems, including contact binaries, common-envelope binaries, cataclysmic variables, and type Ia supernovae.

## Why are accreting objects portrayed with a white flash at the point where the gas stream from the star enters the accretion disk? - Astronomy

You can win yourself up to 2% extra credit by advising me of any science fiction novel or short story, or movie or TV episode, that involves Black Holes or Wormholes, and that is not listed here.

Please note that stories or movies merely involving time travel (like Time Machine or Back To The Future) don't count. Moreover texts, monographs, documentaries, or other works of non-science-fiction on Black Holes do not count.

To gain the full extra credit, you must name a specific novel, short story, movie or episode (not a general series like Deep Space 9), you must have read the story or seen the movie yourself, and you must include a short review of the story/movie in your own words (don't forget the quotes rule). You should provide a link to a place on the web where further information about the story/movie can be found.

To be considered for extra credit, you must make your submission no later than in class on Tuesday May 1, the Tuesday of the last week of classes.

I do not want this to act as a spoiler so I will proceed to the portion in which wormholes become relevant.

At a particular point in the episode Sookie discovers that the realm of the Fae is not what it seems. Rather than beautiful and pure, it is riddled with evil and darkness. As she runs from the creatures within this realm she is presented with a wormhole by a fellow male-Fae who acts as a temporary ally. He instructs her to jump into the wormhole if she wishes to return to Earth. If she does not jump the wormhole will close and she will be forever trapped in the realm of the Fae &mdash never to see her loved ones on Earth again.

Sookie, with only seconds to spare, leaps through the wormhole and is transported back to her life on Earth. Shockingly, what felt like only 15 or so minutes within the realm of Fae was actually 13 months in Earth-time.

Jack Carter, the main character who plays the role of the sheriff in a town that is far from ordinary, is frantically attempting to find the solution to the problem that caused the president's limo to be sliced in half as well as what absorbed a café shop into oblivion. They find a device created by a scientist that absorbs all of the energy from the light spectrum and converts it into reusable energy. However, on this particular day, the town is launching a spacecraft to the Saturn moon Titan. To do this, they use a FTL ion reactor (faster than light), but this coupled with the energy collectors cause black holes to pop up around town. In the end, sheriff Carter uses the collectors to draw all the mini black holes to a lake where a very large black hole forms. Carter launches a bomb loaded with anti-matter into the middle of the black hole to cancel it out, and the crisis is averted.

This episode of the kids' TV show He-Man and the Masters of the Universe begins as usual by introducing the concept and the characters of the program. The scene transitions to Orko exclaiming how happy he is that it is his birthday. The excited Orko goes and asks his friends at Grayskull if they remember what today is. Unfortunately none of them appear to realize it's Orko's birthday. Fortunately they are secretly planning a surprise party for the young sorcerer. Unfortunately during the discussion of the surprise party, they receive a distress call from the nearby Becilia and they must investigate. Upon arriving in Becilia they find an enormous whirlwind terrorizing the region and it's up to He-Man to defend them! Fortunately He-Man stops the whirlwind by “unraveling it with his sword”! Unfortunately his sword vanishes as he puts it into the sheath. During this time in Becilia, Orko goes to the Sorceress to see if she remembers his birthday. Unfortunately he runs into an issue finding her. Shortly thereafter the group that went to Becilia returns to the kingdom of Eternia thinking they have succeeded. They can't figure out who caused the whirlwind and who took Prince Adam's sword, for Skeletor was nowhere to be found! To further the unfortunate predicament, Orko returns hysterical! He explains that he couldn't find the Sorceress because Castle Grayskull has vanished! He-Man, Teela, Man-At-Arms, and Battle Cat head over to Castle Grayskull to find there is a white hole in place of the Castle! Man-At-Arms explains in the episode that “It's like a black hole, only not as dense.” He also explains that Skeletor “utilized the Council of Evil after he located a dying neutron star. ” and He-Man finished the statement with “. and was able to direct it over the castle to use the tremendous suction force from the nova, and pull the castle, with the Sorceress, into another dimension.”

He-Man and Battle Cat jump into the white hole and find themselves in a crazy dimension, which I would assume is the inside of the white hole. Inside the white hole they find various random objects of many colors floating around as well as scary tentacled monsters. He-Man hears the call of the Sorceress and follows her voice. At this time Teela jumps into the white hole, despite He-Man telling her not to. She falls in and lands on a platform, but a short time later she nearly falls off said platform to her doom in a black void below. He-Man comes to her rescue and lifts her to safety and they make their way through the white hole. Eventually they find a wormhole to jump through and it sends them into an anti-verse of their own. In this anti-verse, everything is backwards! The water flows upstream and the trees grow down with their roots high in the sky. Fortunately the find Castle Grayskull! Unfortunately it is being run by Skeletor and the Sorceress is chained up! Fortunately He-Man's sword reappears and Skeletor is quickly dispatched! He-Man then picks up the entire Castle Grayskull and throws it through the white hole and it lands safely back in Eternia! Unfortunately Orko is still upset about everyone “forgetting” his birthday, so he makes a bunch of food for himself and eats it all in a depressed rage. Fortunately someone comes and tells him that King Randor wants to see him. After everyone yells “Surprise” Orko discovers that his friends really do care about him! Unfortunately he ate too much food before the party and collapses from an upset stomach! The episode ends with Orko telling the viewers to watch how many sweets they eat because they have to leave room for the three healthy meals they need to eat everyday!

The episode is a good example of a white hole as a central plot device, even though the science is mostly wrong. The episode is hard to watch, being that the lines are very cheesy and poorly written. The animation is really well done for 1983 and the creativity used in creating the world inside the white hole is astounding. The episode could have used a little more explanation as to how the white hole came to be and how it worked, but being that it's a kid's show, I'm sure it's okay.

“On the Moon” is one of many ridiculous web cartoons from well-known flash animator Mr. Weebl. Amonst his more well-known creations is his flash animation “Badgers,” which became an internet phenomenon in the early to mid-2000s.

Story: The portion of the Star Wars comic that deals with black holes is featured in Acts 4-5 of the 11-act series. The three previous acts deal with Star Fox and crew being hired by General Pepper to defend Cornelia against Andross' forces, saving a passenger ship from an Imperial boarding party, training in the new Arwing fighters on Cornelia, and surviving a trap assault by Imperial fighters on Cornelia. In the last frame of Act IV, Fox proposes using “the black hole” (previously unmentioned) to reach Venom, the home planet of Andross' Empire. At the beginning of Act IV, titled “The Legacy” (a reference to Fox's succession from his father James), Fox counters squadmate Slippy the Frog's argument that the gravity of the black hole would crush an Arwing by referencing the “time-slip theory” which states that a black hole's gravity can be manipulated to create a 4-dimensional warp. General Pepper cryptically cautions Fox against making the same mistake as his father. Fox insists that the Arwing's “gravity diffuser” will allow the Arwing to make the journey where his father's “dinosaur” failed. The story of James McCloud is then told back when Andross had been just a scientist, he had developed a gravity bomb that James had offered to fly to be tested in the asteroid belt. Unfortunately, the bomb was successful so successful that James and much of the belt were sucked into the newly-created black hole. It was this action that drove Andross into exile and caused his transformation into an evil emperor on the fringes of the Lylat system. The rest of Fox's squad sets about altering the Arwing design by “tripling the output of the G induction coils”. At the beginning of Act V, titled “Fixing a Hole” (likely a reference to the Beatles song in addition to the black hole), the squad proceeds into the black hole in their modified Arwings. They are depicted as stretching and spiralling inward to the center of a vortex of visible spectra, likely meant to represent crossing the event horizon. Inside, they are greeted by odd visual phenomena as well as the remnants of wrecked ships that had futilely attempted the feat previously. The squad experiences a dilation of time depicted as “stretched-out” dialogue. Most significantly, Fox has a vision of his father, who says “Junior, follow me!” Fox appears to fly past/through the vision out of the black hole the exit is represented as a white hole. The rest of his squad is also successful Peppy O'Hare states that Slippy's tracking device was able to track Fox through the whole enabling them to follow. The squad proceeds to Venom as planned.

Analysis of Science: Clearly, strict adherence to understood theories of black holes and the possibility of traversing them was not a priority with the Star Fox writers. Obstacles to the success of the operation are explained away with a combination of “future science” and “future tech” like the time-slip theory, gravity diffusers, and G Induction coils. However, I think this comic should be recognized as a valiant effort by the writers to acknowledge that black holes really are as dangerous and difficult to control as they are. The squad encounters a graveyard of ships inside the hole/wormhole, clear signs that even in a society with highly advanced intrastellar space travel, traversing a black hole was still a nearly-impossible endeavor. Additionally, gravity is clearly recognized as the driving force behind the black hole not magic, not “power”, just gravity. The squad is able to tackle the problem head-on by modifying how the Arwing deals with gravity, which is likely what would have to be done for anything to make it through a black hole and survive. The idea that a “gravity bomb” could explode in such a way as to simulate a core-collapse supernova, even in small-scale, is feasible when working with tremendous enough explosions. Indeed, the writers even avoid using the term “wormhole” to describe what happens to the squad inside the black hole, which is good in my mind because of all the quasi- scientific/crackpot theories that the term is connoted with. Finally, it can at least be said that an attempt was made to depict the seemingly inevitable dilation of time that occurs around black holes, even if such a phenomena is unknown to occur in such a way inside a black hole. I won't touch on the plausability of Fox meeting his father's disembodied head inside see “Contact” for more evidence that disembodied human spirits love hanging out around relativistic extrema.

President: “You see Chancellor, the black hole!”

Chancellor: “That's a no-where, a no-place, a void! According to all known laws nothing can exist there.”

Because the Time Lords are busy trying to hold the universe together, they can do nothing, but send first the second, and then the first doctor to help the third one. together these three doctors devise a plan, where they travel through the black hole, to the universe of Antimatter that is located there.

This plane of Antimatter is ruled by a Time Lord, thought long dead, and one who predictable wants revenge on the time lord that abandoned him to this black hole. I must note at this point, they had an understanding that black holes are formed by the explosion of a star. To bring matters to a short conclusion, to exist in this universe, their matter is subtly changed so they are not annihilated, but one object, a recorder remains unchanged. The recorder is dropped, the antimatter universe is annihilated, and the heroes are returned to where they should have been.

The concept of the black hole in this episode seems to be merely cosmetic. There is a clear understanding of both the formation and basic properties of black holes. However the concept of antimatter seems to be lumped in here for little more then to drive the plot.

The usual humor that the Doctor Who series is present as ever. This is the 10 year anniversary episode, and the crossover brings the familiar and loved older doctors together. This adds another dimension of humor, and interest to the story, as the proud and eccentric doctor is forced to endure the experience of dealing with himself, something only others have had to do thus far. The first Doctor: “It is a time bridge . so stop dilly-dallying and cross it.”

Hyperion was a great novel that takes place in the future. The book is mainly made up of the in-depth back stories of pilgrims headed to confront the dreaded mechanical Shrike. Fate hangs in the balance as these average people confront the demigod.

What I liked most about this book was the unique format. Each character's back story was like a little novella on its own, and as each story unfolds the mystery of what links all these characters to each other (and the Shrike) becomes the true conflict of the story. The end of the book leaves the reader with a totally different conception of nearly every character and the true goal of reaching the Shrike.

The sequel to this book, The Fall of Hyperion, is even better because it delves more into the Shrike, the Big Mistake, and the TechnoCore (AIs that run much of society).

The specific reference to a “black hole” is in the scene where “The Romulans drop a 'red matter' bomb into the hole made by the drill. It triggers the formation of a black hole, which collapses the planet and wipes out the Vulcans.” (previous sentence from Bad Astronomy's Review of the Science of 'Star Trek').

Throughout the rest of the series, many more things come out of people's heads including more evil robots, cat ears, and guitars. The whole idea behind getting things out of people's heads is described in episode four. “[They] use the left brain and right brain's distinct thought process to open up an interdimensional channel capable of pulling things through, sometimes from light-years away, in an instant. But you can't just use anyone's head you have to find the right one.” (Commander Amarao episode 4)

The only way to understand this show is to watch it, and even that doesn't help much.

In response to both the Walking Stick's excellent air defense and the writers' inability to produce any more plot involving black holes, the Nadesico crew plan to attack the Walking Stick by ground and blow it up with mines. The catch: they only have one hour. The newly appointed “ground captain”, Akotski, is the handsome, funny-guy stock character that would most likely be played by Matthew McConaughey in a Nadesico feature film. He consoles his worried crew by saying, “If we all work together, I'm sure we can succeed.” The team embarks through thick forests towards the Walking Stick, night having already come and the hour seemingly up. The team confirms that the time is up by stopping to have a campfire, the lyrics “just us friends” being sung in the background. Finally, in the midst of a surprise tank attack on the campfire, the commander radios the crew to tell them they're out of time. The call includes a memorable quote that attempts to fill every gap in the plot and explain the theory behind the black holes:

Commander: “The first black hole dissipated after leaving earth's atmosphere resulting in little residual damage. I'm sure the next one will dissipate within the earth's atmosphere. Our enemies aren't fools, they have less to lose than we do. The next black hole will swallow this entire area and saturate what's left with lethal gamma rays.”

The commander informs the rest of the people at HQ that she is “registering a massive gravitron fluctuation building around the Walking Stick,” and that it will fire soon. Putting aside the fact that a Gravitron is an amusement park ride, the crew panics and realizes that something must be done. They decide to storm the Walking Stick and defeat it right before it shoots another “micro black hole.”

Quotes:
“The stars seem so at peace from a distance, but up close they're so troubled.”

Alien captor: “Black hole is a star that has collapsed in on itself becoming so dense neither matter or light can escape its gravity.”

The climax of this story is that the man's son, after hearing his fathers tale, decided out of curiosity to hold his breath while the attendants were administering the gas to knock him out. The family arrived at their new destination only to find that the boy had gone through the wormhole while conscious, and had gone insane. He rips out his own eyes while screaming, “Longer than you think dad!, longer than you think!”.

The movie revolves around Dr. Rick Marshall, Physicist/Paleontologist who creates a “tachyon amplifier” machine that creates a wormhole to another dimension of existence, “where all timelines converge.” When Dr. Marshall and his team of incompetent scientists/friends first step through the time warp, we see a scene where they fly through a vortex spinning and stretching. They get dropped into a desert where they observe two ancient hominids trying to kill a third. The third is rescued by the group and joins them in their quest. In this new land, the protagonists are faced with the task of saving all of humanity by destroying a group of lizard-men who plan to take over the universe and destroy Earth, and then to return home. They must find the tachyon amplifier before the lizard men do. However, a giant T-Rex is constantly in the way of their plans to succeed. An interesting scene is one where our protagonists stumble upon a vast desert area littered with various random objects that somehow have gotten there through other wormholes in the world.

Despite possessing an immaculate sound track, there are many flaws in the movie Space Jam. In general I like the movie, but I think it's obvious to any discerning critic that this film needs more Charles Barkley. Also, all of the scenes involving Michael's family are entirely forgettable.

The peaceful aliens' home planet has been taken over by two other super-people who are ruling the planet like Gods. These evil super-people are also apparently communists because at one point they declare that they have reorganized the planet's labor structure so that there is “a job for everyone according to his abilities and our needs,” with “cooperation instead of competition.” Their being communists makes them extra evil, so that the viewer does not feel bad about their eventual fate. Inevitably Superman fights the evil super-people for some reason and they subdue him and strap him to a rocket that they intend to shoot into the black hole. Superman escapes with the help of some of the oppressed aliens, and turns the tide on the communists. Ultimately it is they who fall into the black hole, in a swirling mass like water going down a drain. I give the episode 3 out of 5 stars, despite the questionable science, for featuring communists being shot into a black hole.

The graphic novel was left as a cliff hanger. It give a lot of back story to the movie and it gives more meaning to the movie. This is a really thrilling story that you just cant set down and after you finish it then you feel compelled to see the Star Trek movie even if you have seen it already. It was also nice to read what happens to the characters of Star Trek Next Generation.

In this episode, Beowulf Shaffer is attempting to return to earth. Instead, he is stuck on the planet of Jinx for months. He eventually runs into an old friend of his, Carlos Wu. The two reminisce, and Beowulf tells Carlos of his conundrum, that he is stuck on Jinx and desperately wants to return to Earth to see his lover. Carlos tells Beowulf that he has a ride to the Sol system, with the Bureau of Alien affairs, designed to keep relations between earth and Alien nations safe. Beowulf is granted passage by the captain, because of his expertise with Pierson's Puppeteers. It is then that one of the conflicts is introduced, that many ships have been mysteriously disappearing near the Sol system. Most of the journey is uneventful, but when they near Sol, their ship bucks, shakes, and groans from some hidden turbulence and suddenly falls out of hyper drive. They find that the entire hyper drive unit is missing, completely vanished from the ship. Beowulf requests information that might help him figure out the source of the disturbance. After much political red-tape, he finally gets information that he needs and begins to ponder the bizarre recent events. He and Carlos are perplexed to the source of the disturbance and the disappearance of the hyper drive motor. After contacting an astronomer by the name of Dr. Forward, they are invited to his station to await a ferry to earth. After much deliberation, Carlos and Beowulf decide to go to Forward Station, even though the source of the disturbance to be near the “Forward Station.” Once on Forward Station, they learn of Dr. Forward's new contraption: The Grabber. It's a huge arm and bucket contraption that can manipulate ultra dense masses to produce gravitational waves. They are eventually knocked unconscious by Dr. Forward after they realize he is the villain. Dr. Forward had taken a quantum black hole, and used the Grabber to feed a neutronium sphere into it. It created a black hole with a tremendous charge, yet could be moved from place to place due to its ultra-density and his Grabber arm. They eventually are engaged in a struggle with Dr. Forward, and the black hole eventually gets loose. The black hole starts to break the station apart, and the Forward Station starts to disintegrate. Carlos and Beowulf are tied to a pillar and are saved from falling in. Dr. Forward, in a last ditch effort, traps the black hole with the Grabber but falls in right before it is fully trapped. By some turn of events, the ride Beowulf and Carlos were waiting for shows up in the nick of time and they are rescued to earth. As they escape, they see the Forward Station and its asteroid consumed in a big blast of light.

A physics teacher named Eva gains access to a lab with the help of her former lover, Steven, in order to look up Filadyne Corporation's latest project. She realizes that the head of Filadyne, Thomas Abernathy, is planning to create a microscopic sized black hole using a particle accelerator in hopes of creating a new energy source. Unfortunately, he has tried this before, in Luxembourg, and ended up killing a lot of people, including Eva's father. She realizes there is a mathematical mistake that Abernathy made and the black hole created will be unstable &ndash either it will "suck up" the Earth or fall to the core and then explode. Unable to convince Abernathy of his oversight, she convinces Steven and another friend, Lazarus, to help stop Abernathy by sabotaging his experiment, but what begins as a science fiction movie quickly turns into a horror film as people, starting with Lazarus, are killed by Filadyne associates in order to prevent them from getting in the way. In the end, both Eva and Steven end up trapped at the lab while attempting to stop the experiment, and the microscopic black hole is created. The black hole quickly grows in size, "devouring" everything in the lab room, including a couple of scientists, and the entire facility begins to collapse as it is "sucked up" by the black hole. Eva and Steven make it out alive in the end, but I believe Abernathy and just about everyone else is either "devoured" or killed in the explosion, which wipes out a large radius surrounding the facility.

Four officers &ndashPicard, Data, Troi, and La Forge &ndashdiscover the catastrophe while it unfolds, as it were. The Enterprise is in the middle of rescuing the disabled Romulan vessel, and both vessels are caught in the middle of a huge temporal distortion, inside which time is slowed to an indistinguishable halt. The officers are capable of resuming time as normal, but not without resulting in the destruction of the Enterprise.

The viewers do actually get to see the Enterprise explode before Picard’s eyes. He handles it with remarkable composure. He is rather surprised, however, when the explosion suddenly reverses itself.

The crew’s solution, is to make time go backwards, not forwards. Then they can intervene and prevent the incident from occurring. One of the temporal aliens intervenes and botches the entire plan, but as usual Picard experiences a fit of creative genius and all is well. This is not the only episode to say that the Romulans use artificial quantum singularities for power. However, it is never said how they create an artificial singularity, what an artificial singularity IS, how they harness its power, or how they would deal with the disastrous consequences of containing a quantum singularity inside their ship. If it were a quantum singularity, the same as in a black hole, I would imagine the Romulans would use such a thing as a weapon, but the possibility is never mentioned.

Aahhhhhh. When will we finally learn to see the Globetrotters not just as a bunch of cheats who play the same half-witted yokels every game, but as they really are: easy prey for cartoon sitcoms?

Wikipedia: Tesseract states:
"In [A Wrinkle in Time, L'Engle] uses the tesseract as a portal, a doorway which you can pass through and emerge far away from the starting point, as if the two distant points were brought together at one intersection (at the tesseract doorway) by the folding of space-time, enabling near-instantaneous transportation (though this description more closely matches a wormhole)."

Acorna is an alien girl with some special abilities, who is stranded on earth, and is struggling to find her people. Later in the series, she has found her people but must save them from another alien race bent on their destruction. Throughout the series Acorna and her friends make use of wormholes for quick travel and to shortcut across the galaxy. While this use of wormholes is not an integral part of the plot, it still helps the book's storyline move along. Wormhole and space travel are more important in Acorna's People, Acorna's Quest, and Acorna's World than in the other Acorna books. Since time dilation does not exist in the Acorna books, the highest rating I can give them would be 2 stars out of 5.

Signal to Noise is a book about technology, business, conspiracy, and betrayal. The main character ‘Jack’ finds a signal in the background noise in the universe and tracks it back to a ‘being’ named Wheeler. They begin their business with a simple trade. After some trading Jack finds himself with an object that allows him to travel great distances in the blink of an eye. It turns out that this teleporting device uses the rotational power of celestial bodies to transport him.

Signal to Noise is the first book in the series. In the second book, A Signal Shattered, Jack visits the black hole at the center of our galaxy, and he uses some of its power to make copies of himself and send them across the galaxy. I suggest you go read the book to find out why he does that, and what happens.

On the pilot episode of Heroes, "Genesis", one of the characters, Hiro, finds that he has the ability to teleport. At the end of this episode we see Hiro on a subway train in Japan. As Hiro closes his eyes and begins his teleportation from Japan to New York City, we see the digital clock behind him on the train accelerating extremely fast forward in time. However, from Hiro's perspective, time is continuing normally. Shortly thereafter, we see Hiro appear in New York City.

In the second episode, "Don't Look Back", Hiro is a victim of a case of mistaken identity after he is found by the police at a crime scene. While in police custody, Hiro explains to an officer that he is able to manipulate the space-time continuum. In order to try and absolve himself, Hiro calls his friend in Japan who he says can attest to the police that he was in Japan just a day ago. His friend, much to Hiro's dismay, informs the police officer that he has been looking for Hiro for over three weeks. This scene reveals that some time dilation occurs when Hiro teleports.

Heroes, though only two episodes into its existence, has already proven to be an entertaining and intriguing show filled with interesting characters and a mysterious serial-killer villain. Thus far the episodes have led viewers to infer that all of the characters possessing super powers are interconnected, though we are still left to wonder what that connection may be.

I found this episode one of the better Futuramas. The paradoy of the movie Titanic lasted throughout the episode and was pretty funny. Not much scientific detail was mentioned about the black hole, but the viewers did not see the ship freeze at the horizon it just disappeared. Also the crew was able to escape right before the end of the ship hit the horizon, and it is doubtful that the escape pod could move at the speed of light.

The movie Deja Vu is about an ATF agent Doug Carlin who investigates a crime that involves a ferry that is blown up by a terrorist. Carlin is invited to join an "elite" group of members to try and find the terrorist, and to find the killer of a women related to the crime. The elite group happens to have a wormhole generator that views the world 4 days and 6 hours previously. Carlin goes into the wormhole, and saves the day. The movie makes little attempt at scientific accuracy. One somewhat realistic feature is that when Carlin goes through the wormhole, he appears "dead" on arrival and requires reviving. Death is indeed a feature of realistic black holes.

The mission of the mining operation, and thusly the doctor, is to uncover exactly how this is possible, and to learn about the nature of such a civilization that could control such forces. As the mining operation continues, and the story line unfolds, disaster strikes. An ancient alien prisoner is awakened at the bottom of the mines and begins to manipulate the crew via telepathy. This creature identifies itself as the Satan or Devil of every religion throughout the history of the time, and it is revealed that, after a lengthy war, he was imprisoned on the planet to keep the universe safe. Dealing with this foe requires the Doctor to use all of his cunning and genius to keep this beast from escaping and bringing doom to the universe. Soon the Doctor realizes the planet is placed close to the black hole for a reason, so that it will automatically devour the planet (and everyone on it) if the prisoner is ever set free.

Missing from the crew's memories is an encounter with a xenophobic race of aliens whose territory the Enterprise had inadvertantly invaded. Captain Picard asks Data how long the crew was unconscious, and Data replies 30 seconds. When the crew begins to notice discrepancies with this short length of time, Data blames the wormhole as the cause. But the discrepancies keep piling up, and Picard and the others become convinced that Data is lying. Eventually Picard insists on a reason for Data's lies, and he is stunned to learn that it was he that gave Data the order to conceal the true nature of events.

In the end, Picard and his crew learn what really happened, and Picard is then forced into on the spot negotiations with the alien race. In classic Star Trek form, Picard prevents a war, Data remains enigmatic, and the crew resorts to complex calculations to explain apparent time dilation discrepancies.

Donnie Darko, the protagonist, is a troubled and schizophrenic teenager who sleep walks into some very strange situations. On one late night walk, Donnie meets Frank the giant bunny, who informs Donnie that there are 28 days left until the world ends. Frank also tells Donnie about time travel. While Donnie is out, a mysterious jet engine falls on to the Darkos' house. Donnie encounters harsh conflicts on his journey to understand Frank and to overcome his problems. Finally, on the day that the world is going to end, the sky splits in two, and there above is a wormhole. Donnie drives his car into the wormhole, and is transported back to the moment that the jet engine crashes through his house, killing him and thereby saving his girlfriend Gretchen from her fate.

Donnie Darko, the protagonist, is a troubled and schizophrenic teenager who sleep walks into some very strange situations. On one late night walk, Donnie meets Frank the giant bunny, who informs Donnie that there are 28 days left until the world ends. Frank also tells Donnie about time travel. While Donnie is out, a mysterious jet engine falls on to the Darkos' house. Donnie encounters harsh conflicts on his journey to understand Frank and to overcome his problems. Finally, on the day that the world is going to end, the sky splits in two, and there above is a wormhole. Donnie drives his car into the wormhole, and is transported back to the moment that the jet engine crashes through his house, killing him and thereby saving his girlfriend Gretchen from her fate.

So now to introduce the main characters. Coop is the driver of MEGAS. Even with his modifications, he is not completely aware of all of MEGAS' capabilities. The person who is aware of all MEGAS functions is Kiva, MEGAS' creator. The smart red head from the future built the machine but is unaware of how to drive it. Then there is Jamie, Coop's good friend and co-pilot. Jamie is the typical side-kick. These three travel around, train on MEGAS and attract the attention of galactic friends and foes.

This animated series is full of adventure and conflict, and the lasting friendships formed between John Blackstar and his companions. Even though a children's animated series, it could do with more character development.

Though the series premiered as recently as the 1980s, CBS refused to allow the main character to be black, claiming that the public was not ready for this.

## Notes

The mass fraction of H was taken to be (10^<-5>) and other elements were assumed to have their Solar mass fractions.

Although the process of circularisation of the gas stream in white dwarf disruptions is affected by General Relativistic effects in contrast to many main sequence TDEs see the contribution from Bonnerot et al. in this Volume on the formation of the accretion flow after disruption.

The integral of the Schechter function can be expressed in terms of incomplete (Gamma ) functions, (Gamma (p,a)=int _^x^e^<-x>, dx) .

## Formation and evolution of binary and millisecond radio pulsars

We review the various ways in which binary pulsars, millisecond pulsars and pulsars in globular clusters may have formed. To this end the formation processes of neutron stars in interacting binaries, and the subsequent evolution of such systems are discussed.

In section 2 the observed properties of radio pulsars, single as well as in binaries, are briefly reviewed. The peculiar combination of rapid spin and relatively weak magnetic fields of the binary and millisecond pulsars and the high incidence of binaries among millisecond pulsars strongly suggest that many of them (if not all) are old neutron stars that have been “recycled” by the accretion of mass and angular momentum from a companion star in a mass-transfer binary. Recycled pulsars are expected to represent a later evolutionary phase of various observed types of binary X-ray sources.

In section 3 the observed properties of the various types of binary X-ray sources are summarized, and the evolutionary history of close binary systems leading to the formation of X-ray binaries is reviewed. In view of the relevance for the later evolution of X-ray binaries into binary and millisecond pulsars, we discuss in this section also the effect of various types of accretion (from a stellar wind, and by Roche-lobe overflow) on the spin evolution of accreting magnetized neutron stars. Subsequently the later evolution and final evolutionary products of X-ray binaries are discussed. Massive X-ray binaries may in the end either leave (i) a very close binary pulsar consisting of two neutron stars (with an eccentric orbit) or a neutron star and a massive white dwarf with a circular orbit, or (ii) two runaway pulsars, one newborn and one recycled, or (iii) a single low-velocity recycled pulsar. Low-mass X-ray binaries may either leave relatively wide binaries with circular orbits consisting of a low-mass (0.2–0.4M) white dwarf and a recycled neutron star, or a single recycled neutron star which has “evaporated” its companion star, or possibly has merged with it. We also discuss in this section the possible origin of the velocity-magnetic field correlation observed in single radio pulsars. The correlation can be obtained by a combination of close binary evolution and the occurrence of asymmetries in supernova mass ejection or, alternatively, by a combination of close binary evolution and the evaporation of low-mass companions to young pulsars.

Section 4 is devoted to the special formation and evolution processes of close neutron star binaries that operate in globular star clusters. The high incidence of pulsars (mostly binary and/or millisecond pulsars) in globular clusters and the origin of the relatively large fraction (≳ 50%) of single pulsars among them is discussed. From the discussions in section 4 and section 5 it is concluded that so far no clear evidence - nor the need - for the formation of neutron stars (millisecond pulsars) by the accretion-induced collapse of white dwarfs in globular clusters has been presented, although this formation mechanism cannot be excluded. For the formation of low-mass X-ray binaries in the galactic disk this mechanism, however, may make a significant contribution.

In section 5 the statistical properties of the binary and millisecond pulsars in globular clusters and in the general field are discussed in relation to the evolution of neutron star magnetic fields. The following conclusions are drawn: 1.

(i) There is no longer clear evidence that the magnetic fields of isolated neutron stars (radio pulsars) do decay.

(ii) Neutron stars that have been recycled by accretion in close binaries do show clear evidence for magnetic field decay. This field decay may be due to either (a) the accretion process itself or (b) the spin evolution of the neutron stars in binaries which has affected the magnetic field carried by the liquid interior of the neutron star.

(iii) A sizeable fraction of all observed single radio pulsars (of order several tens of per cent) may have been recycled in (mostly massive) close binaries. The presence of this group in the general pulsar population may have created the impression that the magnetic fields of single neutron stars do decay.

In section 6 the findings of earlier sections are summarized and some open problems are listed.

## The Solar Wind

### 1.2 Parker's Solar Wind Model

Apparently inspired by these diverse observations and interpretations, E. Parker, in 1958, formulated a radically new model of the solar corona in which the solar atmosphere is continually expanding outward. Prior to Parker's work most theories of the solar atmosphere treated the corona as static and gravitationally bound to the Sun except for sporadic outbursts of material into space at times of high solar activity. S. Chapman had constructed a model of a static solar corona in which heat transport was dominated by electron thermal conduction. For a 10 6 K corona, Chapman found that even a static solar corona must extend far out into space. Parker realized, however, that a static model leads to pressures at large distances from the Sun that are seven to eight orders of magnitude larger than estimated pressures in the interstellar plasma. Because of this mismatch in pressure at large heliocentric distances, he reasoned that the solar corona could not be in hydrostatic equilibrium and must therefore be expanding. His consideration of the hydrodynamic (i.e., fluid) equations for mass, momentum, and energy conservation for a hot solar corona led him to unique solutions for the coronal expansion that depended on the coronal temperature close to the surface of the Sun. Parker's model produced low flow speeds close to the Sun, supersonic flow speeds far from the Sun, and vanishingly small pressures at large heliocentric distances. In view of the fluid character of the solutions, Parker called this continuous, supersonic, coronal expansion the solar wind. The region of space filled by the solar wind is now known as the heliosphere.

## Chapter 1 (A) Cosmochemistry and properties of light element compounds

Since the “Big Bang”, the elements in the universe have been formed by the process called “nucleosynthesis.” The starting-point is assumed to consist of primordial material of hydrogen mixed with tenth as many atoms of helium. Hydrogen and helium in galaxies could collapse under gravity and the collision of atoms would increase the temperature to such an extent as to start nuclear fusion reactions. These would lead to more conversion of hydrogen to helium with the emission of light and heat. This continues in medium-sized stars, such as the Sun. Apart from hydrogen and most of helium, lithium, and boron, which were produced at the high-temperature, high-density stage occurring during the Big Bang, the remaining elements were synthesized by thermonuclear reactions within stars. In massive stars, the nuclear fusion reactions proceed much faster and hydrogen fuel is consumed in less than a hundred million years. At higher temperature, the conversion of helium takes place to form carbon or oxygen. All the elements present in terrestrial matter must have originated at a later stage by nucleosynthesis in novae, supernovae, or other such cosmic phenomena. Such a process had been responsible for the generation of elements such as fluorine, which is formed predominantly on the surfaces of white dwarf stars into which the larger companion star lost its material.