# How often are new astronomical objects (variable stars, supernovae, comets, etc) discovered by amateurs?

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How often are new astronomical objects (variable stars, supernovae, comets, etc) discovered by amateurs? Where could one report new findings?

The answer is: frequently. There are many amateur astronomers that make it their ambition to discover new supernovae or to observe and report on new variable stars.

As an example, let me cite amateurs Robert Evans, who apparently holds the record for most supernovae found by visual observation, or Tom Boles, who holds the record for supernova discoveries by an individual.

Observations of variable stars can be reported to the Information Bulletin of Variable Stars (IBVS). Supernovae discovery or new comets would normally be reported to the International Astronomical Union Circulars.

## Astronomy

Astronomy (from Greek: ἀστρονομία , literally meaning the science that studies the laws of the stars) is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, and comets. Relevant phenomena include supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, astronomy studies everything that originates outside Earth's atmosphere. Cosmology is a branch of astronomy that studies the universe as a whole. [1]

Astronomy is one of the oldest natural sciences. The early civilizations in recorded history made methodical observations of the night sky. These include the Babylonians, Greeks, Indians, Egyptians, Chinese, Maya, and many ancient indigenous peoples of the Americas. In the past, astronomy included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars. Nowadays, professional astronomy is often said to be the same as astrophysics. [2]

Professional astronomy is split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects. This data is then analyzed using basic principles of physics. Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. These two fields complement each other. Theoretical astronomy seeks to explain observational results and observations are used to confirm theoretical results.

Astronomy is one of the few sciences in which amateurs play an active role. This is especially true for the discovery and observation of transient events. Amateur astronomers have helped with many important discoveries, such as finding new comets.

### Neuroscience - What is a inhibitory tone when talking about neurons?

Neurons communicate electrochemically. That is, when a signal arrives to a neuron it fires a series of electrical signals, called action potentials.
Action potentials are depolarization events that propagate along the neuronal membrane, down to the neuronal terminal.
The terminal of a neuron is (with some exceptions) in contact with another neuron, via a structure called synapse.
When the depolarization arrives at the terminal, it allows the entry of calcium, which then mediates the release of a chemical substance, a neurotransmitter into the synaptic space. Finally, neurotransmitters act on the postsynaptic neuron, by binding to specific receptors on its cell membrane and can either stimulate it, in which case the postsynaptic neuron will fire more and release more of its neurotransmitter or inhibit it, in which case the opposite happens.

The textbook example of a stimulatory neurotransmitter is glutamate, and the inhibitory one is GABA 1 .

In various areas of the brain certain neurons are constantly receiving inputs from GABAergic afferents. This means that those neurons are constantly receiving a GABA stimulus that inhibits them, and are thus under a constant inhibitory tone. This will prevent their firing until a sufficiently potent stimulatory stimulus arrives or until the inhibitory tone is somehow released (for instance if the inhibitory GABAergic afferents are themselves inhibited by some of their own afferents).

1 note that this is not always true: GABA can also be stimulatory in various situations

### Galaxy - Mass resolution - Astronomy

Usually, in an N-body or SPH simulation the term "mass resolution" refers to the mass of a single particle, which usually all have the same mass.

A single particle can always be "detected" in the simulation, since we have control of the coordinates of all particles, but a structure of several particles becomes ill-defined if the number of particles is too small

The mass of the smallest resolved structure depends on your definition of "resolved". That could be, say, 10 or 100 particles, depending on what quantity you are interested in measuring, and how accurately you want it. For instance, to define the mass of the structure, you need to be able to count the number of particles in the structure. But how does one know where to stop counting in a more or less continuous field of particles? One way is to determine an approximate center of mass $x_mathrm$ (approximate, since it can only be exact once all associated particles are defined), calculate the average density $langlerhorangle$ inside a sphere centered on $x_mathrm$ (which will be higher than the global average, since you started out on an overdensity), and increase the radius until $langlerhorangle$ falls below some threshold (e.g. 200 times the global average). If the number of particles is too small, $langlerhorangle$ will change a lot between each iteration, making your mass inaccurate. I think for this purpose, at least ten particles are needed.

If you're interested in the structure of the interstellar medium in a galaxy in a hydrodynamic (i.e. SPH) simulation, you probably need more particles than this. But I thinks it's fair to say that people disagree on how many particles are needed to resolve a galaxy.

### How do we know Milky Way is a 'barred' spiral galaxy?

There are several different lines of evidence which together form a coherent picture: that of a barred galaxy. Moreover, as most disc galaxies are barred, we should expect the same from the Milky Way. The various evidences are:

The observed light distribution (2MASS) shows a left-right assymmetry in brightness and the vertical height. This is explained by the near end of the bar being located on that side.

The observed gas velocities show velocities which are "forbidden" in an axisymmetric or near-axisymmetric (spiral arms only) galaxy. These velocities occur naturally from the orbits of gas in a barred potential

The velocity distribution of stars in the Solar neighbourhood shows some asymmetries and clumping which is most naturally explained by orbital resonance with the bar rotation.

The extent, pattern speed, and orientation of the bar is consistent between all three of these.

## Amateur astronomy

Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the unaided eye, binoculars, or telescopes. Even though scientific research may not be their primary goal, some amateur astronomers make contributions in doing citizen science, such as by monitoring variable stars,[1] double stars,[2] sunspots,[3] or occultations of stars by the Moon[4] or asteroids,[4] or by discovering transient astronomical events, such as comets,[5] galactic novae[6] or supernovae in other galaxies.[7]

Amateur astronomers do not use the field of astronomy as their primary source of income or support, and usually have no professional degree in astrophysics or advanced academic training in the subject. Most amateurs are hobbyists, while others have a high degree of experience in astronomy and may often assist and work alongside professional astronomers.[8] Many astronomers have studied the sky throughout history in an amateur framework however, since the beginning of the twentieth century, professional astronomy has become an activity clearly distinguished from amateur astronomy and associated activities.[9]

Amateur astronomers typically view the sky at night, when most celestial objects and astronomical events are visible, but others observe during the daytime by viewing the Sun and solar eclipses. Some just look at the sky using nothing more than their eyes or binoculars, but more dedicated amateurs often use portable telescopes or telescopes situated in their private or club observatories. Amateurs can also join as members of amateur astronomical societies, which can advise, educate or guide them towards ways of finding and observing celestial objects. They can also promote the science of astronomy among the general public.[10]

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An image of the Cat's Paw Nebula created combining the work of professional and amateur astronomers. The image is the combination of the 2.2-metre MPG/ESO telescope of the La Silla Observatory in Chile and a 0.4-meter amateur telescope.

Collectively, amateur astronomers observe a variety of celestial objects and phenomena. Common targets of amateur astronomers include the Sun, the Moon, planets, stars, comets, meteor showers, and a variety of deep sky objects such as star clusters, galaxies, and nebulae. Many amateurs like to specialise in observing particular objects, types of objects, or types of events which interest them. One branch of amateur astronomy, amateur astrophotography, involves the taking of photos of the night sky. Astrophotography has become more popular with the introduction of far easier to use equipment including, digital cameras, DSLR cameras and relatively sophisticated purpose built high quality CCD cameras.

Most amateur astronomers work at visible wavelengths, but a small minority experiment with wavelengths outside the visible spectrum. An early pioneer of radio astronomy was Grote Reber, an amateur astronomer who constructed the first purpose built radio telescope in the late 1930s to follow up on the discovery of radio wavelength emissions from space by Karl Jansky. Non-visual amateur astronomy includes the use of infrared filters on conventional telescopes, and also the use of radio telescopes. Some amateur astronomers use home-made radio telescopes, while others use radio telescopes that were originally built for astronomical research but have since been made available for use by amateurs. The One-Mile Telescope is one such example.
Common tools

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Places like Paranal Observatory offer crystal clear skies for observing astronomical objects with or without instruments.[11]

Amateur astronomers use a range of instruments to study the sky, depending on a combination of their interests and resources. Methods include simply looking at the night sky with the naked eye, using binoculars, and using a variety of optical telescopes of varying power and quality, as well as additional sophisticated equipment, such as cameras, to study light from the sky in both the visual and non-visual parts of the spectrum. Commercial telescopes are available, new and used, but it is also common for amateur astronomers to build (or commission the building of) their own custom telescopes. Some people even focus on amateur telescope making as their primary interest within the hobby of amateur astronomy.

Although specialized and experienced amateur astronomers tend to acquire more specialized and more powerful equipment over time, relatively simple equipment is often preferred for certain tasks. Binoculars, for instance, although generally of lower power than the majority of telescopes, also tend to provide a wider field of view, which is preferable for looking at some objects in the night sky.

Amateur astronomers also use star charts that, depending on experience and intentions, may range from simple planispheres through to detailed charts of very specific areas of the night sky. A range of astronomy software is available and used by amateur astronomers, including software that generates maps of the sky, software to assist with astrophotography, observation scheduling software, and software to perform various calculations pertaining to astronomical phenomena.

Amateur astronomers often like to keep records of their observations, which usually takes the form of an observing log. Observing logs typically record details about which objects were observed and when, as well as describing the details that were seen. Sketching is sometimes used within logs, and photographic records of observations have also been used in recent times. The information gathered is used to help studies and interactions between amateur astronomers in yearly gatherings. Although not professional information or credible, it is a way for the hobby lovers to share their new sightings and experiences.

The popularity of imaging among amateurs has led to large numbers of web sites being written by individuals about their images and equipment. Much of the social interaction of amateur astronomy occurs on mailing lists or discussion groups. Discussion group servers host numerous astronomy lists. A great deal of the commerce of amateur astronomy, the buying and selling of equipment, occurs online. Many amateurs use online tools to plan their nightly observing sessions, using tools such as the Clear Sky Chart.
Common techniques

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While a number of interesting celestial objects are readily identified by the naked eye, sometimes with the aid of a star chart, many others are so faint or inconspicuous that technical means are necessary to locate them. Although many methods are used in amateur astronomy, most are variations of a few specific techniques.[according to whom?]
Star hopping
Main article: Star hopping

Star hopping is a method often used by amateur astronomers with low-tech equipment such as binoculars or a manually driven telescope. It involves the use of maps (or memory) to locate known landmark stars, and "hopping" between them, often with the aid of a finderscope. Because of its simplicity, star hopping is a very common method for finding objects that are close to naked-eye stars.

More advanced methods of locating objects in the sky include telescope mounts with setting circles, which assist with pointing telescopes to positions in the sky that are known to contain objects of interest, and GOTO telescopes, which are fully automated telescopes that are capable of locating objects on demand (having first been calibrated).
Mobile apps

The advent of mobile applications for use in smartphones has led to the creation of many dedicated apps.[12][13] These apps allow any user to easily locate celestial objects of interest by simply pointing the smartphone device in that direction in the sky. These apps make use of the inbuilt hardware in the phone, such as GPS location and gyroscope. Useful information about the pointed object like celestial coordinates, the name of the object, its constellation, etc. are provided for a quick reference. Some paid versions give more information. These apps are gradually getting into regular use during observing, for the alignment process of telescopes.[14]
Setting circles
Main article: Setting circles

Setting circles are angular measurement scales that can be placed on the two main rotation axes of some telescopes.[citation needed] Since the widespread adoption of digital setting circles, any classical engraved setting circle is now specifically identified as an "analog setting circle" (ASC). By knowing the coordinates of an object (usually given in equatorial coordinates), the telescope user can use the setting circle to align (i.e., point) the telescope in the appropriate direction before looking through its eyepiece. A computerized setting circle is called a "digital setting circle" (DSC). Although digital setting circles can be used to display a telescope's RA and Dec coordinates, they are not simply a digital read-out of what can be seen on the telescope's analog setting circles. As with go-to telescopes, digital setting circle computers (commercial names include Argo Navis, Sky Commander, and NGC Max) contain databases of tens of thousands of celestial objects and projections of planet positions.

To find a celestial object in a telescope equipped with a DSC computer, one does not need to look up the specific RA and Dec coordinates in a book or other resource, and then adjust the telescope to those numerical readings. Rather, the object is chosen from the electronic database, which causes distance values and arrow markers to appear in the display that indicate the distance and direction to move the telescope. The telescope is moved until the two angular distance values reach zero, indicating that the telescope is properly aligned. When both the RA and Dec axes are thus "zeroed out", the object should be in the eyepiece. Many DSCs, like go-to systems, can also work in conjunction with laptop sky programs.[citation needed]

Computerized systems provide the further advantage of computing coordinate precession. Traditional printed sources are subtitled by the epoch year, which refers to the positions of celestial objects at a given time to the nearest year (e.g., J2005, J2007). Most such printed sources have been updated for intervals of only about every fifty years (e.g., J1900, J1950, J2000). Computerized sources, on the other hand, are able to calculate the right ascension and declination of the "epoch of date" to the exact instant of observation.[15]
GoTo telescopes
Main article: GoTo (telescopes)

GOTO telescopes have become more popular since the 1980s as technology has improved and prices have been reduced. With these computer-driven telescopes, the user typically enters the name of the item of interest and the mechanics of the telescope point the telescope towards that item automatically. They have several notable advantages for amateur astronomers intent on research. For example, GOTO telescopes tend to be faster for locating items of interest than star hopping, allowing more time for studying of the object. GOTO also allows manufacturers to add equatorial tracking to mechanically simpler alt-azimuth telescope mounts, allowing them to produce an overall less expensive product. GOTO telescopes usually have to be calibrated using alignment stars in order to provide accurate tracking and positioning. However, several telescope manufacturers have recently developed telescope systems that are calibrated with the use of built-in GPS, decreasing the time it takes to set up a telescope at the start of an observing session.
Remote-controlled telescopes

With the development of fast Internet in the last part of the 20th century along with advances in computer controlled telescope mounts and CCD cameras "Remote Telescope" astronomy is now a viable means for amateur astronomers not aligned with major telescope facilities to partake in research and deep sky imaging. This enables anyone to control a telescope a great distance away in a dark location. The observer can image through the telescope using CCD cameras. The digital data collected by the telescope is then transmitted and displayed to the user by means of the Internet. An example of a digital remote telescope operation for public use via the Internet is the Bareket observatory, and there are telescope farms in New Mexico,[16] Australia and Atacama in Chile.[17]
Imaging techniques

Amateur astronomers engage in many imaging techniques including film, DSLR, LRGB, and CCD astrophotography. Because CCD imagers are linear, image processing may be used to subtract away the effects of light pollution, which has increased the popularity of astrophotography in urban areas. Narrowband filters may also be used to minimize light pollution.
File:Milky way -route 292 shiga kusatsu road- 1920x1080.webmPlay media
Video of the night sky taken with DSLR cameras in Japan.
Scientific research

Scientific research is most often not the main goal for many amateur astronomers, unlike professional astronomers. Work of scientific merit is possible, however, and many amateurs successfully contribute to the knowledge base of professional astronomers. Astronomy is sometimes promoted as one of the few remaining sciences for which amateurs can still contribute useful data. To recognize this, the Astronomical Society of the Pacific annually gives Amateur Achievement Awards for significant contributions to astronomy by amateurs.

The majority of scientific contributions by amateur astronomers are in the area of data collection. In particular, this applies where large numbers of amateur astronomers with small telescopes are more effective than the relatively small number of large telescopes that are available to professional astronomers. Several organizations, such as the American Association of Variable Star Observers and the British Astronomical Association, exist to help coordinate these contributions.

Amateur astronomers often contribute toward activities such as monitoring the changes in brightness of variable stars and supernovae, helping to track asteroids, and observing occultations to determine both the shape of asteroids and the shape of the terrain on the apparent edge of the Moon as seen from Earth. With more advanced equipment, but still cheap in comparison to professional setups, amateur astronomers can measure the light spectrum emitted from astronomical objects, which can yield high-quality scientific data if the measurements are performed with due care. A relatively recent role for amateur astronomers is searching for overlooked phenomena (e.g., Kreutz Sungrazers) in the vast libraries of digital images and other data captured by Earth and space based observatories, much of which is available over the Internet.

In the past and present, amateur astronomers have played a major role in discovering new comets. Recently however, funding of projects such as the Lincoln Near-Earth Asteroid Research and Near Earth Asteroid Tracking projects has meant that most comets are now discovered by automated systems long before it is possible for amateurs to see them.
Telescope set up in Brooklyn Bridge Park for a public stargazing session
Societies
Main article: List of astronomical societies

There are a large number of amateur astronomical societies around the world, that serve as a meeting point for those interested in amateur astronomy. Members range from active observers with their own equipment to "armchair astronomers" who are simply interested in the topic. Societies range widely in their goals and activities, which may depend on a variety of factors such as geographic spread, local circumstances, size, and membership. For example, a small local society located in dark countryside may focus on practical observing and star parties, whereas a large one based in a major city might have numerous members but be limited by light pollution and thus hold regular indoor meetings with guest speakers instead. Major national or international societies generally publish their own journal or newsletter, and some hold large multi-day meetings akin to a scientific conference or convention. They may also have sections devoted to particular topics, such as lunar observation or amateur telescope making.
Notable amateur astronomers
Main page: Category:Amateur astronomers
Sir Patrick Moore was one of the world's leading popularisers of astronomy.

George Alcock, discovered several comets and novae.
Thomas Bopp, shared the discovery of Comet Hale-Bopp in 1995 with unemployed PhD physicist Alan Hale.
Robert Burnham Jr. (1931–1993), author of the Celestial Handbook.
Andrew Ainslie Common (1841–1903), built his own very large reflecting telescopes and demonstrated that photography could record astronomical features invisible to the human eye.
Robert E. Cox (1917–1989) who conducted the "Gleanings for ATMs" column in Sky & Telescope magazine for 21 years.
John Dobson (1915–2014), whose name is associated with the Dobsonian telescope.
Robert Owen Evans is an amateur astronomer who holds the all-time record for visual discoveries of supernovae.
Clinton B. Ford (1913–1992), who specialized in the observation of variable stars.
John Ellard Gore (1845–1910), who specialized in the observation of variable stars.
Edward Halbach (1909–2011), who specialized in the observation of variable stars.
Will Hay, the famous comedian and actor, who discovered a white spot on Saturn.
Walter Scott Houston (1912–1993) who wrote the "Deep-Sky Wonders" column in Sky & Telescope magazine for almost 50 years.
Albert G. Ingalls (1888–1958), editor of Amateur Telescope Making, Vols. 1–3 and "The Amateur Scientist".
Peter Jalowiczor (born in 1966) discovered four exoplanets
David H. Levy discovered or co-discovered 22 comets including Comet Shoemaker-Levy 9, the most for any individual.
Terry Lovejoy discovered five comets in the 21st century and developed modifications to DSLR cameras for astrophotography.
Sir Patrick Moore (1923–2012), presenter of the BBC's long-running The Sky at Night and author of many books on astronomy.
Leslie Peltier (1900–1980), a prolific discoverer of comets and well-known observer of variable stars.
John M. Pierce (1886–1958) was one of the founders of the Springfield Telescope Makers.
Russell W. Porter (1871–1949) founded Stellafane and has been referred to as the "founder"
Grote Reber (1911–2002), pioneer of radio astronomy constructing the first purpose built radio telescope and conducted the first sky survey in the radio frequency.
Isaac Roberts (1829–1904), early experimenter in astronomical photography.

Discoveries with major contributions by amateur astronomers

Cygnus A (1939) is a radio galaxy and one of the strongest radio sources on the sky.
Dramatic period decrease in T Ursae Minoris using AAVSO observations (1995)
McNeil's Nebula (2004) is a variable nebula
XO-1b (2006) is an exoplanet
tidal streams around NGC 5907 (2008)
Voorwerpjes (2009) is a type of quasar ionization echo.
Pea Galaxies (2009) are a type of galaxy.
Most recent (2010) outburst of U Scorpii
Kronberger 61 (2011) is a planetary nebula.
Speca (2011) is a spiral galaxy containing contain DRAGNs (Double Radio-source Associated with Galactic Nucleus).
2011 HM102 (2013) is a Neptune Trojan.
PH1b (2013) is an extrasolar planet in a circumbinary orbit in a quadruple star system.
PH2b (2013) is an extrasolar gas giant planet located in its parent star's habitable zone.
J1649+2635 (2014) is a spiral galaxy containing contain DRAGNs (Double Radio-source Associated with Galactic Nucleus).
Yellowballs (2015)[18] are a type of compact star-forming region.
9Spitch (2015) is a distant gravitationally lensed galaxy with high star-forming rate.
NGC 253-dw2 (2016) is a dwarf spheroidal (dSph) galaxy candidate undergoing tidal disruption around the nearby galaxy NGC 253. The galaxy was discovered by an amateur astronomer with a small-aperture amateur telescope.
KIC 8462852 (2016) is an F-type star showing unusual dimming events.
HD 74389 (2016) contains a debris disk. It is the first debris disk discovered around a star with a companion white dwarf.
AWI0005x3s (2016) is the oldest M-dwarf with a debris disk detected in a moving group at the time of the discovery.
PSR J1913+1102 (2016)[19] is a binary neutron star with the highest total mass at the time of the discovery.
Donatiello I (2016) a nearby spheroidal dwarf galaxy discovered by the Italian amateur astronomer Giuseppe Donatiello. It is also the first galaxy to be named after an amateur astronomer.
Transiting Exocomets (2017) are comets in an extrasolar system blocking some of the starlight while transiting in front of the extra-solar star.
K2-138 (2018) is a planetary system with five confirmed planets in an unbroken 3:2-resonance chain.
Supernova 2016gkg (2018) was observed by an amateur astronomer shortly after it began to erupt.
PSR J1744−7619 (2018)[20] is the first Pulsar to be detected only in gamma-rays and not in radio-waves.
STEVE (2018) is an atmospheric phenomenon.
K2-288Bb (2019) is an extrasolar planet in the habitable zone around a M-star, which belongs to a binary system.
LSPM J0207+3331 (2019) is an old white dwarf containing a debris disk with two components.
Interstellar Comet 2I/Borisov (2019) is the first interstellar comet.
Kojima-1Lb (confirmed in 2019) is a Neptune-sized exoplanet discovered by an amateur astronomer with the microlensing method. Kojima-1 is the brightest microlensing host discovered.[21]
WISE2150-7520AB (2019/2020) is a pair of brown dwarfs with the lowest binding energy at a total mass smaller than 0.1 solar masses not associated with a young cluster.[22]
GJ 3470 c (2020) is the first exoplanet candidate completely discovered by amateurs. Unlike Peter Jalowiczor, Kojima-1Lb and XO-1b, GJ 3470 c was fully discovered by an amateur in a project led by amateur astronomers.[23]

Prizes recognizing amateur astronomers

Amateur Achievement Award of Astronomical Society of the Pacific
Chambliss Amateur Achievement Award

Astronomical object
Caldwell catalogue A list of astronomical objects for observation by amateur astronomers compiled by Sir Patrick Caldwell-Moore.
Clear Sky Chart Weather forecasts designed for amateur astronomers.
List of astronomical societies
List of telescope parts and construction
Messier catalogue A set of astronomical objects catalogued by the French astronomer Charles Messier in 1771, which is still used by many amateurs as an observing list.
Observation
Observational astronomy
Sidewalk astronomy
Skygazing
Star party

"American Association of Variable Star Observers : The AAVSO Research Portal". Retrieved September 17, 2017.
Heintz, W. D. (1978). Double Stars. D. Reidel Publishing Company, Dordrecht. pp. 4–10. ISBN 90-277-0885-1.
Wilkinson, John (2012). New Eyes on the Sun: A Guide to Satellite Images and Amateur Observation. Springer Science & Business Media. ISBN 978-3-642-22839-1.
"International Occultation Timing Association (IOTA) : Introduction to Observing Occultations". Retrieved September 17, 2017.
Clay Sherrod, P. Clay Koed, Thomas L. (1981). A Complete Manual of Amateur Astronomy: Tools and Techniques for Astronomical Observations. p. 66. ISBN 978-0-486-15216-5.
Marsden, B.G. (1988). Dunlop, Storm Gerbaldi, Michèle (eds.). Stargazers : The Contribution of Amateurs to Astronomy : Amateur Astronomers and the IAU Central Bureau for Astronomical Telegrams and Minor Planet Center. Springer-Verlag. p. 68. doi:10.1007/978-3-642-74020-6. ISBN 978-3-540-50230-2.
Zuckerman, Ben Malkan, Matthew A. (1996). The Origin and Evolution of the Universe. Jones & Bartlett Learning. p. 68. ISBN 0-7637-0030-4.
"Sky & Telescope : Pro-Am Collaboration". Retrieved September 17, 2017.
Meadows, A.J. (1988). Dunlop, Storm Gerbaldi, Michèle (eds.). Stargazers : The Contribution of Amateurs to Astronomy : Twentieth-Century Amateur Astronomers. Springer-Verlag. p. 20. doi:10.1007/978-3-642-74020-6. ISBN 978-3-540-50230-2.
Motta, M. (2006). "Contributions of Amateur Astronomy to Education". Journal of the American Association of Variable Star Observers. 35 (1): 257. Bibcode:2006JAVSO..35..257M.
"Beneath the Milky Way". European Southern Observatory. Archived from the original on September 6, 2017. Retrieved March 29, 2016.
Amateur Stargazing With a GPS Tour Guide
Turn Your Smartphone into an Astronomy Toolbox with Mobile Apps
Daylight Polar Alignment Made Easy
"Argo Navis : User Manual 10" (PDF). p. 93. Retrieved January 28, 2018.
"Remote Observatories". www.nmskies.com.
Maury, Alain. "SPACE : A cost effective solution for your observatory" (PDF).
Kerton, C. R. Wolf-Chase, G. Arvidsson, K. Lintott, C. J. Simpson, R. J. (January 26, 2015). "The Milky Way Project: What Are Yellowballs?". The Astrophysical Journal. 799 (2): 153. arXiv:1502.01388. Bibcode:2015ApJ. 799..153K. doi:10.1088/0004-637x/799/2/153. ISSN 1538-4357. S2CID 119196894.
"Neutron stars on the home PC". www.mpg.de. Retrieved December 11, 2019.
[email protected] discovers first millisecond pulsar visible only in gamma rays". www.mpifr-bonn.mpg.de. Retrieved December 11, 2019.
Fukui, A. Suzuki, D. Koshimoto, N. Bachelet, E. Vanmunster, T. Storey, D. Maehara, H. Yanagisawa, K. Yamada, T. Yonehara, A. Hirano, T. (November 2019). "Kojima-1Lb Is a Mildly Cold Neptune around the Brightest Microlensing Host Star" (PDF). AJ. 158 (5): 206. arXiv:1909.11802. Bibcode:2019AJ. 158..206F. doi:10.3847/1538-3881/ab487f. ISSN 0004-6256. S2CID 202888719.
Faherty, Jacqueline K. Goodman, Sam Caselden, Dan Colin, Guillaume Kuchner, Marc J. Meisner, Aaron M. Gagne', Jonathan Schneider, Adam C. Gonzales, Eileen C. Gagliuffi, Daniella C. Bardalez Logsdon, Sarah E. (2020). "WISE2150-7520AB: A very low mass, wide co-moving brown dwarf system discovered through the citizen science project Backyard Worlds: Planet 9". The Astrophysical Journal. 889 (2): 176. arXiv:1911.04600. Bibcode:2020ApJ. 889..176F. doi:10.3847/1538-4357/ab5303. S2CID 207863267.

"The Extrasolar Planet Encyclopaedia — GJ 3470 c". exoplanet.eu. Retrieved August 5, 2020.

Timothy Ferris (2002). Seeing in the Dark: How Backyard Stargazers Are Probing Deep Space and Guarding Earth from Interplanetary Peril. New York: Simon & Schuster. ISBN 978-0-684-86579-9.
P. Clay Sherrod Thomas L. Koed (2003). A Complete Manual of Amateur Astronomy: Tools and Techniques for Astronomical Observations. Mineola, N.Y.: Dover Publications. ISBN 978-0-486-42820-8.
Mousis, O. et al. (2014). "Instrumental methods for professional and amateur collaborations in planetary astronomy". Experimental Astronomy. 38 (1–2): 91–191. arXiv:1305.3647. Bibcode:2014ExA. 38. 91M. doi:10.1007/s10686-014-9379-0. S2CID 118513531.

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## EIU Astro

In a preview if the new superautomated survey telescopes that are appearing in the next few years…this from Palomar observatory. The venerable 48″ Schmidt telescope, the largest in the world, that was responsible for the grandfather of all surveys , the original Palomar sky survey has been automatated and given a new life as the 48-inch Samuel Oschin Telescope.

Edwin Hubble using the 48" Schmidt Telescope

An innovative sky survey has begun returning images that will be used to detect unprecedented numbers of powerful cosmic explosions-called supernovae-in distant galaxies, and variable brightness stars in our own Milky Way. The survey also may soon reveal new classes of astronomical objects.

All of these discoveries will stem from the Palomar Transient Factory (PTF) survey, which combines, in a new way, the power of a wide-field telescope, a high-resolution camera, and high-performance networking and computing, with rapid follow-up by telescopes around the globe, to open windows of discovery for astronomers. The survey has already found 40 supernovae and is gearing up to switch to a robotic mode of operation that will allow objects to be discovered nightly without the need for human intervention.

The Palomar Transient Factory is a collaboration of scientists and engineers from institutions around the world, including the California Institute of Technology (Caltech) the University of California, Berkeley, and the Lawrence Berkeley National Laboratory (LBNL) Columbia University Las Cumbres Observatory the Weizmann Institute of Science in Israel and Oxford University.

During the PTF process, the automated wide-angle 48-inch Samuel Oschin Telescope at Caltech’s Palomar Observatory scans the skies using a 100-megapixel camera.

The flood of images, more than 100 gigabytes every night, is then beamed off of the mountain via the High Performance Wireless Research and Education Network¬-a high-speed microwave data connection to the Internet-and then to the LBNL’s National Energy Scientific Computing Center. There, computers analyze the data and compare it to images previously obtained at Palomar. More computers using a type of artificial intelligence software sift through the results to identify the most interesting “transient” sources-those that vary in brightness or position.

Within minutes of a candidate transient’s discovery, the system sends its coordinates and instructions for follow-up observations using the Palomar 60-inch telescope and other instruments.

Soon all of the steps in the process will be completely automated, including decisions about which transients merit a second look. When follow-up observations indicate that candidate transient detections show promise, a prioritized list of candidates is brought to the attention of astronomers from the PTF member institutions. Finally, an astronomer becomes personally involved, by performing detailed observations using telescopes such as Palomar’s 200-inch Hale Telescope, a Keck Telescope in Hawaii, or other partner telescopes around the world.

Upgraded 48" Oschin Telescope

The PTF is designed to search for a wide variety of transient sources with characteristic timescales ranging from minutes to months, giving astronomers one of their deepest and most comprehensive explorations of the universe in the time domain.

“By looking at the sky in a new way, we are ushering in a new era of astronomical discovery,” says PTF principal investigator Shrinivas Kulkarni, MacArthur Professor of Astronomy and Planetary Science at Caltech and director of the Caltech Optical Observatories. “Nimble automated telescopes and impressive computing power make this possible.”

“No one has looked on these timescales with this sensitivity before. It’s entirely possible that we will find new astronomical objects never before seen by humans,” says Nicholas Law of Caltech, the project scientist for PTF.

Because it looks for anything changing in the sky, the PTF survey covers a vast variety of different astronomical targets. The wide range of the survey extends across the entire universe. Astronomers expect to discover everything from stars exploding millions of light-years away to near-Earth asteroids that could someday impact our planet.

Much of the survey’s time is spent searching for so-called Type Ia supernovae. These supernovae, formed from the explosion of a class of dead star known as a white dwarf, are very useful to astronomers because they can help determine the distance to galaxies located across the universe. Those distances allow astronomers to probe the origin, structure, and even the ultimate fate of the universe.

By operating more rapidly than previous surveys, PTF will also detect objects of a completely different nature, such as pulsating stars, different types of stellar explosions, and possibly planets around other stars.

PTF’s innovative survey techniques also have raised astronomers’ expectations of finding new, unexpected, astronomical objects.

The PTF already has found many new cosmic explosions, including 32 Type Ia supernovae, eight Type II supernovae, and four cataclysmic variable stars. Intriguingly, PTF also has found several objects with characteristics that do not exactly match any other objects that have been seen before. PTF astronomers are eagerly watching these objects to see how they change, and to determine what they might be.

The quantity and quality of incoming data have astonished astronomers working in the field. On one recent night, PTF patrolled a section of the sky about five times the size of the Big Dipper-and found 11 new objects. “Today I found five new supernovae before breakfast,” says Caltech’s Robert Quimby, a postdoctoral scholar and leader of the PTF software team. “In the previous survey I worked on, I found 30 in two years.”

## Beyond the Boundaries of Time and Space

One of the most common phrases used by academic papers in Astronomy and Astrophysics is “we need more data and will know more with better equipment in the future”. Undoubtedly, more advanced telescopes and deeper detections will help uncover a more complete perspective of the universe. While we are grateful for the technology we have today and look forward to more technological development in the future, we should also appreciate and feel amazed by the astounding contributions our ancestors made to science thousands of years ago without any “better equipment”. More importantly, these ancient records and findings still have a deep impact on modern science.

Although the system of modern astronomy is believed to be built on Greek findings in the western world, the earliest recorded discoveries of astronomical objects originated in the eastern world of Ancient China. In fact, the exploration of astronomy has been deeply embedded in Chinese culture and history. The word for “universe” in Chinese is comprised with two characters – “宇宙”, one meaning space and one meaning time. The theoretical support of this definition first appeared in 1907 when Albert Einstein developed the concept of general relativity. However, the Chinese word itself was invented before 200 B.C., and ever since has continued to demonstrate that the story of the universe has no boundaries in time or space.

Of all the astronomical objects traveling through time and space, comets are among the most popular documented by sky observers, including ancient Chinese.

As early as 168 B.C., Chinese ancestors constructed a complex set of illustrations of comet observations. Out of many scrolls discovered in 1973 from the Mawangdui Silk Texts, one specifically describes astronomical and meteorological phenomena, including 29 drawings of comets with different morphologies (Figure 1). Scientists estimate that, in order to produce such a figure, the observers might have kept a full record of

100 comets. If our ancestors observed comets as frequently as we do nowadays, the observations may have taken

1000 years and several generations of consistent documentation!

Figure 1: Classifications of comets based on morphology, discovered in the Mawangdui Silk Texts in 1973. No. 29 illustrates a very rare active comet. The original scroll was made around 168 B.C.. Adopted from Figure 3.4 in “4000 ans D’astronomi Chinoise” by Jean-Marc Bonnet-Bidaud published in 2020.

Although our ancestors didn’t understand the physical nature of astronomical objects as well as we do now, their astonishing records are still meaningful to modern astronomy.

Although many comets follow periodical trajectories, they lose a small part of their compositions each time they arrive close to the sun. Therefore, the dynamics and morphology of comets change over time. No. 29 in Figure 1 is a schematic illustration of an active comet. In this case, the rare active comet ejects so much gas and dust that it starts whirling on its axis. As many comets we know nowadays have lost their momentum (e.g., Encke’s Comet), we can take a glance at their previous active phases through these ancient documents. Moreover, in 648 A.D., at the beginning of the Tang dynasty in China (or Byzantine era in Rome), some observers had already studied the relation between the tail of the comet and its location in respect to the sun in the Book of Jin. After 900 years, the same relation was also discovered by European astronomers.

Up till now, historians have found evidence of amazing astronomical records in all ancient civilizations, including the visits of the well-known Halley’s comet. What makes ancient China stand out is not just the earliest records but also the scientific accuracy.

The New Book of Tang, a historical work covering the entire Tang dynasty, records activities of Halley’s comet from March 22 to April 28 in the year of 837 A.D.. During this period, the documents detail the observed time, location, length, shape and change of Halley’s comet every day until it disappeared. Although Halley’s comet was visible to the entire world at that time, historical records in other regions of the world show more poetic descriptions rather than Chinese observers’ scientific descriptions. In fact, the first “scientific documentation” in Europe as complete as the one in the New Book of Tang only occurred in 1456.

Furthermore, the timing of astronomical events recorded by ancient Chinese observers are very precise due to the usage of two calendars (lunisolar and sexagesimal) simultaneously. Moreover, historians can also use the evolution of Chinese characters and languages to further confirm the era of the discovered tablets, scrolls and papers. Therefore, it is surprisingly accurate to convert the dates of 29 observations on Halley’s comet over 2200 years from different Chinese dynasties to the modern calendar we use now.

Thanks to these detailed ancient records with precise dates, astronomers were able to revise the trajectory of Halley’s comet in 1982 after combining all previous observations dating back to 240 B.C.. They concluded that the period of Halley’s comet is not constant as 76 years but ranges from 76 to 79 years.

In addition to comets, ancient Chinese have made countless significant scientific contributions to modern astronomy:

• The earliest record of a supernova explosion in 185 A.D. (Han dynasty of China) supports the recent X-ray observational results on supernova remnant ‘RCW 86‘ in 2006.
• The thorough sky maps (Figure 2) made in 320 A.D. (Jin dynasty of China) led to an early estimate of axial precession as 1° change in 50 years without any modern equipment, while the accurate rate accepted today is 1° in 72 years.
• Among six different theories of the structure of the universe emerged in ancient China, “Xuan Ye Shuo”, proposed before 648 A.D., believes that the stars and the Milky Way are made of gas and float in the endless universe, which is surprisingly close to modern astronomy suggests!

Compared to the vastness of the universe, the life expectancy of humans can be ignored. However, looking back in history, it is clear that curiosity about the universe has remained constant across the boundaries of time and space. We, humans, look up into the same starry sky with wonder regardless of time, location, nationality, race, or any other difference. Meanwhile, every inch of the universe embraces us all equally.

Thanks to all the valuable ancient documents, we can now envision the sky our ancestors once looked up to. Those diligent and rigorous scientists in the early days of human civilization made unimaginable contributions to the history of astronomy. They began to compose human intelligence and devotion into a story of time and space, that we are continuing to write.

Reference: “4000 ans D’astronomi Chinoise” by Jean-Marc Bonnet-Bidaud

## Classification

As part of the attempt to understand supernovae, astronomers have classified them according to the absorption lines of different chemical elements that appear in their spectra. The first element for a division is the presence or absence of a line caused by hydrogen. If a supernova's spectrum contains a line of hydrogen (known as the Balmer series in the visual portion of the spectrum) it is classified Type II otherwise it is Type I. Among those types, there are subdivisions according to the presence of lines from other elements and the shape of the light curve (a graph of the supernova's apparent magnitude as a function of time).

TypeCharacteristics
Type I
Type IaLacks hydrogen and presents a singly ionized silicon (Si II) line at 615.0 nm (nanometers), near peak light.
Type IbNon-ionized helium (He I) line at 587.6 nm and no strong silicon absorption feature near 615 nm.
Type IcWeak or no helium lines and no strong silicon absorption feature near 615 nm.
Type II
Type IIPReaches a "plateau" in its light curve
Type IILDisplays a "linear" decrease in its light curve (linear in magnitude versus time).

## How often are new astronomical objects (variable stars, supernovae, comets, etc) discovered by amateurs? - Astronomy

RR Lyrae Stars – marvelous candles

1. Introduction – a house of cards, measuring astronomical distances

We find our place in the Universe by using overlapping “yardsticks” to measure astronomical distances near and far. However, these yardsticks resemble a "house of cards." They are all based upon the preceding yardstick, and they are ultimately founded on having precise parallax measurements for the nearest objects. Parallax methods can directly measure the distance of objects “close” to the Earth, including Solar System objects and the nearest stars out to 300 + light years (Hipparcos). Gaia (European Space Agency) is now active and performing extremely precise astrometry, photometry, and radial velocity measurements of measurements of millions of stellar objects (Gaia, 2019). It will extend parallax methods approximately 200 times farther than Hipparcos.

Parallax measurements then support the use of stellar “standard candles” (Cepheid variable stars and RR Lyrae stars) on the next rung of the ladder for estimating more distant Milky Way objects and for measuring close by galaxies. Finally, very distant yardsticks (type 1a supernovae, spiral galaxy surface brightness fluctuations, and red shift determinations) are used for examining remote galaxy clusters and quasars (see Ferdie et al., 2004). This house of cards technique of overlapping distance scales means we can take a ruler to the Universe, but it is fraught with uncertainty, and the errors add up as we extend our measurements to greater and greater distances. Figure one and table one summarize these overlapping yardsticks:

Figure 1. Overlapping yardsticks for measuring the Universe. From Ferdie, 2004.

Table 1 – The Distance Ladder

The series of techniques employed to obtain distances to progressively more remote astronomical objects.

Cepheids, Main Sequence Fitting

Cepheids, Supernovae, OB stars

HST Cepheids, OB stars, Supernovae

Brightest Galaxies, Tully-Fisher

Standard candles are the next rung on the distance ladder after parallax measurements. At this point, we leave direct measurement techniques and begin to extend our distance scale to millions of light years using indirect techniques. Standard candles represent any astronomical object with a consistent, well known intrinsic luminosity. The observed brightness of a standard candle in the Milky Way or in another galaxy can be compared with its known intrinsic brightness to estimate its distance. Cepheid variable stars, RR Lyrae stars, and type Ia supernovae are the classic standard candles, though there are several other objects or techniques that can be used as standard candles as shown in figure one and Table 1.

Cepheid variable stars are named for Delta Cephei, their prototype. They are giant variable stars whose individual periods can be directly correlated with their intrinsic luminosities. The longer the period, the greater the star’s luminosity. This period-luminosity relationship was discovered by Henrietta Leavitt (1868-1921) in 1912. It has been well established, and these stars are the most important stellar candles for short and intermediate astronomical distances out to 50-100 million light years. Type 1a supernovae are a particular type of supernova with a characteristic spectrum and light curve. Their peak luminosities are almost exactly the same, and they can be used as standard candles for measuring the most distant reaches of the Universe. This essay examines the use of RR Lyrae stars as standard candles. Their importance for measuring distances within the Milky Way and nearby galaxies is second only to that of Cepheid variable stars, and they provide a complementary cross check for Cepheid distance measurements.

The prototype for this class of stars, RR Lyrae (also known as RR Lyra), was discovered to be a short period variable star by Williamina Fleming (1857-1911) at Harvard in 1899. She noted its changing brightness on photographic plates taken over a period of several days. It was also noted to have a period nearly the same as a large number of similar such stars found in globular clusters by Solon Bailey (1854-1931) in 1893 ( Moore, 2002). RR Lyrae itself was at first thought to have escaped from a globular cluster, but later other RR Lyrae like stars were found as isolated stars apart from the many RR Lyrae stars associated with globular clusters (Moore, 2002). RR Lyrae has a period of 13 hours and 36 minutes. It varies from magnitude 7.1 to 8.1. Figure 2A shows RR Lyrae’s location with respect to the constellation Lyra. Figure 3 compares the periods of Cepheid and RR Lyrae stars. Figure 4 shows a typical period of a Cepheid variable star, and figure 5A shows a typical period for a RR Lyrae variable star.

RR Lyrae stars can be grouped into two categories based on the shape of their light curves (Sarajedin, 2020). The most common light curve is sawtooth like that shown in Figure 5A. The other class of light curves is roughly sinusodal with shorter periods. Both of these curves are well illustrated in Figure 5B taken Sarajedin, 2020.

Figure 2A. RR Lyrae and environs. From: http://www.exn.ca/Stories/1998/06/22/55.asp

Figure 2B. Close-up view of RR Lyrae (arrow pointing upward) and nearby stars. North is at the top and East is to the left. The central star marked with the arrow pointing to the right has a magnitude of

8.8. The star marked with the arrow pointing to the left has a magnitude of 7.2. Ninety-second exposure with Canon 20Da digital camera using an 85 mm f/3.5 lens, ISO 800. Image courtesy James McGaha.

Figure 3. Comparison of Cepheid and RR Lyrae stars.

Figure 4. Typical light curve for a Type I (Classical) Cepheid variable star.

Figure 5 A. Typical light curve for a RR Lyrae variable star.

Figure 5B. Sawtooth (RRa and RRb) RR Lyrae light curves and sinusodal (RRc) RR Lyrae light curve (from Astronomy, July 2020, page 58).

Figure 6. Luminosity temperature (Hertzsprung-Russell [HR] diagram showing RR Lyrae stars on the horizontal branch (from Astronomy, July 2020, page 59).

## Nova in Scutum, supernova in Pisces and a comet

I just found this article in S&T with very important and exciting news about bright new astronomical objects:

### #2 VanJan

Observed the nova in Scutum last evening (29/7/17) with my 90mm refractor at 36X and 130X. Guesstimated the magnitude at 8.5 - a more precise estimation difficult due to lack of comparison stars. Seemed reddish to me as well.

Kudos to Bob King and the the S&T web site for drawing my attention to this nova, now sufficiently bright to observe with my scope under light-polluted skies. And a for the OP, too.

Edited by VanJan, 30 July 2017 - 04:07 PM.

### #3 BrooksObs

Indeed, Nv Sct '17 appears to be a most unusual sort of nova. About the time of its discovery, toward in the last week in June, it was about magnitude 12.2 and still slowly rising in brightness. This brightening trend continued, but steadily slowed until about July 20th when it appeared that it might have reached its peak. But then, sudden on July 22nd a new and far faster brightening trend set in raising the nova to the mid 8's by last evening!

Obvious a member of the category of "slow novae" and one's who will often display peculiar behavior and protracted intervals of brightness, Nv Sct '17 is easy to locate and would probably be an easy candidate to large binoculars were it not for the waxing moon's approach right now.

### #4 andrew hampton

Yes, thanks to the article I also observed the Nova on Fri. 28th 2017 with a reddish glow around 8.5 mag. and again on 30th. It seemed slightly less bright last night though - perhaps nearer 9th magnitude.

Edited by andrew hampton, 31 July 2017 - 09:49 AM.

### #5 BrooksObs

As a follow-up to my previous post I would add the following for the benefit to those seriously interested in novae. Most definitely Nv Sct '17 belongs to the class of slow novae, by far the most unpredictable in behavior, subject to erratic variability and for visual observers to my mind the most "entertaining" of all the nova types. While probably still a bit early to conjecture Nv Sct '17 future, I might speculate that so far its lightcurve is suggestive of a class J nova (per Sterope, Schaefer & Hendren). Rather similar early behavior was displayed by V723 Cas and HR Del in the past. If indeed similar, the star might remain reasonably near maximum brightness, while perhaps subject to striking fluctuations, all the way until it nears solar conjunction. or possibly even longer!

### #6 Aquarellia

Thank you for those precisions BrooksObs !

Indeed this is a very strange object, I did already 14 estimations about this Nova, tonight my visual magn gives around +9.0.

My first estimation was done just one month ago +11.4.

I post my observation in the sketching forum here : https://www.cloudyni. 60-nova-scutum/

Edited by Aquarellia, 01 August 2017 - 03:25 AM.

### #7 vakulenko_sergiy

Last evening (1 aug) I've observed Nova in Scutum

ED80, Atik383L, Baader L: 20x30sec:

### #8 andrew hampton

Just had another view of the Nova tonight 2 Aug. and I'd say it looked slightly less bright again than on the 30th July. A bit fainter than 9th magnitude around 9.2

### #9 toranaga

Hm. comet ASASSN1? Sounds bit like assassin.

### #10 Aquarellia

Hm. comet ASASSN1? Sounds bit like assassin.

On purpose. All Sky Automated Survey for SuperNovae (ASAS-SN) number 1 for the firstncomet discovered by this system.

For the nova, my estimation (just done 15 minutes ago) was between 9.2 and 9.3

### #11 jodemur

Yeah. I saw that assassin star on the 28th also. I printed out the AAVSO Wider field chart and had no trouble finding it at 60x. It was bright orange. 120x really made it pop.

I couldn't estimate the mag. but I would say it was all of 9th mag. anyhow. The orangest thing in that patch of sky at 11:30 EDT.

I tried to find it at same time on the 29th to show off to my wife but our skies had degraded some and the moon was brighter and further S. It wasn't as bright but still showed orange. That's my first nova.

### #12 BrooksObs

I find it particularly interesting to see the virtually total lack of notice Nv Sct '17 has garnered on the General Observing and Astronomy section of this forum. Years back a nova this bright (mid 8's), easy to locate, and well placed in the summer evening sky would have generated countless sightings from even casual observers. This could hardly be from lack of notification given that S&T devoted a full article to it not long ago on their web page. Makes me wonder about the future of the hobby.

Be that as it may, it now appears that the nova peaked in brightness on July 31 and has been slowly, but steadily, declining since. I made it out to be of magnitude 9.8 as the near full moon was rising last evening, down by nearly 1.5 magnitudes so far from its maximum.

### #13 DHEB

I find it particularly interesting to see the virtually total lack of notice Nv Sct '17 has garnered on the General Observing and Astronomy section of this forum. Years back a nova this bright (mid 8's), easy to locate, and well placed in the summer evening sky would have generated countless sightings from even casual observers. This could hardly be from lack of notification given that S&T devoted a full article to it not long ago on their web page. Makes me wonder about the future of the hobby.

BrooksObs, may be a lot of people who frequent this forum are nowadays very focused on preparations for the coming North American eclipse?

Thanks for your observations & reports!

### #14 BrooksObs

I find it particularly interesting to see the virtually total lack of notice Nv Sct '17 has garnered on the General Observing and Astronomy section of this forum. Years back a nova this bright (mid 8's), easy to locate, and well placed in the summer evening sky would have generated countless sightings from even casual observers. This could hardly be from lack of notification given that S&T devoted a full article to it not long ago on their web page. Makes me wonder about the future of the hobby.

BrooksObs, may be a lot of people who frequent this forum are nowadays very focused on preparations for the coming North American eclipse?

Thanks for your observations & reports!

I highly doubt that the upcoming eclipse would deter folks from spending perhaps 5 minutes to spot such an easy, bright, nova during a night's observing session. In fact, I wouldn't imagine that the coming event has had far less impact on the regular observing habits of hobbyists in general, especially several weeks out. More likely, i'd venture that the majority of casual hobbyists here aren't even particularly aware of the nova's presence!

### #15 DHEB

I highly doubt that the upcoming eclipse would deter folks from spending perhaps 5 minutes to spot such an easy, bright, nova during a night's observing session. In fact, I wouldn't imagine that the coming event has had far less impact on the regular observing habits of hobbyists in general, especially several weeks out. More likely, i'd venture that the majority of casual hobbyists here aren't even particularly aware of the nova's presence!

BrooksObs

Yes, I understand what you mean. In a sense it is not a surprise because variable stars are by no means a central interest for most amateurs. David Levy also complained about that perceived lack of interest in variable stars (*). However, I do not perceive this as a worrying trend. I believe there are plenty of amateurs interested in novas and supernovas. Also this is a hobby with many sides and there is no right or wrong.

* In his book Observing variable stars. A guide for the beginner, first edition (1989), chapter 28, page 157.

Edited by cincosauces, 09 August 2017 - 02:54 PM.

### #16 goodricke1

I find it particularly interesting to see the virtually total lack of notice Nv Sct '17 has garnered on the General Observing and Astronomy section of this forum. Years back a nova this bright (mid 8's), easy to locate, and well placed in the summer evening sky would have generated countless sightings from even casual observers. This could hardly be from lack of notification given that S&T devoted a full article to it not long ago on their web page. Makes me wonder about the future of the hobby.

Nova Sgr 2 was just a couple of years ago and peaked more than 20 times brighter than Nv Sct 17, remaining at mag 6-7 during the summer months. Maybe the current nova suffers by comparison. Also with mag 12-15 Supernovae readily accessible in amateur scopes, perhaps a piffling mag 8-9 nova is no longer as capable of provoking excitability. More generally, the exoplanet and alien life frenzy seems to be the prime motivation for a lot of amateur (and professional) interest in the subject nowadays, with other areas deemed less trendy.

Speaking for myself, July was a complete cloudout here in Ireland and opportunities for observing have been vanishingly rare. Hopefully I can put that right tonight.

### #17 Redbetter

I find it particularly interesting to see the virtually total lack of notice Nv Sct '17 has garnered on the General Observing and Astronomy section of this forum. Years back a nova this bright (mid 8's), easy to locate, and well placed in the summer evening sky would have generated countless sightings from even casual observers. This could hardly be from lack of notification given that S&T devoted a full article to it not long ago on their web page. Makes me wonder about the future of the hobby.

Be that as it may, it now appears that the nova peaked in brightness on July 31 and has been slowly, but steadily, declining since. I made it out to be of magnitude 9.8 as the near full moon was rising last evening, down by nearly 1.5 magnitudes so far from its maximum.

I don't see that it says much about the general future of the hobby. Instead it is perhaps a better reflection of the history of the hobby. I come at this from the perspective of a visual observer rather than an imager. While I was aware of this nova, I didn't look for it since I didn't intend to track its brightness over several nights. Instead I had countless galaxies beckoning for attention in a dark sky.

My impression of the history of the hobby is that visual variable star observing made more sense in a time period of smaller apertures and before extensive use of CCD's changed the nature of tracking variables. Over the past several decades apertures have greatly increased and so have the range of accessible targets. I doubt there are that many visual variable star observers left, and would expect that fractionally they would make up a far smaller portion now.

It has become easier these days to track down supernovae in other galaxies. Two decades ago information on such targets for the casual observer was often less timely, and it took more effort to find good images, create a finder chart for an obscure galaxy, etc. Now I sift through half a dozen or more candidates each month just to decide which ones will be easier targets.

I haven't been observing quasars either. I skipped the recent outburst that had a lot of folks excited. On the other hand I did manage an impression of the lensed galaxy in Andromeda, driven as much by curiosity as anything else.

It is also easier to generate charts to track even the most difficult planetary satellites. And it is more feasible to sort them out from field stars too because the readily available data for field stars has improved so much. This has allowed me to go back and confirm decades old observations.

There are a number of comets to search for if one is so inclined. And tens of thousands of galaxies. One of my fellow observers has been working through a lengthy asteroid list.

So in perspective this nova seems like less of a "big deal" now than it might have seemed in the past. That isn't because it is inherently less important, but because the range of targets accessible has so greatly expanded. It is vying for attention with many times as much competition as it would have once had.

Another aspect that I come across is that a nova in an otherwise dim star is more difficult for the casual observer today to locate in the field. It won't be in their push-to/go-to database and even if it were, the person wouldn't know which field star was the right one. I see this with comets too. They can be a bit confusing to those who don't visually star hop and they seem to have trouble with RA/Dec.

### #18 andrew hampton

I've just had another look at the Nova tonight and made it 9.8 to 9.9 magnitude. Perhaps Comet C/2017 O1 will generate some enthusiasm as in brightens in the late summer skies !

### #19 goodricke1

Just about captured it tonight with a 55mm lens in heavy moonlight yes dipping to

### #20 smithrrlyr

I find it particularly interesting to see the virtually total lack of notice Nv Sct '17 has garnered on the General Observing and Astronomy section of this forum. Years back a nova this bright (mid 8's), easy to locate, and well placed in the summer evening sky would have generated countless sightings from even casual observers. This could hardly be from lack of notification given that S&T devoted a full article to it not long ago on their web page. Makes me wonder about the future of the hobby.

Be that as it may, it now appears that the nova peaked in brightness on July 31 and has been slowly, but steadily, declining since. I made it out to be of magnitude 9.8 as the near full moon was rising last evening, down by nearly 1.5 magnitudes so far from its maximum.

I don't see that it says much about the general future of the hobby. Instead it is perhaps a better reflection of the history of the hobby. I come at this from the perspective of a visual observer rather than an imager. While I was aware of this nova, I didn't look for it since I didn't intend to track its brightness over several nights. Instead I had countless galaxies beckoning for attention in a dark sky.

My impression of the history of the hobby is that visual variable star observing made more sense in a time period of smaller apertures and before extensive use of CCD's changed the nature of tracking variables. Over the past several decades apertures have greatly increased and so have the range of accessible targets. I doubt there are that many visual variable star observers left, and would expect that fractionally they would make up a far smaller portion now.

It is certainly true that some aspects of variable star astronomy are no longer within the realm of the visual observer. One would rarely observe an eclipsing variable or an RR Lyrae star visually in this day of CCD observing. However, I hope that visual observers do not entirely give up on variable stars. There are, for example, long period variables and cataclysmic variables that are more poorly monitored today than they were twenty or thirty years ago when visual observers kept them under frequent watch. For now, there is still value in visual observing. It can also be fun to check on what some variable star is up to. A star that you might have followed for decades can still surprise you.

### #21 BrooksObs

I lot of misconceptions in your latest post, I'm afraid brother Redbetter.

Firstly, amateur CCD efforts are far from taking over the job of monitoring most categories of variable stars to date. Outside of time-series work, they account for only a fraction of the data employed in AAVSO lightcurves across the board. Were visual monitoring to cease today the professional variable star community would find itself in serious straights indeed. The value of such long enduring associations as the AAVSO, BAA VSS, et al, is that their data represents the monitoring of a great many stars' activities, selected in a single fashion (visually), spanning many decades. What CCD or other "modern" data is available offers little more than a recent snapshot of a star's behavior. Neither are the photometric values obtained by these modern approaches necessarily congruent with visual data, not being the true equivalent of visual. AAVSO has had a difficult time in attempting to marry amateur CCD data with their visual work. and for some stars it just seems impossible!

The employing of smaller scopes in the past represents no limitations as the AAVSO's program has evolved to match the stars monitored with the scopes available. Incidentally, relatively few variable star observers are today employing scopes larger that 12" aperture, not even double the typical aperture of 8 decades ago!

For the life of me, I cannot see how straining to glimpse a SN at magnitude 12-15 in a galaxy is more exciting than observing a regular nova of 8th magnitude? At best, the SN might be followed by the hobbyist at near the limits of his scope for a couple of weeks, while the nova can be watched fading for perhaps several months. especially one as conveniently positioned as Nv Sct '17!

As to comets, except for a virtual handful of true enthusiasts, just look at the number of poster sightings here unless the comet breaks 7th magnitude. Most folks here do not bother looking for such objects fainter than about 8th magnitude at all!. So that typically limits their nightly choice of these to no more than one and more often zero.

What worries my about the hobby is the general malaise and redirection that has come over the amateur community in the United Stats during the past 20 or so years. Rather than continuing the interest shown by their predecessor to observe at least occasionally in some meaningful way, they have increasingly reverted to the simple stargazing approach favored by those with crude, handmade instruments, of 3 or 4 generations ago. You, yourself point out that perhaps many folks currently aren't looking at Nv Sct '17 because its position is not to be found in their GoTo scope's memory bank. That is a very sad commentary indeed on the state of of hobbyists and their capabilities today.

Thankfully, outside the United States hobbyists continue to practice far more serious forms of observing, making the U.S. something of a backwater in hobbyist circles these days.

Edited by BrooksObs, 10 August 2017 - 09:07 AM.

### #22 DHEB

I agree with BrooksObs that CCD observations alone are far from enough to cover all variable stars for which it is important to keep systematic time series of many stars, like Miras, irregular and cataclysmic variables. I believe that visual estimations are still important for many of these stars and I see no reason why this type of observation will stop being important at least in the near future.

On the other hand, I also agree see that Smithrrlyr and Redbetter have a point when they mean that at least for some types of variability, like RR Lyrae, Delta Cephei or T Tauri stars, where either the magnitude range is small or the periodicity short, CCD observations do provide a better means than the human eye.

I only see possibilities here. The sky is a big place and astronomy is a wide field. There is lots of room for everyone's approach to them! Let's enjoy

Edited by cincosauces, 10 August 2017 - 01:05 PM.

### #23 BrooksObs

Yes, cincosauces, you are absolutely correct, CCD measurements are highly suited and desirable in the monitoring of small range, unusual & rapidly varying stars (particularly eclipsers and RR Lyraes), together with time series-critical observations of certain dwarf novae. Likewise, it is even more critical to maintain the visual monitoring of a host of others stars of very diverse types. It would take perhaps half a dozen times the number of CCD observers active today in variable star work to equal the current output of their visual counterparts and, as I've mentioned previously, CCD magnitudes are often not in good agreement with what human eyes see. and in some instances, way off. For many stars it is a record of their long-term behavior that is important, not the last year's ultra-precise CCD figures for them. All I can say is thank goodness for European, Far Eastern and other hobbyists outside the good old USA, because the torch has certainly been passed on to them to carry on in this important field.

### #24 Redbetter

Seeing a SN in another galaxy is more interesting to me because I also am frequently introduced to a new (to me) galaxy along the way, particularly for some of the 15+ magnitude hosts. The SN/galaxy provides a jumping off point for any galaxies or other DSO's in the neighborhood. Certainly the most recent NGC 6964 has been interesting, and within a visually complex galaxy. This SN is one I have been able to show at outreach. The SN has remained in reach of a 4" refractor since May. I have not been estimating magnitudes on it, but it is one that would have been a good target for tracking to see what its peak and decay looked like. Seeing individual stars in ever more distant galaxies (rather than just a few in the local group) is a treat.

It is no surprise to me that few variable star observers would be employing large apertures. It gets right back to the competition argument that aperture introduces. Regularly observe the same individual stars or examine a host of new targets each time? As the aperture increases so does the presentation of DSO's and the number of targets increases geometrically as well. Whole groups of galaxies emerge from what looked like a single galaxy before.

From my perspective, appreciating variables and novae requires tracking a number of them. They aren't the sort of things one can look at once or twice to appreciate. The need for some record keeping/logging already shifts them out of the quick surveys of many casual observers. If I was doing imaging, then I would be interested in variables and being able to contribute useful data.

Less time dependent is tracking some of the close double stars. These make for easy casual observation at a leisurely pace for follow up. I look through my newbie notes from over two decades ago and see the change in separation and angle of pairs like Porrima, Zeta Bootes, and even Sirius (which could not be separated visually by me back then.) The contrast of binary stars attracts my attention more than variables. It is something that can be appreciated in a single session.

Tracking the movement of dwarf planets, asteroids, and planetary satellites even holds more consistent appeal to me. Again, it is a matter of variables competing with other objects for observing time.

As for comets, there are always a few about. I was spoiled starting out with Hyakutake and Hale-Bopp. I don't keep close track of comets although I observe them down to about 12th magnitude at times. I will look at a few when well positioned. I have caught a number with binoculars or scope in town against twilight skies. Mostly comets are oversold (like the Perseid meteor shower every year) and under deliver, so it doesn't surprise me that folks ignore them until they break into the visual magnitude ranges. The light curves are guesses, so they are frequently much dimmer than projected at the time they are well positioned. That usually doesn't pose too much trouble for me, but many amateurs try to track such things from skies that are too bright to make it rewarding.

## Contents

Astronomy (from the Greek ἀστρονομία from ἄστρον astron, "star" and -νομία -nomia from νόμος nomos, "law" or "culture") means "law of the stars" (or "culture of the stars" depending on the translation). Astronomy should not be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects. [4] Although the two fields share a common origin, they are now entirely distinct. [5]

### Use of terms "astronomy" and "astrophysics"

"Astronomy" and "astrophysics" are synonyms. [6] [7] [8] Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties," [9] while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena". [10] In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject. [11] However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics. [6] Some fields, such as astrometry, are purely astronomy rather than also astrophysics. Various departments in which scientists carry out research on this subject may use "astronomy" and "astrophysics", partly depending on whether the department is historically affiliated with a physics department, [7] and many professional astronomers have physics rather than astronomy degrees. [8] Some titles of the leading scientific journals in this field include The Astronomical Journal, The Astrophysical Journal, and Astronomy & Astrophysics.

### Ancient times

In early historic times, astronomy only consisted of the observation and predictions of the motions of objects visible to the naked eye. In some locations, early cultures assembled massive artifacts that possibly had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops and in understanding the length of the year. [12]

Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye. As civilizations developed, most notably in Mesopotamia, Greece, Persia, India, China, Egypt, and Central America, astronomical observatories were assembled and ideas on the nature of the Universe began to develop. Most early astronomy consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the Universe were explored philosophically. The Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the Universe, or the Ptolemaic system, named after Ptolemy. [13]

A particularly important early development was the beginning of mathematical and scientific astronomy, which began among the Babylonians, who laid the foundations for the later astronomical traditions that developed in many other civilizations. [15] The Babylonians discovered that lunar eclipses recurred in a repeating cycle known as a saros. [16]

Following the Babylonians, significant advances in astronomy were made in ancient Greece and the Hellenistic world. Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena. [17] In the 3rd century BC, Aristarchus of Samos estimated the size and distance of the Moon and Sun, and he proposed a model of the Solar System where the Earth and planets rotated around the Sun, now called the heliocentric model. [18] In the 2nd century BC, Hipparchus discovered precession, calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the astrolabe. [19] Hipparchus also created a comprehensive catalog of 1020 stars, and most of the constellations of the northern hemisphere derive from Greek astronomy. [20] The Antikythera mechanism (c. 150–80 BC) was an early analog computer designed to calculate the location of the Sun, Moon, and planets for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical astronomical clocks appeared in Europe. [21]

### Middle Ages

Medieval Europe housed a number of important astronomers. Richard of Wallingford (1292–1336) made major contributions to astronomy and horology, including the invention of the first astronomical clock, the Rectangulus which allowed for the measurement of angles between planets and other astronomical bodies, as well as an equatorium called the Albion which could be used for astronomical calculations such as lunar, solar and planetary longitudes and could predict eclipses. Nicole Oresme (1320–1382) and Jean Buridan (1300–1361) first discussed evidence for the rotation of the Earth, furthermore, Buridan also developed the theory of impetus (predecessor of the modern scientific theory of inertia) which was able to show planets were capable of motion without the intervention of angels. [22] Georg von Peuerbach (1423–1461) and Regiomontanus (1436–1476) helped make astronomical progress instrumental to Copernicus's development of the heliocentric model decades later.

Astronomy flourished in the Islamic world and other parts of the world. This led to the emergence of the first astronomical observatories in the Muslim world by the early 9th century. [23] [24] [25] In 964, the Andromeda Galaxy, the largest galaxy in the Local Group, was described by the Persian Muslim astronomer Abd al-Rahman al-Sufi in his Book of Fixed Stars. [26] The SN 1006 supernova, the brightest apparent magnitude stellar event in recorded history, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and Chinese astronomers in 1006. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include Al-Battani, Thebit, Abd al-Rahman al-Sufi, Biruni, Abū Ishāq Ibrāhīm al-Zarqālī, Al-Birjandi, and the astronomers of the Maragheh and Samarkand observatories. Astronomers during that time introduced many Arabic names now used for individual stars. [27] [28]

It is also believed that the ruins at Great Zimbabwe and Timbuktu [29] may have housed astronomical observatories. [30] In Post-classical West Africa, Astronomers studied the movement of stars and relation to seasons, crafting charts of the heavens as well as precise diagrams of orbits of the other planets based on complex mathematical calculations. Songhai historian Mahmud Kati documented a meteor shower in August 1583. [31] [32] Europeans had previously believed that there had been no astronomical observation in sub-Saharan Africa during the pre-colonial Middle Ages, but modern discoveries show otherwise. [33] [34] [35] [36]

For over six centuries (from the recovery of ancient learning during the late Middle Ages into the Enlightenment), the Roman Catholic Church gave more financial and social support to the study of astronomy than probably all other institutions. Among the Church's motives was finding the date for Easter. [37]

### Scientific revolution

During the Renaissance, Nicolaus Copernicus proposed a heliocentric model of the solar system. His work was defended by Galileo Galilei and expanded upon by Johannes Kepler. Kepler was the first to devise a system that correctly described the details of the motion of the planets around the Sun. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. [38] It was Isaac Newton, with his invention of celestial dynamics and his law of gravitation, who finally explained the motions of the planets. Newton also developed the reflecting telescope. [39]

Improvements in the size and quality of the telescope led to further discoveries. The English astronomer John Flamsteed catalogued over 3000 stars, [40] More extensive star catalogues were produced by Nicolas Louis de Lacaille. The astronomer William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found. [41]

During the 18–19th centuries, the study of the three-body problem by Leonhard Euler, Alexis Claude Clairaut, and Jean le Rond d'Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Joseph-Louis Lagrange and Pierre Simon Laplace, allowing the masses of the planets and moons to be estimated from their perturbations. [42]

Significant advances in astronomy came about with the introduction of new technology, including the spectroscope and photography. Joseph von Fraunhofer discovered about 600 bands in the spectrum of the Sun in 1814–15, which, in 1859, Gustav Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to the Earth's own Sun, but with a wide range of temperatures, masses, and sizes. [27]

The existence of the Earth's galaxy, the Milky Way, as its own group of stars was only proved in the 20th century, along with the existence of "external" galaxies. The observed recession of those galaxies led to the discovery of the expansion of the Universe. [43] Theoretical astronomy led to speculations on the existence of objects such as black holes and neutron stars, which have been used to explain such observed phenomena as quasars, pulsars, blazars, and radio galaxies. Physical cosmology made huge advances during the 20th century. In the early 1900s the model of the Big Bang theory was formulated, heavily evidenced by cosmic microwave background radiation, Hubble's law, and the cosmological abundances of elements. Space telescopes have enabled measurements in parts of the electromagnetic spectrum normally blocked or blurred by the atmosphere. [ citation needed ] In February 2016, it was revealed that the LIGO project had detected evidence of gravitational waves in the previous September. [44] [45]

The main source of information about celestial bodies and other objects is visible light, or more generally electromagnetic radiation. [46] Observational astronomy may be categorized according to the corresponding region of the electromagnetic spectrum on which the observations are made. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or outside the Earth's atmosphere. Specific information on these subfields is given below.

Radio astronomy uses radiation with wavelengths greater than approximately one millimeter, outside the visible range. [47] Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves can be treated as waves rather than as discrete photons. Hence, it is relatively easier to measure both the amplitude and phase of radio waves, whereas this is not as easily done at shorter wavelengths. [47]

Although some radio waves are emitted directly by astronomical objects, a product of thermal emission, most of the radio emission that is observed is the result of synchrotron radiation, which is produced when electrons orbit magnetic fields. [47] Additionally, a number of spectral lines produced by interstellar gas, notably the hydrogen spectral line at 21 cm, are observable at radio wavelengths. [11] [47]

A wide variety of other objects are observable at radio wavelengths, including supernovae, interstellar gas, pulsars, and active galactic nuclei. [11] [47]

### Infrared astronomy

Infrared astronomy is founded on the detection and analysis of infrared radiation, wavelengths longer than red light and outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets, circumstellar disks or nebulae whose light is blocked by dust. The longer wavelengths of infrared can penetrate clouds of dust that block visible light, allowing the observation of young stars embedded in molecular clouds and the cores of galaxies. Observations from the Wide-field Infrared Survey Explorer (WISE) have been particularly effective at unveiling numerous Galactic protostars and their host star clusters. [49] [50] With the exception of infrared wavelengths close to visible light, such radiation is heavily absorbed by the atmosphere, or masked, as the atmosphere itself produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places on Earth or in space. [51] Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space more specifically it can detect water in comets. [52]

### Optical astronomy

Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy. [53] Images of observations were originally drawn by hand. In the late 19th century and most of the 20th century, images were made using photographic equipment. Modern images are made using digital detectors, particularly using charge-coupled devices (CCDs) and recorded on modern medium. Although visible light itself extends from approximately 4000 Å to 7000 Å (400 nm to 700 nm), [53] that same equipment can be used to observe some near-ultraviolet and near-infrared radiation.

### Ultraviolet astronomy

Ultraviolet astronomy employs ultraviolet wavelengths between approximately 100 and 3200 Å (10 to 320 nm). [47] Light at those wavelengths is absorbed by the Earth's atmosphere, requiring observations at these wavelengths to be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue stars (OB stars) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include planetary nebulae, supernova remnants, and active galactic nuclei. [47] However, as ultraviolet light is easily absorbed by interstellar dust, an adjustment of ultraviolet measurements is necessary. [47]

### X-ray astronomy

X-ray astronomy uses X-ray wavelengths. Typically, X-ray radiation is produced by synchrotron emission (the result of electrons orbiting magnetic field lines), thermal emission from thin gases above 10 7 (10 million) kelvins, and thermal emission from thick gases above 10 7 Kelvin. [47] Since X-rays are absorbed by the Earth's atmosphere, all X-ray observations must be performed from high-altitude balloons, rockets, or X-ray astronomy satellites. Notable X-ray sources include X-ray binaries, pulsars, supernova remnants, elliptical galaxies, clusters of galaxies, and active galactic nuclei. [47]

### Gamma-ray astronomy

Gamma ray astronomy observes astronomical objects at the shortest wavelengths of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the Compton Gamma Ray Observatory or by specialized telescopes called atmospheric Cherenkov telescopes. [47] The Cherenkov telescopes do not detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere. [54]

Most gamma-ray emitting sources are actually gamma-ray bursts, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars, and black hole candidates such as active galactic nuclei. [47]

### Fields not based on the electromagnetic spectrum

In addition to electromagnetic radiation, a few other events originating from great distances may be observed from the Earth.

In neutrino astronomy, astronomers use heavily shielded underground facilities such as SAGE, GALLEX, and Kamioka II/III for the detection of neutrinos. The vast majority of the neutrinos streaming through the Earth originate from the Sun, but 24 neutrinos were also detected from supernova 1987A. [47] Cosmic rays, which consist of very high energy particles (atomic nuclei) that can decay or be absorbed when they enter the Earth's atmosphere, result in a cascade of secondary particles which can be detected by current observatories. [55] Some future neutrino detectors may also be sensitive to the particles produced when cosmic rays hit the Earth's atmosphere. [47]

Gravitational-wave astronomy is an emerging field of astronomy that employs gravitational-wave detectors to collect observational data about distant massive objects. A few observatories have been constructed, such as the Laser Interferometer Gravitational Observatory LIGO. LIGO made its first detection on 14 September 2015, observing gravitational waves from a binary black hole. [56] A second gravitational wave was detected on 26 December 2015 and additional observations should continue but gravitational waves require extremely sensitive instruments. [57] [58]

The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known as multi-messenger astronomy. [59] [60]

### Astrometry and celestial mechanics

One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation (the use of celestial objects to guide navigation) and in the making of calendars.

Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations, and an ability to determine past and future positions of the planets with great accuracy, a field known as celestial mechanics. More recently the tracking of near-Earth objects will allow for predictions of close encounters or potential collisions of the Earth with those objects. [61]

The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the Universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, as their properties can be compared. Measurements of the radial velocity and proper motion of stars allow astronomers to plot the movement of these systems through the Milky Way galaxy. Astrometric results are the basis used to calculate the distribution of speculated dark matter in the galaxy. [62]

During the 1990s, the measurement of the stellar wobble of nearby stars was used to detect large extrasolar planets orbiting those stars. [63]

Theoretical astronomers use several tools including analytical models and computational numerical simulations each has its particular advantages. Analytical models of a process are better for giving broader insight into the heart of what is going on. Numerical models reveal the existence of phenomena and effects otherwise unobserved. [64] [65]

Theorists in astronomy endeavor to create theoretical models and from the results predict observational consequences of those models. The observation of a phenomenon predicted by a model allows astronomers to select between several alternate or conflicting models as the one best able to describe the phenomena.

Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency between the data and the model's results, the general tendency is to try to make minimal modifications to the model so that it produces results that fit the data. In some cases, a large amount of inconsistent data over time may lead to the total abandonment of a model.

Phenomena modeled by theoretical astronomers include: stellar dynamics and evolution galaxy formation large-scale distribution of matter in the Universe origin of cosmic rays general relativity and physical cosmology, including string cosmology and astroparticle physics. Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for black hole (astro)physics and the study of gravitational waves.

Some widely accepted and studied theories and models in astronomy, now included in the Lambda-CDM model are the Big Bang, dark matter and fundamental theories of physics.

A few examples of this process:

 Physical process Experimental tool Theoretical model Explains/predicts Gravitation Radio telescopes Self-gravitating system Emergence of a star system Nuclear fusion Spectroscopy Stellar evolution How the stars shine and how metals formed The Big Bang Hubble Space Telescope, COBE Expanding universe Age of the Universe Quantum fluctuations Cosmic inflation Flatness problem Gravitational collapse X-ray astronomy General relativity Black holes at the center of Andromeda Galaxy CNO cycle in stars The dominant source of energy for massive star.

Along with Cosmic inflation, dark matter and dark energy are the current leading topics in astronomy, [66] as their discovery and controversy originated during the study of the galaxies.

### Astrophysics

Astrophysics is the branch of astronomy that employs the principles of physics and chemistry "to ascertain the nature of the astronomical objects, rather than their positions or motions in space". [67] [68] Among the objects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and the cosmic microwave background. [69] [70] Their emissions are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density, temperature, and chemical composition. Because astrophysics is a very broad subject, astrophysicists typically apply many disciplines of physics, including mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.

In practice, modern astronomical research often involves a substantial amount of work in the realms of theoretical and observational physics. Some areas of study for astrophysicists include their attempts to determine the properties of dark matter, dark energy, and black holes whether or not time travel is possible, wormholes can form, or the multiverse exists and the origin and ultimate fate of the universe. [69] Topics also studied by theoretical astrophysicists include Solar System formation and evolution stellar dynamics and evolution galaxy formation and evolution magnetohydrodynamics large-scale structure of matter in the universe origin of cosmic rays general relativity and physical cosmology, including string cosmology and astroparticle physics.

### Astrochemistry

Astrochemistry is the study of the abundance and reactions of molecules in the Universe, and their interaction with radiation. [71] The discipline is an overlap of astronomy and chemistry. The word "astrochemistry" may be applied to both the Solar System and the interstellar medium. The study of the abundance of elements and isotope ratios in Solar System objects, such as meteorites, is also called cosmochemistry, while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of molecular gas clouds is of special interest, because it is from these clouds that solar systems form.

Studies in this field contribute to the understanding of the formation of the Solar System, Earth's origin and geology, abiogenesis, and the origin of climate and oceans.

### Astrobiology

Astrobiology is an interdisciplinary scientific field concerned with the origins, early evolution, distribution, and future of life in the universe. Astrobiology considers the question of whether extraterrestrial life exists, and how humans can detect it if it does. [72] The term exobiology is similar. [73]

Astrobiology makes use of molecular biology, biophysics, biochemistry, chemistry, astronomy, physical cosmology, exoplanetology and geology to investigate the possibility of life on other worlds and help recognize biospheres that might be different from that on Earth. [74] The origin and early evolution of life is an inseparable part of the discipline of astrobiology. [75] Astrobiology concerns itself with interpretation of existing scientific data, and although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories.

This interdisciplinary field encompasses research on the origin of planetary systems, origins of organic compounds in space, rock-water-carbon interactions, abiogenesis on Earth, planetary habitability, research on biosignatures for life detection, and studies on the potential for life to adapt to challenges on Earth and in outer space. [76] [77] [78]

### Physical cosmology

Cosmology (from the Greek κόσμος (kosmos) "world, universe" and λόγος (logos) "word, study" or literally "logic") could be considered the study of the Universe as a whole.

Observations of the large-scale structure of the Universe, a branch known as physical cosmology, have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the Big Bang, wherein our Universe began at a single point in time, and thereafter expanded over the course of 13.8 billion years [79] to its present condition. [80] The concept of the Big Bang can be traced back to the discovery of the microwave background radiation in 1965. [80]

In the course of this expansion, the Universe underwent several evolutionary stages. In the very early moments, it is theorized that the Universe experienced a very rapid cosmic inflation, which homogenized the starting conditions. Thereafter, nucleosynthesis produced the elemental abundance of the early Universe. [80] (See also nucleocosmochronology.)

When the first neutral atoms formed from a sea of primordial ions, space became transparent to radiation, releasing the energy viewed today as the microwave background radiation. The expanding Universe then underwent a Dark Age due to the lack of stellar energy sources. [81]

A hierarchical structure of matter began to form from minute variations in the mass density of space. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars, the Population III stars. These massive stars triggered the reionization process and are believed to have created many of the heavy elements in the early Universe, which, through nuclear decay, create lighter elements, allowing the cycle of nucleosynthesis to continue longer. [82]

Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually, organizations of gas and dust merged to form the first primitive galaxies. Over time, these pulled in more matter, and were often organized into groups and clusters of galaxies, then into larger-scale superclusters. [83]

Various fields of physics are crucial to studying the universe. Interdisciplinary studies involve the fields of quantum mechanics, particle physics, plasma physics, condensed matter physics, statistical mechanics, optics, and nuclear physics.

Fundamental to the structure of the Universe is the existence of dark matter and dark energy. These are now thought to be its dominant components, forming 96% of the mass of the Universe. For this reason, much effort is expended in trying to understand the physics of these components. [84]

### Extragalactic astronomy

The study of objects outside our galaxy is a branch of astronomy concerned with the formation and evolution of Galaxies, their morphology (description) and classification, the observation of active galaxies, and at a larger scale, the groups and clusters of galaxies. Finally, the latter is important for the understanding of the large-scale structure of the cosmos.

Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into spiral, elliptical and Irregular galaxies. [85]

As the name suggests, an elliptical galaxy has the cross-sectional shape of an ellipse. The stars move along random orbits with no preferred direction. These galaxies contain little or no interstellar dust, few star-forming regions, and older stars. Elliptical galaxies are more commonly found at the core of galactic clusters, and may have been formed through mergers of large galaxies.

A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation within which massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the Milky Way and one of our nearest galaxy neighbors, the Andromeda Galaxy, are spiral galaxies.

Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.

An active galaxy is a formation that emits a significant amount of its energy from a source other than its stars, dust and gas. It is powered by a compact region at the core, thought to be a supermassive black hole that is emitting radiation from in-falling material.

A radio galaxy is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit shorter frequency, high-energy radiation include Seyfert galaxies, Quasars, and Blazars. Quasars are believed to be the most consistently luminous objects in the known universe. [86]

The large-scale structure of the cosmos is represented by groups and clusters of galaxies. This structure is organized into a hierarchy of groupings, with the largest being the superclusters. The collective matter is formed into filaments and walls, leaving large voids between. [87]

### Galactic astronomy

The Solar System orbits within the Milky Way, a barred spiral galaxy that is a prominent member of the Local Group of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.

In the center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a supermassive black hole at its center. This is surrounded by four primary arms that spiral from the core. This is a region of active star formation that contains many younger, population I stars. The disk is surrounded by a spheroid halo of older, population II stars, as well as relatively dense concentrations of stars known as globular clusters. [88]

Between the stars lies the interstellar medium, a region of sparse matter. In the densest regions, molecular clouds of molecular hydrogen and other elements create star-forming regions. These begin as a compact pre-stellar core or dark nebulae, which concentrate and collapse (in volumes determined by the Jeans length) to form compact protostars. [89]

As the more massive stars appear, they transform the cloud into an H II region (ionized atomic hydrogen) of glowing gas and plasma. The stellar wind and supernova explosions from these stars eventually cause the cloud to disperse, often leaving behind one or more young open clusters of stars. These clusters gradually disperse, and the stars join the population of the Milky Way. [90]

Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A dark matter halo appears to dominate the mass, although the nature of this dark matter remains undetermined. [91]

### Stellar astronomy

The study of stars and stellar evolution is fundamental to our understanding of the Universe. The astrophysics of stars has been determined through observation and theoretical understanding and from computer simulations of the interior. [92] Star formation occurs in dense regions of dust and gas, known as giant molecular clouds. When destabilized, cloud fragments can collapse under the influence of gravity, to form a protostar. A sufficiently dense, and hot, core region will trigger nuclear fusion, thus creating a main-sequence star. [89]

Almost all elements heavier than hydrogen and helium were created inside the cores of stars. [92]

The characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it fuses its hydrogen fuel into helium in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to evolve. The fusion of helium requires a higher core temperature. A star with a high enough core temperature will push its outer layers outward while increasing its core density. The resulting red giant formed by the expanding outer layers enjoys a brief life span, before the helium fuel in the core is in turn consumed. Very massive stars can also undergo a series of evolutionary phases, as they fuse increasingly heavier elements. [93]

The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse supernovae [94] while smaller stars blow off their outer layers and leave behind the inert core in the form of a white dwarf. The ejection of the outer layers forms a planetary nebula. [95] The remnant of a supernova is a dense neutron star, or, if the stellar mass was at least three times that of the Sun, a black hole. [96] Closely orbiting binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova. [97] Planetary nebulae and supernovae distribute the "metals" produced in the star by fusion to the interstellar medium without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone. [98]

### Solar astronomy

At a distance of about eight light-minutes, the most frequently studied star is the Sun, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 billion years (Gyr) old. The Sun is not considered a variable star, but it does undergo periodic changes in activity known as the sunspot cycle. This is an 11-year oscillation in sunspot number. Sunspots are regions of lower-than- average temperatures that are associated with intense magnetic activity. [99]

The Sun has steadily increased in luminosity by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth. [100] The Maunder minimum, for example, is believed to have caused the Little Ice Age phenomenon during the Middle Ages. [101]

The visible outer surface of the Sun is called the photosphere. Above this layer is a thin region known as the chromosphere. This is surrounded by a transition region of rapidly increasing temperatures, and finally by the super-heated corona.

At the center of the Sun is the core region, a volume of sufficient temperature and pressure for nuclear fusion to occur. Above the core is the radiation zone, where the plasma conveys the energy flux by means of radiation. Above that is the convection zone where the gas material transports energy primarily through physical displacement of the gas known as convection. It is believed that the movement of mass within the convection zone creates the magnetic activity that generates sunspots. [99]

A solar wind of plasma particles constantly streams outward from the Sun until, at the outermost limit of the Solar System, it reaches the heliopause. As the solar wind passes the Earth, it interacts with the Earth's magnetic field (magnetosphere) and deflects the solar wind, but traps some creating the Van Allen radiation belts that envelop the Earth. The aurora are created when solar wind particles are guided by the magnetic flux lines into the Earth's polar regions where the lines then descend into the atmosphere. [102]

### Planetary science

Planetary science is the study of the assemblage of planets, moons, dwarf planets, comets, asteroids, and other bodies orbiting the Sun, as well as extrasolar planets. The Solar System has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of the Sun's planetary system, although many new discoveries are still being made. [103]

The Solar System is divided into the inner Solar System (subdivided into the inner planets and the asteroid belt), the outer Solar System (subdivided into the outer planets and centaurs), comets, the trans-Neptunian region (subdivided into the Kuiper belt, and the scattered disc) and the farthest regions (e.g., boundaries of the heliosphere, and the Oort Cloud, which may extend as far as a light-year). The inner terrestrial planets consist of Mercury, Venus, Earth, and Mars. The outer giant planets are the gas giants (Jupiter and Saturn) and the ice giants (Uranus and Neptune). [104]

The planets were formed 4.6 billion years ago in the protoplanetary disk that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that, with time, became protoplanets. The radiation pressure of the solar wind then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up, or eject, the remaining matter during a period of intense bombardment, evidenced by the many impact craters on the Moon. During this period, some of the protoplanets may have collided and one such collision may have formed the Moon. [105]

Once a planet reaches sufficient mass, the materials of different densities segregate within, during planetary differentiation. This process can form a stony or metallic core, surrounded by a mantle and an outer crust. The core may include solid and liquid regions, and some planetary cores generate their own magnetic field, which can protect their atmospheres from solar wind stripping. [106]

A planet or moon's interior heat is produced from the collisions that created the body, by the decay of radioactive materials (e.g. uranium, thorium, and 26 Al), or tidal heating caused by interactions with other bodies. Some planets and moons accumulate enough heat to drive geologic processes such as volcanism and tectonics. Those that accumulate or retain an atmosphere can also undergo surface erosion from wind or water. Smaller bodies, without tidal heating, cool more quickly and their geological activity ceases with the exception of impact cratering. [107]

Astronomy and astrophysics have developed significant interdisciplinary links with other major scientific fields. Archaeoastronomy is the study of ancient or traditional astronomies in their cultural context, utilizing archaeological and anthropological evidence. Astrobiology is the study of the advent and evolution of biological systems in the Universe, with particular emphasis on the possibility of non-terrestrial life. Astrostatistics is the application of statistics to astrophysics to the analysis of a vast amount of observational astrophysical data.

The study of chemicals found in space, including their formation, interaction and destruction, is called astrochemistry. These substances are usually found in molecular clouds, although they may also appear in low-temperature stars, brown dwarfs and planets. Cosmochemistry is the study of the chemicals found within the Solar System, including the origins of the elements and variations in the isotope ratios. Both of these fields represent an overlap of the disciplines of astronomy and chemistry. As "forensic astronomy", finally, methods from astronomy have been used to solve problems of law and history.

Astronomy is one of the sciences to which amateurs can contribute the most. [108]

Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with equipment that they build themselves. Common targets of amateur astronomers include the Sun, the Moon, planets, stars, comets, meteor showers, and a variety of deep-sky objects such as star clusters, galaxies, and nebulae. Astronomy clubs are located throughout the world and many have programs to help their members set up and complete observational programs including those to observe all the objects in the Messier (110 objects) or Herschel 400 catalogues of points of interest in the night sky. One branch of amateur astronomy, amateur astrophotography, involves the taking of photos of the night sky. Many amateurs like to specialize in the observation of particular objects, types of objects, or types of events that interest them. [109] [110]

Most amateurs work at visible wavelengths, but a small minority experiment with wavelengths outside the visible spectrum. This includes the use of infrared filters on conventional telescopes, and also the use of radio telescopes. The pioneer of amateur radio astronomy was Karl Jansky, who started observing the sky at radio wavelengths in the 1930s. A number of amateur astronomers use either homemade telescopes or use radio telescopes which were originally built for astronomy research but which are now available to amateurs (e.g. the One-Mile Telescope). [111] [112]

Amateur astronomers continue to make scientific contributions to the field of astronomy and it is one of the few scientific disciplines where amateurs can still make significant contributions. Amateurs can make occultation measurements that are used to refine the orbits of minor planets. They can also discover comets, and perform regular observations of variable stars. Improvements in digital technology have allowed amateurs to make impressive advances in the field of astrophotography. [113] [114] [115]

Although the scientific discipline of astronomy has made tremendous strides in understanding the nature of the Universe and its contents, there remain some important unanswered questions. Answers to these may require the construction of new ground- and space-based instruments, and possibly new developments in theoretical and experimental physics.

• What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses—the initial mass function—apparently regardless of the initial conditions? [116] A deeper understanding of the formation of stars and planets is needed.
• Is there other life in the Universe? Especially, is there other intelligent life? If so, what is the explanation for the Fermi paradox? The existence of life elsewhere has important scientific and philosophical implications. [117][118] Is the Solar System normal or atypical?
• What is the nature of dark matter and dark energy? These dominate the evolution and fate of the cosmos, yet their true nature remains unknown. [119]
• What will be the ultimate fate of the universe? [120]
• How did the first galaxies form? [121] How did supermassive black holes form? [122]
• What is creating the ultra-high-energy cosmic rays? [123]
• Why is the abundance of lithium in the cosmos four times lower than predicted by the standard Big Bang model? [124]
• What really happens beyond the event horizon? [125]
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