# What do the fusing 'onion layers' of a pre-supernova star look like to scale?

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I'm sure we've all seen the diagrams of various layers of element fusion from hydrogen to silicon in a star that's just about to go supernova.

(Picture from courses.lumenlearning.com)

I suspect these pictures grossly exaggerate the relative radii at which these fusing layers occur for the sake of readability. What would a more accurate to-scale picture of these fusing layers look like?

Now, I assume the answer will vary significantly based on the mass of the star, and I suspect in certain mass regimes some layers will not be fusing at all. I can think of a few other variables that may affect the answer too.

So in CYA fashion, I'll leave it up to the answerer to identify specific illustrative or interesting cases, as I'm not after a specific answer but rather a general feel for how large some of the layers are in comparison to the others.

Pre- supernova models often characterise the compactness of the core using a "compactness parameter" defined as $$upsilon = frac{(M/M_{odot})}{R(M)/1000 { m km}},$$ where $$M$$ is usually chosen to be $$2.5M_{odot}$$ and $$R(M)$$ is the radius within which $$M$$ is contained.

Pre supernova models by Farmer et al. (2016) show that the central $$2.5M_{odot}$$ of a massive star included the Carbon-burning core in a $$15M_{odot}$$ (initial mass) star, but only contained the oxygen-burning core in a more massive star (models are presented up to 30$$M_{odot}$$. i.e. Your answer is going to be mass and composition dependent (these models are for a solar metallicity initial composition).

In the $$15M_{odot}$$ model (with mass-loss), $$upsilon sim 0.08$$ at core collapse, which means the carbon burning core would have been contained within 31,000 km. The $$30M_{odot}$$ model is more compact with $$upsilon= 0.58$$, indicating that the oxygen-burning core would have been contained within 4,300 km.

At collapse the iron core might have a mass of 1.4-1.8$$M_{odot}$$, will be supported by electron degeneracy pressure and should have a size a bit smaller than a typical carbon white dwarf (radius of a few thousand km).

You can compare these sizes with the size of the entire red supergiant star, which might have a radius of a few au (e.g. Betelgeuse).

The shell-burning regions would be found out to slightly larger radii than this I think, but these numbers are a reasonable estimate. Looking in detail at the models I don't think that the final stages of pre- supernova evolution look anything like the strictly stratified onion-shell picture that is seen all over the internet.

Rob jeffies gave the results for what the core looks like. But for completeness that hydrogen shell envelope is big, very big. Its of order 1000 times the radius of the Sun. Or in other words if it replaced the Sun it would extend out to about Jupiter.

Answer seems to be : In Red Super Giats the star can have 300 - 1000 the diameter of the sun and its core where nucleosynthesis occurs is only the diameter of the earth. This would explain why after 13.8 Billion years still 98% of the universe is made of hydrogen and helium

See image on page 42 of this slide show https://slideplayer.com/slide/13959320/

## What do the fusing 'onion layers' of a pre-supernova star look like to scale? - Astronomy

Currently, the most popular theory states that the nuclei of hydrogen and helium, the lightest and most abundant elements in the visible universe, were created in the moments following the Big Bang. All other naturally occurring elements were — and continue to be — generated in the high temperature and pressure conditions present in stars. Elements are composed of tiny particles called atoms that are indivisible under normal conditions. However, when exposed to high heat and pressure, atoms can either break apart or fuse together. Under these conditions, the nucleus of one element can fuse with the nucleus of a different element, creating the nucleus of a heavier element. When elements lighter than iron form, the mass of the new nucleus is less than the combined mass of the two original nuclei. The difference in mass between the two is released as energy. In stars, this kind of reaction is referred to as stellar nucleosynthesis, but it is more commonly known as nuclear fusion. Nuclear fusion is used today on Earth in the nuclear explosives called hydrogen bombs. Many people hope that one day nuclear fusion will be used for peaceful energy production.

Stars are fueled by nuclear fusion reactions, which take place in their deep interiors, or cores. Hydrogen nuclei fuse, forming helium nuclei. The energy produced by these fusion reactions prevents the star from collapsing under its own gravity. Mature stars contain enough hydrogen nuclei to last billions of years. When a star's hydrogen fuel supply is spent, however, its core begins to contract. The contraction is so intense that it creates conditions under which helium nuclei fuse. In this way, helium becomes the star's next fuel source. The fusion of helium nuclei produces carbon and oxygen nuclei, and in the process sufficient energy is released to temporarily sustain the star.

Once helium runs out, the nuclei of carbon, oxygen, and other elements begin to fuse. These new fuel sources are depleted at faster and faster rates. Since the heaviest element created in a star by nuclear fusion reactions is iron, a large iron core eventually forms at the center of everything. At this point, gravity becomes overwhelming, the core collapses, and an explosion occurs, during which outer layers of gas and heavy elements are ejected to space. Such explosions, called supernovas, occur about once a century in our galaxy. The energy created by supernovas produces nuclei heavier than iron. This process is known as supernova nucleosynthesis.

1. Where are elements created? How are heavier elements created from hydrogen and helium?

2. What evidence do scientists use to tell which elements are released when a supernova explodes?

3. How do the elements in the atmosphere of a supernova help scientists identify types of supernovas?

All matter is made up of atoms -- elements comprised of smaller particles such as protons, neutrons, and electrons. The number of protons within the nucleus -- the central component of the atom -- determines the type of element. An element can have different forms, called isotopes, based on the number of neutrons in the nucleus. For example, an ordinary hydrogen nucleus contains just one proton. But deuterium, an isotope of hydrogen, has one proton and one neutron in its nucleus.

The entire universe shares a common set of elements. In the very early universe, the only elements were hydrogen and helium. But since the formation of stars, lighter elements within the stars began fusing to create heavier elements, producing all the other naturally occurring elements. Under the extremely high temperatures and pressures within the core of stars, atoms collide at high enough speeds to overcome the usual electromagnetic repulsion of nuclei, allowing nuclear fusion to occur.

All stars live by fusing hydrogen into helium. In the first step of the process, two hydrogen atoms fuse to form deuterium. In the next step, another hydrogen atom fuses with the deuterium, creating a rare isotope of helium that has two protons and one neutron in its nucleus. In the third step, two of the rare helium atoms fuse to create a single normal helium atom and two hydrogen atoms. The fusion pathway described above requires six hydrogen atoms to create one helium atom -- however, there are two hydrogen atoms left over at the end of the process. The net result is that it takes four hydrogen atoms to make one helium atom. The energy that fuels a star is a result of the difference in mass between the original four hydrogen atoms and the resulting helium atom. Following Einstein's mass-energy relationship, E=mc2, the missing mass is converted to energy.

At even higher temperatures and pressures, heavier elements are able to form. Many are made from a process called "helium capture," in which a heavier element fuses with a helium atom. For example, helium fuses with carbon to make oxygen, and helium fuses with oxygen to make neon. Heavier nuclei can also fuse with each other, such as when carbon and oxygen fuse to make silicon or two silicon atoms fuse to make iron. Eventually, the interior of a massive star begins to resemble an onion, with different elements being created in different layers. However, elements heavier than iron are only produced in the extraordinary conditions created by the collapse and explosion of a star -- a supernova.

Now, watch a short movie on how elements are forged in stars and answer the following questions:

1. How does a star get its energy to glow?

2. What elements make up young stars?

3. What causes a star to become a supernova?

4. Why do you think it takes a tremendous amount of heat and pressure to create helium (and then carbon, etc.)?

5. What could you infer about the age of a star if you were to find evidence of iron being present?

## NuSTAR Sheds New Light on Supernova Explosions

This is the first map of radioactivity in a supernova remnant, the blown-out bits and pieces of a massive star that exploded. The blue color shows radioactive material mapped in high-energy X-rays using NuSTAR. Heated, non-radioactive elements previously imaged by Chandra using low-energy X-rays are shown in red, yellow, and green. Image Credit/Caption: NASA/JPL-Caltech/CXC/SAO

New and exciting observations from NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, hint at a possible solution to one of the most intriguing mysteries of modern astrophysics: What is the exact mechanism that drives supernova explosions?

Supernova explosions are among the most energetic phenomena in the Universe, signifying the death of big, supermassive stars. During the explosion, most of the star’s mass is violently ejected into interstellar space, seeding the interstellar medium with all the heavy elements that were created inside the star’s core, allowing for the creation of a newer generation of stars and planetary systems. What is left behind after the explosion is the former star’s core, having a mass several times that of the Sun compacted into an area no bigger than San Francisco. The ejected inner layers of the star create an expanding shock wave of material that collides with and sweeps through the interstellar medium, creating structures called supernova remnants, or SNRs, that can span many light-years across interstellar space.

The two main causes of supernova explosions are the accretion of matter around white dwarfs in binary star systems and the core collapse of a very massive star. In the latter case, when the star’s core runs out of fuel and can no longer sustain nuclear fusion reactions at its center, the star collapses onto itself from its own gravity. During the collapse, the star’s inner layers bounce off the now burnt-out core, triggering such a violent and energetic explosion that during its peak it can outshine an entire galaxy.

This chart depicts the electromagnetic spectrum, highlighting the X-ray portion. Chandra sees X-rays with energies between 0.1 and 10 kiloelectron volts (keV), the “red” part of the spectrum, while NuSTAR sees the highest-energy, or “bluest,” X-ray light, with energies between 3 and 70 keV. Image Credit/Caption: NASA/JPL-Caltech

Supernova explosions and their remnants have been studied extensively within the Milky Way and other galaxies by many different ground- and space-based observatories in multiple wavelengths during the last several decades. NASA’s Chandra X-ray Observatory in particular has provided astronomers with very detailed views of SNRs in the high-energy part of the electromagnetic spectrum, helping to reveal their elemental composition. Yet the exact processes that drive these cataclysmic cosmic events have for the most part remained unknown. Astronomers always knew that supernovae explode, but they were uncertain as to how. The reason is that previous X-ray observatories like Chandra and XMM-Newton didn’t have the capability to observe in the energies needed to peer into the cores of these explosions and unravel their hidden mechanisms. Astronomers were in need of a space telescope able to observe the high-energy X-rays that were invisible to previous observatories.

Their needs were met with the launch of NASA’s NuSTAR X-ray telescope in June 2012. Whereas Chandra and XMM-Newton can observe at the low-energy part of X-ray wavelengths between 0.1 and 10 KeV, NuSTAR can see all the way from 3 to 79 KeV, at the high-energy part of X-rays. This superior observing capability allows NuSTAR to study high-energy physics phenomena in the Universe like never before, around supernovae, black holes, active galactic nuclei, and other exotic celestial objects.

During a media teleconference that was hosted by NASA on February 19, the NuSTAR science team detailed the fascinating observations made by the X-ray observatory of Cassiopeia A, a supernova remnant inside our Milky Way galaxy, located approximately 11,000 light-years away in the constellation Cassiopeia. As per science publishing guidelines, the team’s study was published the same day in the journal Nature.

“The results we’re unveiling today are the first-ever map of radioactive material in the remnants of a star that exploded in an incredibly powerful event, called a supernova,” said a jubilant Fiona Harisson, principal investigator for NuSTAR. “This is helping us to untangle the mysteries of how stars explode and in particular what’s happening at the very heart of the explosion. No other telescope could make this map, because previous imaging telescopes don’t go to high-enough energy.”

NuSTAR, has, for the first time, imaged the radioactive “guts” of a supernova remnant, the leftover remains of a star that exploded. The NuSTAR data are blue, and show high-energy X-rays. Yellow shows non-radioactive material detected previously by NASA’s Chandra X-ray Observatory in low-energy X-rays. Image Credit/Caption: NASA/JPL-Caltech/CXC/SAO

NuSTAR was able to map the distribution of the radioactive isotope Titanium-44 at the heart of Cassiopeia A by detecting the element’s X-ray emission. Titanium-44 is a highly unstable isotope that decays into calcium-44, while emitting very-high energy X-rays in the process. Its presence was invisible to previous observations by other X-ray telescopes because it only glows in the high-energy X-ray wavelengths that NuSTAR can observe.

The progenitor stars of supernova explosions that are caused by core collapse develop a layered internal structure throughout their lifetime that resembles that of an onion. These stars start their lives by fusing hydrogen into helium, deep in their cores. When the hydrogen is depleted they continue to burn the helium that has been created previously, converting it to carbon, which in turn is converted to neon and so forth, with the star’s core fusion process progressing up through the periodic table of elements, producing progressively heavier atomic nuclei all the way to iron. When the star reaches this stage, it can no longer sustain any fusion reactions and it collapses under its own weight, resulting in a supernova explosion. Although Cassiopeia A most probably exploded sometime between 1667 and 1680, with the supernova remnants expanding away ever since, NuSTAR’s observations have for the first time revealed to astronomers the presence of the actual elements that have been created during the time of the supernova explosion itself. “With NuSTAR we have new forensic tools to investigate the ashes left behind when the star exploded,” said Caltech astronomer Brian Grefenstette and member of the NuSTAR team, during the teleconference. “When it starts to explode, it leaves behind clues that we can use to figure out what happens when the star collapses and produces the elements that make up us. Now, one of the clues is the image of radioactive titanium, that we see with NuSTAR, which can tell us what was going on down at the guts of the explosion.”

The pattern of radioactive titanium observed by NuSTAR (right) does not match the pattern of heated iron seen by NASA’s Chandra X-ray Observatory (left). Image Credit/Caption: NASA/JPL-Caltech/CXC/SAO

An unexpected result to come out of NuSTAR’s observations is that the distribution of titanium-44 doesn’t match the distribution of iron that had been detected inside Cassiopeia A from previous observations with Chandra. Researchers had hoped that since both iron and titanium are created together at the time, their distributions inside the supernova remnant would match exactly. Yet that weren’t the case. “We were surprised when we looked at the images and found that the two maps obviously didn’t match,” said Grefenstette. “This means that either we were wrong and that there’s iron that’s hidden in the center of Cassiopeia A that is unseen by Chandra because the material is too cool, or something more exotic is going on, that is changing how the elements were formed in the supernova explosion. Either way, it’s a new puzzle for us to solve.”

What wasn’t so unexpected was that the clumps of radioactive material observed by NuSTAR weren’t elongated toward a certain direction at the sky. Previous models have suggested that a supernova explosion might be driven by elongated jets of material, streaming out from a rapidly rotating star. Yet many of these models have frustratingly failed to produce any supernova explosions in previous computer simulations. There was something really missing in astrophysicists’ understanding of how supernovae really work. NuSTAR’s observations of radioactive titanium-44 inside Cassiopeia A didn’t reveal the presence of any elongated jets. Instead, they indicated that the most possible solution to the puzzle is that prior to the explosion the star’s material near the core is sloshing around like boiling water inside a pressure cooker. And just like steam blowing the top of a pressure cooker, this sloshing motion inside the star creates a shockwave that rips it apart. “Stars are spherical balls of gas, and so you might think that when they end their lives and explode, that explosion would look like a uniform ball expanding out with great power,” said Harrison. “Our new results show how the explosion’s heart, or engine, is distorted, possibly because the inner regions literally slosh around before detonating.”

Video Credit: NASA/JPL-Caltech/Christian Ott

“Now, all this sloshing mechanism has been simulated in a computer before, but the NuSTAR image is the first evidence that this kind of explosion is occurring in nature, and [it means] that we may be on the right track to try to understand how massive stars explode,” added Grefenstette.

Robert Kirshner, a professor of Science at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., although not directly involved with the NuSTAR’s team study, nevertheless provided valuable insight during the teleconference. “NuSTAR is living up to its name in two ways: this is not just nuclear, but it’s new,” remarked Dr. Kirshner. “It is pioneering science and you have to expect, that when you get new results, it’ll open up as many questions as you answer. Now, you should care about this. Supernovae make the chemical elements. So, if you bought an American car, it wasn’t made in Detroit 2 years ago. The iron atoms in that steel, were manufactured in an ancient supernova explosion that took place 5 billion years ago. And NuSTAR shows that the titanium that’s in your uncle Jack’s replacement hip, were made in those explosions too. So, we’re all star dust and NuSTAR is showing as where we came from, including our replacement parts. So, you should care about this, and so should your uncle Jack.”

During the Q&A session of the conference, I had the chance to ask Dr. Kirshner whether supernova explosions were possible during the very early Universe, approximately 15 million years after the Big Bang, that could have led to the creation of the first generation of habitable planets capable of supporting life, as argued in a recent study by Professor Abraham Loeb, also of the Harvard-Smithsonian Center for Astrophysics.

A full-scale model of the James Webb Space Telescope at the Goddard Space Flight Center, together with team that has worked on it. Image Credit: NASA

Dr. Kirshner’s answer was as inspiring as it was thought-provoking: “That’s a very interesting question. You know, the human mind is a fallible thing and people’s imagination usually turns out to be less strange than the real world. So, even when something sounds a little strange, nevertheless you shouldn’t be too quick to dismiss it. I don’t think that there’s any question that there will be massive stars that form out of the primordial soup of hydrogen and helium—and we know that something like that must have happened—but this is mostly in the realm if the unobserved. So, if I would have my dream, I would build a really big infrared telescope in space!,” stressed Kirshner, with the excitement evident in his voice. “And of course, that is exactly what we’re doing! We’re building the James Webb Space Telescope, and it will have a pretty good chance I would say, of seeing this first generation of stars—the ones that made the first elements. And we’re gonna do it! Before too long, in a few years, we’re gonna have that JWST, we’re gonna see these stars exploding, we will be able to measure their chemistry, or estimate their properties anyway. I’m very hopeful that we’re gonna go beyond our current knowledge.”

The NuSTAR team plans to further study other supernova remnants as well to try and see whether what was observed in Cassiopeia A is typical of supernova explosions in general. With NuSTAR, astronomers now have for the first time the capability to check theoretical predictions for how exactly massive stars end their lives and how the material from these explosions enriches the interstellar medium with the stuff of life. “Supernovae produce and eject into the cosmos most of the elements are important to life as we know it,” says Dr. Alex Filippenko, Professor of astronomy at the University of California, Berkeley. “These results are exciting because for the first time we are getting information about the innards of these explosions, where the elements are actually produced.”

With all the fascinating discoveries coming from astrophysics observatories currently in orbit like NuSTAR, and from the ones that are scheduled to be launched during the next several years, like the James Webb Space Telescope, we can look forward to a deeply transformational decade in our scientific and human understanding of our place in the Cosmos.

Want to keep up-to-date with all things space? Be sure to “Like” AmericaSpace on Facebook and follow us on Twitter: @AmericaSpace

## What do the fusing 'onion layers' of a pre-supernova star look like to scale? - Astronomy

Hardly anything notable befalls a star during most of its lifetime. Provided the nuclear events at its core continue to offset the relentless onslaught of gravity, nothing spectacular happens to the star as a whole. Predictably, its core fuses hydrogen into helium, its surface erupts in flares and storms, and its atmosphere passess vast amounts of radiation. But, by and large, stars experience no sudden changes while in equilibrium, which is actually a kind of "dynamic steady state." They simply consume hydrogen during this, the longest phase in the history of all stars, a duration lasting

99% of their total lifetimes.

Main-sequence Equilibrium Actually, stars maintain hydrostatic equilibrium, not thermodynamic equilibrium, as illustrated in Figure 3.22. The former pertains to the structural integrity of a normal star, noting again its delicate balance in the tug-of-war between gravity pulling in and heat pushing out. Technically, it’s not heat that pushes out as much as gas pressure, heat being a form of energy whereas pressure—the product of temperature and density—is more akin to a force. Hydrostatic equilibrium—as in a “compressible fluid,” which is the way stars are modeled—tends to stabilize a star at every point within the star, to keep it from collapsing or exploding, in either case catastrophically. By contrast, thermodynamic equilibrium occurs when temperatures are uniform throughout, a state most definitely not achieved by any star. In fact, stars have a clear and obvious temperature gradient, from their fiery cores to their cooler (but still hot) surfaces. What’s more, such gradients grow as stars age, driving them further from thermal equilibrium. It is this temperature variation that establishes decidedly non-equilibrium conditions, thermodynamically, in all stars.

 FIGURE 3.22 — A steadily burning star on the main sequence has its inward pull of gravity counterbalanced by the outward pressure of its hot gases. This is true at any point within the star, guaranteeing its stability. (Prentice Hall)

Note also, to make another clarification, that even in hydrostatic equilibrium, a star like the Sun continues to change its luminosity—that is, its rate of energy flow—ever so slightly over the course of its lifetime. Specifically, for the Sun’s case, that amounts to an increase in brightness of

1% every 100 million years. Although that seems minute, extrapolating back some 3-4 billion years means that the early Sun was probably

1 /3 less luminous than it is today—and that might pose a problem understanding the origin and maintenance of early life if planet Earth were at the time too cold for water to be liquefied. We shall return to discuss this “faint-Sun paradox” later in the seventh, CULTURAL EPOCH.

Stars, then, in their normal, balanced state, continue to produce energy indefinitely, pending some drastic change. The great struggle between heat and gravity remains under control, typically for billions of years. Eventually, however, something drastic does occur: All stars eventually exhaust their fuel.

Computer simulations are again our foremost guide to the specific changes experienced by any star near death. Identifying numerous physical and chemical factors and adjusting their values repeatedly, theoreticians have built models to describe the wide variety of stars seen in the real Universe. Let’s first detail the death plunge of a star like our Sun, after which we can extrapolate to all stars, large and small. Keep in mind, though, that all these fatal events occur within the last 1% of a star’s lifetime.

Hydrogen Depletion As the Sun ages, its hydrogen steadily depletes, at least within a small, central core having a few percent of the star’s full crossectional size. After nearly 10 billion years of slow and steady burning, little hydrogen will remain within the innermost fusion zone. The star literally runs out of gas. Much like an automobile cruising along a highway at a constant speed for many hours without a care in the world, its engine starts to cough and sputter as the gas gauge approaches empty. Unlike automobiles, though, stars aren’t easy to refuel.

Widespread exhaustion of hydrogen in the stellar core causes the nuclear reactions there to cease. Hydrogen combustion continues unabated in the star's intermediate layers, above its core though well below its surface. But the core itself normally provides the bulk of the support in any star, acting as a foundation and guaranteeing its stability. By contrast, the lack of core burning assures instability because, although the outward gas pressure weakens in the cooling core, the inward pull of gravity most assuredly does not. Gravity never lets up it’s relentless. Once the outward push against gravity is relaxed—even a little—structural changes in the star become inevitable.

Generation of more heat could bring the aged star back into hydrostatic balance. If, for example, the helium at the core began fusing into some heavier element such as carbon, then all would be well once again, for energy would be recreated as a by-product to help reestablish the outward gas pressure. But the helium there cannot burn—not yet, anyway. Despite the phenomenal temperature of millions of kelvins, the core is just too “cold” for helium to fuse into any heavier elements.

Recall that a temperature of at least 10 7 K is needed to initiate the hydrogen --> helium fusion cycle. That’s what it takes for two colliding hydrogen nuclei (protons) to get up enough speed to ram each other violently and thus overwhelm the repulsive electromagnetic force between two like charges. Otherwise, the nuclei cannot penetrate the domain of the nuclear binding force and the fusion process simply doesn’t work. Even 10 7 K, however, is insufficient for helium fusion, since each helium nucleus (2 protons + 2 neutrons) has a net charge twice that of the hydrogen nucleus, making the repulsive electromagnetic force greater. To ensure successful fusion by means of a violent collision between helium nuclei, even higher temperatures are needed. How high? About a hundred million kelvins󈟚 8 K.

Lacking that degree of heat, the star’s core of helium “ash” doesn’t remain idle for long. Its hydrogen fuel spent, the core begins contracting. It has to there’s not enough pressure to hold back gravity. However, this very shrinkage allows the gas density to increase, thereby creating more heat as gas particle collisions become ever more frequent. Once again, it’s gravity, in the guise of gravitational potential energy converting to frictional heat energy, that drives this process—indeed drives up the temperature.

The increasingly hot core continues to roil the overlying layers of this stellar furnace. It’s very much like a domestic thermostat that calls for more heat in our homes, thereby keeping the air temperature comfortably stable. In an aged star, Nature seeks more energy to restabilize events, and when the star generates enough of it, negative feedback terminates the contraction—at least for a while. (“Feedback” because, as in a central heating system, a change in the effect is fed back to modify its cause, and “negative” because the feedback loop controlling the process ensures that the effect doesn’t increase or decrease without limit.) But first, higher temperatures—at this stage, well >10 7 K—cause hydrogen nuclei in the star’s intermediate layers to fuse even more furiously than in the core before. All the while, helium ash continues to pile up around the core. Figure 3.23 depicts this rather peculiar condition where hydrogen is burning at a fantastic rate around the nonburning helium ash.

 FIGURE 3.23 — As a star's core becomes progressively depleted of hydrogen, the hydrogen fusion reactions continue to burn in its intermediate layers, high above the nonburning helium ash. (Prentice Hall)

The aged star is really in a predicament now. Its days are numbered. The core is unbalanced and shrinking, on its way toward generating enough heat for helium fusion. The intermediate layers are also scrambling to maintain some semblance of poise, fusing hydrogen into helium at faster-than-normal rates. Alas, the gas pressure exerted by this enhanced hydrogen burning does build up, forcing the star’s outermost layers to expand not even gravity can stop them. So, although the core is shrinking, the overlying layers are expanding! Clearly, the star’s structural stability is completely ruined.

Observational Consequences Two observable aspects of such a perverse star are interesting. To a viewer far away, this celestial object would seem gigantic, nearly 100 times larger than usual. Captured radiation would also imply that the star’s surface was a little cooler (

1000 K) than normal. This is not to say that the act of ballooning and chilling of an aged star could be observed directly during any one human lifetime. The transition from a normal, solar-mass star to an elderly giant still takes

These large-scale changes in the disposition of an aged 1-solar-mass star can be traced on the HR diagram. Figure 3.24 shows the resulting path away from the main sequence. As illustrated, the luminosity of this giant star—again, R 2 T 4 —becomes

100 times the current brightness of our Sun.

 FIGURE 3.24 — As the core of helium ash shrinks and the intermediate stellar layers expand, the star leaves the main sequence. Labeled stage 8, it's on its way to becoming a red-giant star. (Lola Chaisson)

The second change—surface cooling—is a direct result of the first change—increased size. As the star expands, the sum total of its heat spreads throughout a much larger stellar volume. Hence, visible radiation emitted from such a cooling, yet still-hot, surface shifts in color. Like a white-hot piece of metal that turns red while cooling, the whole extended star displays a reddish tint. Over the course of time, again long by human though short by stellar standards, a star of normal size and yellow color slowly changes into one of giant size and red color. The bright normal star has evolved into a dim red-giant star.

Figure 3.25 compares the relative sizes of our Sun and a red-giant star. The typical giant star is huge, having swollen to

100 times its main-sequence size. By contrast, the helium core is surprisingly small, probably

1000 times smaller than the entire star. This makes the core only a few times larger than Earth.

The density in the core is now huge. Continued shrinkage of the red giant's core has compacted its helium gas to

10 5 g/cm 3 . This value may be contrasted with

10 -6 g/cm 3 in the outermost layers of the red-giant star, with

5 g/cm 3 average Earth density, or with

150 g/cm 3 in the core of the present Sun. Owing to this greatly compressed helium state,

25% of the mass of the entire star is packed into its small core.

To recapitulate these momentous events and give them some local relevamce, once the Sun exhausts its hydrogen fuel supply at its core, instability is sure to set in. Its core will shrink as its overlying layers swell, all the while equilibrium is shot. As such, the Sun is destined to become a bloated sphere hundreds of times its normal size, perhaps large enough to engulf many of the planets, including Mercury and Venus, and maybe even Earth and Mars as well.

Humans need not panic, not yet at any rate. Provided the theory of stellar evolution is reasonably correct as described here, we can be sure that our Sun will not swell to this red-giant stage for another 5 billion years. Whether life can remain viable on Earth that long is debatable there are two competing arguments: First, owing to the faint-Sun paradox noted a few pages earlier, the future Sun seems likely to increase its luminosity by

10% in a “mere” billion years, possibly rendering our planet unsuitable for life well before the Sun itself expires. Planet Earth will eventually get quite steamy regardless of any global pollution caused by humankind. Second, countering that long-term heating is a natural cooling forecast by the expected outward migration of the planets’ orbits as the Sun loses mass and lessens its gravitational grip. Hard to believe, the Sun is shedding its own matter (in a “solar wind”) at the prodigious rate of about a million tons each second, yet even in a billion years will have lost <0.1% of its total mass, which might not be enough for the planets to drift away much. Whether the resulting cooling trend caused by the receding planets can offset the heating trend caused by increased sunlight is an unsolved problem—a rare astronomical enigma with life-and-death terrestrial implications. Whichever, life’s days on Earth are surely numbered, its oceans destined to evaporate and its atmosphere dissipate, our planet eventually resembling a ceramic-encrusted Mercury. Not to worry, such a hell-on-Earth will not commence for nearly another trillion or so days.

Red-giant stars are not the fiction of some theoretician’s mind. They really do exist, scattered in numerous places across the sky. Even the naked eye can perceive the most famous of all red giants—the bright star Betelgeuse, that swollen, elderly, distinctly reddish member of the constellation Orion—a prominent beacon in the northern hemisphere’s winter sky. This star is so luminous, it can be seen even through the smog and light pollution of our biggest cities. Look up!

Helium Fusion Should the inherent imbalance of a red-giant star be maintained unabated, the core would eventually implode, while the rest of the star drifts into space. Various forces and pressures at work inside such a decrepit star would literally, though slowly, pull it apart. Fortunately for the stellar veteran, this tortuous shrinkage-expansion doesn't continue indefinitely. Within 100 million years after the star first begins to panic for lack of hydrogen fuel, something else happens—helium ignites in thermonuclear burning. Accordingly, the star's natural thermostat shuts off the flow of additional heat as the core stabilizes once more. Though this seems like a whole new lease on life, it amounts to only a brief reprieve.

Deep down inside a red-giant star, the density increases as the interior pressure builds. Once the matter in the star’s core becomes

1000 times denser than that of a normal star (i.e.,

10 5 g/cm 3 ), collisions among the gas particles are violent and frequent enough to generate sufficient heat, via friction, to reach the 10 8 K temperature needed for helium fusion. Helium nuclei henceforth collide, trigger the central fires once again, and begin transforming into carbon. Thereafter for a period of a few hours, the helium burns ferociously, like an uncontrolled bomb. This onset of helium burning is such a sudden and rapid event in the history of a star that astronomers give it a special name—“helium flash.” It’s remarkable that the star doesn’t explode.

Despite their brevity, these renewed nuclear events release an enormous flood of new energy. The energy is potent enough to etherealize the core matter somewhat, thereby lowering its density and relieving some of the pent-up pressure among the charged nuclei. This small expansive adjustment of the core halts the gravitational contraction of the star, reestablishing an equilibrium of sorts—in this case, a balance occurring at the quantum level among the densely packed electrons whose tiny point-like spheres are essentially touching one another, thereby physically holding up the aged star against gravity.

To make yet another clarifying technical comment, note that, in actuality, the nuclear reaction that changes helium into carbon occurs in two steps known as the “triple-alpha process.” First, two helium nuclei (which are also termed alpha particles) combine to form beryllium, which is a very unstable nucleus that would normally break right back down (in less than a microsecond) into two helium nuclei—causing the process to be stuck in an endless cycle that yields nothing heavier than helium. However (and this is the second step), the huge densities in the helium ash guarantee that a third helium nucleus sometimes collides with newly made beryllium before it has a chance to decay. This is not a miracle, or some sort of “anthropic principle,” implying that a supernatural being designed it that way to permit heavy elements and therefore life. Rather, given the very high densities in a red-giant’s core, the time scale for collision and then fusion among three helium nuclei (hence the name, "triple") is naturally shorter than for the fusion and breakdown of beryllium. The result is carbon, the nucleus of a vitally important element in the later CHEMICAL EPOCH of our cosmic-evolutionary story.

Once the helium --> carbon fusion reactions commence, thus stabilizing the core, the hydrogen --> helium fusion reactions churning in the layers above subside (but do not stop). Theoretical computations implies that the star expands its outer layers a bit too rapidly, overshooting the distance at which it reestablishes a relaxed structural balance. The entire star is then able to shrink a little, losing some of its swollen appearance. This slight shrinkage of the outer layers causes the luminosity to decrease and the surface temperature to increase, reversing the star's evolutionary path once again, as shown in Figure 3.26. Like all the other evolutionary changes in the early or late phases of a star, this slight size adjustment onto the "horizontal branch" is made quickly—at least by cosmic standards—namely, in

 FIGURE 3.26 — After a large increase in luminosity, a red-giant star finally settles down into another equilibrium state at stage 10, on the so-called horizontal branch. (Lola Chaisson)

Though the time scales for marked stellar change are deemed rapid for a star’s birth in gas and dust as well as its thrust toward an end-fate, all these transient durations are still long compared to human lifespans. Observers have little hope of watching a given star move through all, or even some, of the evolutionary paces underway in the STELLAR EPOCH. Instead, much as before, astronomers search the Galaxy for evidence of diverse cosmic objects at different stages of their evolutionary cycles, trying to position them like puzzle pieces into a self-consistent picture. Or, to use another metaphor, like social behaviorists charged with the task of unraveling the population dynamics of animals, astronomers are finding that the deeper they peer into galactic lairs, the more instructive the menagerie of stellar inhabitants becomes. In the end, we always rely on mathematical modeling to match (and adjust) the theory of stellar evolution with the observations of the many varied stages in the birth and death of stars.

Table 3-2 summarizes a computer calculation done for a 1-solar-mass object. It's a continuation of the previous compilation listed in Table 3-1, except that the density units have been switched from particles/cm 3 to g/cm 3 , to reflect the growing densities (there being about 10 24 atomic particles in 1 gram of matter). The previous table ended with stage 7, a main-sequence object fusing hydrogen into helium over the course of

10 billion years. The new table here begins with stage 8, the evolutionary path away from the main sequence. Stage 9 describes an established red-giant star fusing helium into carbon at its core.

As for the physical quantities listed in Table 3-1, those describing each of the stages of Table 3-2 cannot be specified with high accuracy. The temperature, density, size, and luminosity, as well as the precise evolutionary path, are not completely understood at this time. Each of these quantities depends on the initial conditions used for the mass and composition of a star, as well as on the rate of nuclear burning deep inside.

Solar Neutrinos This reliance on computer modeling is exactly what made the results of an important experiment so disturbing—until recently. The one experiment that bears directly on the physical events inside stars didn’t jibe well with the predictions for a star like our Sun. For decades, scientists were puzzled by the number of neutrino elementary particles found in the solar radiation reaching Earth. Derived from an Italian word meaning “little neutral one,” neutrinos are known from experiments on Earth to be virtually massless and chargeless, and to travel at (or very close to) the velocity of light. Interacting with almost nothing, neutrinos are ghost-like particles endowed with an ability to pass freely through several light-years of lead! Hence, they should be able to escape unhesitatingly from the solar core, where they are created in copious amounts as by-products of nuclear reactions. Ordinary radiation scatters around (or “random walks”) in the solar interior for about a million years before being emitted from the Sun’s surface into space, but neutrinos should pierce the solar surface in 2 seconds and arrive at Earth a mere 8 minutes after being made at the core. They thus comprise the only direct test of the nuclear events responsible for powering the Sun.

Solar neutrinos nonchalantly penetrate Earth all the time. Some 5 million neutrinos pepper every cm 2 of our bodies each second, though we are neither aware of nor harmed by them. Despite their elusiveness, however, the effects of neutrinos can be studied with carefully built instruments made of rare materials. One of those materials is a chemical with the tongue-twisting name of tetrachloroethylene. As toxic as it sounds, C2Cl4 is a safe fluid often used in the dry-cleaning industry. So a “neutrino telescope” was built in the 1970s at the bottom of a South Dakota gold mine by filling a large tank with 400,000 liters (

100,000 gallons) of this stuff. In that way, some of the solar neutrinos arriving at Earth can be counted and analyzed, though actually only 1 is detected for every 10 15 of them streaming through the tank. The underground location of this laboratory and its unique telescope is essential to shield the experiment from interference due to cosmic rays and other elementary particles hailing from non-solar sources such as ancient supernovae. Although the equipment seems to have worked properly for decades, the rate of neutrino detection has often been consistently less than theory predicts they are seen about twice per week, rather than once per day—about a three-fold discrepancy.

Astrophysicists have wrestled with these puzzling results for many years. Both theorists and experimentalists are reluctant to blame any underabundance of solar neutrinos on conceptual errors in the theory of stellar evolution. No one wants to discard what seems like a good understanding of solar fusion, all other aspects of which agree so well with observations. Some researchers (mostly theorists) suspect the experimental gear perhaps it wasn’t quite tuned properly, and in any case a factor of

3 isn’t usually a large issue in astronomy. Others (mostly experimenters) are leery about the computer models if the Sun’s core were only 10% cooler than theory maintains, the predicted number of solar neutrinos would be less. Still others argue that we don’t yet know enough about the odd neutrino particle itself the physical properties of the neutrinos might make them the culprit, especially if they turn out to have even minute amounts of mass.

More recently, in the first decade of the 21st century, this factor-of-three discrepancy seems to have been resolved during experiments in new underground laboratories located in Japan and Canada, the latter using a 1000-ton sphere of ultrapure water suspended more than a kilometer beneath the surface and surrounded by 10,000 neutrino sensors. The new results do indicate that neutrinos have minute amounts of mass—roughly a millionth (10 -6 ) the mass of an electron, which is itself nearly 2000 times lighter than a proton. However, even this ultra-tiny mass is enough to cause the apparently schizophrenic neutrinos to change their properties, even to transform them into other particles, during their 8-minute journey from the Sun to the Earth. And that’s what most astronomers now think is happening: Neutrinos are likely produced in the Sun at the rate predicted by theory, but some of them change into something else—probably morphing into other types of neutrinos—while en route to Earth. The original experiments were insensitive to these changes, but the newer experiments are detecting evidence of them. At issue now is the need to fix up the standard model of particle physics, in which neutrinos are expected to have precisely zero mass—or to begin a whole new search to solve another potential contradiction between quantum theory and delicate experimentation.

Assuming these latest results are correct—namely, that neutrinos have both intrinsic mass and mutable properties—we once again wonder if the neutrinos could be the solution to the elusive dark-matter quandary. Given the tremendous number of neutrinos likely flooding our Galaxy—both leftovers from particle interactions in the early Universe as well as new ones created in all the stars of the Milky Way—it still seems doubtful. Although neutrinos are surely part of the cosmic mix, their total accumulation likely amounts to <1% of the overall mass of the Galaxy.

In any event, few researchers regard the surprising solution to the solar-neutrino problem as a threat to our understanding of the way that stars shine. This decades-old neutrino dilemma now seems to have been more of a problem with the physics of the particle than with the astronomy of the Sun. By checking and double-checking both theory and experiment, all the while continuing to address the issue with reason and skepticism—which is exactly the way science progresses—what once loomed as a serious misunderstanding of stellar fusion has apparently now been resolved.

Nagging Issues Uncertainties limit our understanding of every epoch of cosmic evolution. Here in the STELLAR EPOCH, as elsewhere, we seem able to identify the broad outlines of many possible events, but the fine details aren’t always in hand. What causes flaring on our Sun, resulting in huge prominences of matter and radiation that escape our star and impact our planet? How does the

11-year solar cycle work, turning surface sunspots off and on at nearly decade intervals? Can we explain satisfactorily the million-degree corona, or outer atmosphere of the Sun, when its surface is only 6000 K? What’s the role of magnetic fields in the origin, maintenance, and demise of all stars?

Even the brightest star in the nighttime sky seems a little puzzling, at least as regards the historical record. Sirius A, only 9 light-years away, appears twice as luminous as any other visible star (excluding the Sun) and has been prominently observed by many ancient civilizations. Cuneiform texts of the Babylonians refer to this star as far back as 1000 B.C., and historians know that the star strongly influenced the agriculture and religion of the Egyptians of about 3000 B.C. So, given the lengthy record of observations of Sirius, here’s an object for which we might have a chance to study slight evolutionary changes, despite the long time scales usually needed to produce such changes. Yet herein lies the puzzle.

Sirius A does seem to have changed its appearance over the ages written historical records clearly imply it. But the naked-eye observations of the ancients are confusing. Every piece of information about Sirius recorded between the years 100 B.C. and A.D. 200 claims that this star was red. In contrast, modern observations now show it to be white or bluish white, but definitely not red. Accordingly to the theory of stellar evolution, no star should be able to change its color from red to blue-white so dramatically in such a short time—even over thousands of years. Any change of this sort should take at least

100,000 years, and in any case would more likely change from blue to red.

Astronomers have offered many explanations for the rather sudden change in Sirius A. These include the possibility that some ancient observers were wrong and other scribes copied them. Or perhaps a galactic dust cloud passed between Sirius and Earth

2000 years ago, reddening the star much as Earth’s dusty atmosphere often does for our Sun at dusk. Or maybe a companion to Sirius A, namely Sirius B, was a red giant and dominant star of this double-star system 2000 years ago and has since expelled its outer envelope to reveal the small (white-dwarf) star that we now observe as Sirius B.

None of these explanations seems plausible, however. How could the color of the sky’s brightest star be incorrectly recorded for hundreds of years? Where’s the intervening galactic cloud now? Where’s the shell of the former red giant? We are left with the uneasy feeling that the night’s brightest star doesn’t seem to fit well into the currently accepted scenario of stellar evolution.

As if that were not enough, our resolute navigational beacon, Polaris the North Star, is also a bit of a conundrum. Despite Shakespeare's classic line for Julius Ceasar, "But I am constant as the Northern Star," the light from Polaris is not so steady, yet the sky is replete with variable stars so that is perhaps alright. Alas, the extent of its variability is also changing and quickly too, and that is indeed puzzling. Greek astronomers of 2000 years ago claimed that Polaris' average brightness was 3 times dimmer than now, a rate of change, if real, much greater than that predicted by current models of stellar evolution. That not all the loose ends are yet tied up isn’t meant to imply major cracks in our understanding of stars rather that plenty of work remains to be done regarding those picky little details that often serve to fine-tune that understanding.

These subtle yet bothersome issues aside, stellar evolution is judged one of the great success stories of modern astrophysics. Theory and observation have advanced hand in hand over the last many decades, refining our knowledge of stars as they proceed from cradle to grave. Today, the subject of stellar evolution is a cornerstone of the cosmic-evolutionary narrative, a key part of that broadest view of the biggest picture that we’ve come to know rather well.

Carbon Core Nuclear reactions in an old star’s helium core churn on, but not for long. Whatever helium exists in the core is rapidly consumed. The helium --> carbon fusion cycle, like the hydrogen --> helium cycle before it, runs at a rate proportional to the temperature the greater the core heat, the faster the reactions proceed. Under these very high temperatures, helium fuel simply doesn’t last long—probably less than a few million years after its initial “flash.”

Buildup of carbon ash in the inner core causes a series of physical events similar to those in the earlier helium core. Helium first becomes depleted at the star’s very center, after which fusion there ceases, the temperature being too low for carbon detonation. The carbon core then shrinks and heats a little, as Nature’s thermostat kicks in again while searching for more energy from renewed gravitational infall. This, in turn, causes the hydrogen and helium burning cycles to ramp up in the middle layers of the star. Such an aged star begins to resemble a huge onion, with different shells of progressively heavier elements toward its center. All this additional heating causes its outer envelope ultimately to expand, much as it did earlier, making the star once again a swollen red giant. Figures 3.27 and 3.28 depict the star's interior and the evolutionary path followed during these latest events.

 FIGURE 3.27 — Within a few million years after the onset of helium burning, carbon ash accumulates in the inner core of a star, above which hydrogen and helium are still burning. (Prentice Hall)

 FIGURE 3.28 — A carbon-core star eventually heads back toward higher luminosities—technically along an "asymptotic giant branch"—for the same reason it evolved there in the first place: lack of nuclear fusion at the core, causing contraction of the core and expansion of the overlying layers. (Lola Chaisson)

Provided the core temperature does become high enough for the fusion of two carbon nuclei, or more likely a union of carbon and helium nuclei, even heavier products can be synthesized. Newly generated energy supports the star at each stage in the nuclear chain, returning the star to its accustomed hydrostatic equilibrium. Again, this isn’t a thermodynamic equilibrium, for such decrepit old stars develop steep thermal and elemental gradients from core to surface. For this reason, such aged stars are decidedly more complex than their younger counterparts. Ironically, as the fusion process advances, old stars continue getting brighter, all the while they are dying.

This contracting-heating-fusing-cycle is generally the way that many of the heavy elements are fashioned within the last gasps of stellar cores. All elements heavier than carbon are created within the final 1% of some stars’ lifetimes. Our Sun, however, is not one of them it's too small.

Mass Loss Stars of all spectral types are known to be active and to have stellar winds, much as the active Sun displays most evidently every

11 years during its periods of increased sunspots, flares, and prominences. Consider the highly luminous, hot, blue stars (O- and B-types) that have by far the strongest winds. Observations of their ultraviolet spectra with telescopes on rockets and satellites have shown that their wind speeds (or gales!) often reach 3000 km/s (or several million mph). The corresponding mass-loss rates approach and sometimes exceed 10 -5 solar mass per year this is equivalent to an entire solar mass (yet typically only about a tenth of the total mass in these bigger stars) being carried off into space in the relatively short span of 100,000 years.

Observations made by the International Ultraviolet Explorer satellite operating in Earth orbit during the 1980s proved that to produce such great winds, the pressure of hot coronal gases (which drive the solar wind) does not suffice. Instead, the winds of the luminous hot stars must be driven directly by the pressure of the ultraviolet radiation emitted by these stars. The same mechanism has been theorized to eject gas from the cores of some particularly active galaxies, a subject touched on briefly in the previous GALACTIC EPOCH .

Such powerful stellar winds hollow out vast cavities in the interstellar medium, pushing outward expanding shells of galactic matter resembling those generated by exploded stars, as discussed both at the end of the GALACTIC EPOCH and in the next section of this STELLAR EPOCH. Aside from the well-known fact that copious quantities of ultraviolet radiation are available from luminous hot stars to drive such stellar winds, the details of the process are not well understood. Whatever is going on, it’s surely convoluted, for the ultraviolet spectra of the stars tend to vary with time, implying that the wind is unsteady. Apparently, stellar instabilities of some kind or another are at the heart of the issue.

Observations made more recently with radio and infrared as well as optical telescopes prove that luminous cool stars (e.g., K- and M-type giants) lose mass at rates comparable to those of the luminous hot stars their wind velocities, however, are much lower, averaging 30 km/s (or "merely" 70,000 mph). Because luminous red stars are inherently cool objects (

3000 K surface temperature), they emit no detectable ultraviolet radiation, so the mechanism driving the winds probably differs from that in luminous hot stars. We can only surmise that gas turbulence and/or magnetic forces in the atmospheres of these stars are somehow responsible. Unlike the hot stars, winds from these cool stars are rich in dust particles and molecules. Since nearly all stars more massive than the Sun eventually evolve into such red giants, these winds, pouring forth from vast numbers of stars, provide a major source of new gas and dust in interstellar space. Thus, the recently discovered stellar winds provide a vital link in the cycle of star formation and galactic evolution. As with the hot stars, astronomers are unsure what affect these winds and mass losses have on the subsequent evolution of the stars themselves.

## Kepler Observes Supernova's Shockwave in Visible Light for First Time

An artist’s impression of a supernova explosion. With the help of NASA’s Kepler space telescope, astronomers were able to observe for the first time the exact moment when the shockwave from a supernova reaches the surface of the progenitor star just before the latter explodes. Image Credit: ESO/M. Kornmesser

One of the most impressive deep-sky spectacles for professional and seasoned amateur astronomers alike are supernova explosions. Signifying the end stages in the lives of stars that are more massive than the Sun, supernovae are transient powerhouses of tremendous force that are integral in the perpetual cosmic cycle of life and death and the recycling of interstellar material that eventually gives rise to the next generation of stars and planetary systems. Even though astronomers have gained much understanding about the physical processes that drive these cosmic fireworks in the last couple of decades, many of the underlying details have remained sketchy to date. NASA’s Kepler space telescope recently added one important piece of knowledge to astronomers’ picture of supernovae, by directly observing its shockwave in visible wavelengths for the first time—the bright flash that immediately precedes the explosion itself before the progenitor star is completely torn apart.

All stars go through their lives by fusing hydrogen into helium deep inside their cores, a process that maintains them in hydrostatic equilibrium, balancing the inward pressure of the star’s mass itself with the outward pressure of the radiation and light that is produced from the nuclear fusion that takes place inside its core. When stars exhaust their hydrogen supplies during the end of their lives, they are no longer able to counteract the force of gravity and their cores start to collapse under their own weight, heating up the surrounding stellar layers and causing them to expand. At that point, the stars’ initial mass determines their eventual fate.

The light curve of supernova KSN 2011d as was observed by Kepler. The supernova’s shockwave breakout is clearly visible just before the star began to explode. Image Credit: NASA Ames/W. Stenzel

For low- and medium-mass ones up to approximately eight solar masses, when during core collapse temperatures and pressures are sufficiently high, the core begins to fuse helium into heavier elements like carbon and oxygen, leading to a huge inflation of the star which evolves to become a red giant. These types of stars are not massive enough for fusion to continue beyond that point and the red giant eventually blows off its outer layers into space, forming a planetary nebula. More massive stars, on the other hand, that have at least eight times the mass of the Sun go on to fuse even heavier elements when hydrogen is depleted. The nuclear fusion process of these stars slowly progresses up through the periodic table of elements, producing increasingly heavier atomic nuclei from carbon all the way to iron which are deposited on the star’s interior on successive layers above the core in an onion-like fashion, all the while the star inflates and becomes a red supergiant with a radius that is typically hundreds of times that of the Sun. When the star reaches the stage of iron fusion it can no longer sustain any further nuclear reactions and it begins its final collapse under its own weight. In a matter of seconds, the imploding stellar layers hit the core at tremendous speeds that can reach over 20 percent the speed of light and then rebound creating a ferocious shock wave that propagates outwards in an exploding manner, taking the star’s layers with it away from the core in what essentially constitutes the beginning of a supernova explosion.

Theoretical models had predicted that at this point the shock wave would traverse the star’s interior and reach the surface within a time span of less than an hour, before propagating farther out into the surrounding interstellar medium. In the exact instance that the shock wave would reach the photosphere, it would produce a sudden flash of light across multiple wavelengths. This flash, called a “shock breakout” would be detectable as a characteristic spike in brightness in the supernova’s overall light curve. Yet, since these shock waves travel through the dying star in such a short amount of time, detecting their photometric signature represented an overwhelming challenge for astronomers since they would have to be looking at the right place at the right time, just before a supernova would go off. Not to be deterred, their efforts paid off handsomely when NASA’s Kepler space telescope observed just such an event as it unfolded during one of its long observing campaigns.

Better known as a prolific planet-hunting mission, Kepler has been responsible for largely revolutionising the search for exoplanets around other stars since it was launched in March 2009, having already detected more than 4,700 exoplanet candidates and 1,040 confirmed discoveries to date. The space telescope was able to achieve these results by continuously staring at approximately 150,000 stars at a fixed field of view in the sky, searching for the characteristic dips in brightness that would signify the passage of an exoplanet across the stars’ disk. It turns out that the telescope’s pointing at a fixed place in the sky was exactly what was needed in the search for other transient astrophysical phenomena as well, like supernovae explosions. With that in mind, an international team of astronomers, led by Peter Garnavich, a professor of astrophysics at the University of Notre Dame in Indiana, set out in 2011 to monitor approximately 500 galaxies that were positioned in Kepler’s field of view, in the hunt for any brightness variations that would be indicative of a supernova. And find they did, when the space telescope observed two red supergiant stars in 30-minute intervals before and after they exploded.

In its new K2 mission, Kepler has been repurposed to point in a direction that is parallel to its orbital path around the Sun, close to the plane of the eccliptic. This allows the spacecraft to gaze at different areas of the sky above and below the galactic buldge during its orbit, which provides astronomers with the chance to conduct many different astrophysical observations in addition to the mission’s plane-hunting duties. Image Credit: NASA/ Ames

The stars observed by Kepler were real behemoths, with a radii approximately 280 and 490 times that of the Sun respectively. For context, if both were placed at the center of the Solar System they would easily envelope all of the terrestrial planets. “To put their size into perspective, Earth’s orbit about our Sun would fit comfortably within these colossal stars,” says Garnavich. The study of their light curves showed that they were typical Type II-P supernovae, a subclass of Type II supernovae which are characterised by the presence of strong hydrogen emission lines in their spectra. These types of stellar explosions have been studied extensively by astronomers, and the progression of their light curves is well understood. Type II-P supernovae in particular reach peak brightness on a timescale of one to two weeks, which they can maintain for an extensive period of time, typically lasting for several months (a period that is represented as a characteristic “plateau” in the light curve) before slowly fading out.

Kepler’s constant gaze on these two stars, named KSN 2011a and KSN 2011d, allowed astronomers to track their light curves in detail despite their great distances of 700 million and 1.2 billion light-years respectively, showing that they matched well with theoretical predictions of how Type II generally behave. Yet, most importantly, in the case of KSN 2011d the space telescope was able to observe a distinctive small spike in the star’s brightness just before the latter went supernova. Lasting no more than 20 minutes, this surge in brightness, which was recorded immediately before the star’s light curve began to rise toward maximum, was a clear sign of the long-theorised shock breakout which indicates the moment in time when the shockwave from the exploding supernova reaches the star’s surface.

“In order to see something that happens on timescales of minutes, like a shock breakout, you want to have a camera continuously monitoring the sky,” says Garnavich. “You don’t know when a supernova is going to go off, and Kepler’s vigilance allowed us to be a witness as the explosion began.”

“It is a thrill to be a part of theoretical predictions becoming an observed and tested phenomenon,” adds Ed Shaya, an associate research scientist at the University of Maryland, College Park and member of Garnavich’s team. “We now have more than just theory to explain what happens when a supernova shock wave reaches the surface of a star as that star is totally torn apart.”

Despite the fact that both KSN 2011a and d displayed a similar energy output that was typical of Type II supernovae, the former’s light curve surprisingly lacked a similar signature of a shock breakout. Furthermore, KSN 2011a’s rise time to peak brightness was somewhat shorter (in the order of 10 days compared to 14 for KSN 2011a), which indicates that even though the overall driving mechanisms for Type II supernovae are the same, some of the details may differ. “That is the puzzle of these results,” comments Garnavich. “You look at two supernovae and see two different things. That’s maximum diversity.”

The most probable explanation for this discrepancy that fits theoretical predictions, according to the researchers, is that in the case of KSN 2011a the shock wave either didn’t travel all the way out to the photosphere due to the star’s much larger size, or if it did it was dispersed more evenly, possibly due to the presence of circumstellar material around the star, stealing away its luminosity that would otherwise have registered in the supernova’s light curve. “No fast shock breakout emission is seen in KSN2011a, but this is likely due to the circumstellar interaction suspected in the early light curve,” writes Garnavich’s team in their study , which was accepted for publication in the Astrophysical Journal. “The rapid rise in KSN2011a … suggests the supernova shock continued to propagate into circumstellar material allowing it to convert more kinetic energy into luminosity and di ffuse the shock breakout over a longer time … KSN2011d does show excess emission at the time expected for shock breakout with a brightness of 12% that of supernova peak in the Kepler band. The time-scale and brightness observed for the breakout is consistent with model predictions.”

A collage of images of the Type II supernova SN 1987A in the neighboring Large Magellanic Cloud, that were taken by the Hubble space telescope over a period of 12 years. The photos show a ring of material around the supernova progressively being lit up as it was hit by the shock wave from the initial explosion. Image Credit: NASA, ESA, Pete Challis, and Robert Kirchner

Kepler’s direct discovery of a supernova shock breakout showcases the fact that even though the space telescope was primarily designed as an exoplanet hunting mission, it is nevertheless a first-rate astrophysical observatory as well, which could play an equally important role in astrophysical research besides its planet-hunting duties in the years to come. Now well in its K2 “Second Light” mission, which followed the loss of its second reaction wheel in May 2013, the space telescope has been repurposed in pointing in a direction that is parallel to its orbital path around the Sun near the ecliptic plane, which has opened a host of new opportunities for research in many different fields of astrophysics. “We’re no longer an exoplanet mission,” said John Troeltzsch, program manager for Ball Aerospace, the prime contractor for Kepler which also devised the telescope’s K2 extended mission, during a presentation at the Laboratory for Atmospheric and Space Physics in Boulder, Colo. “We’re a general-purpose astrophysics observatory [for] astroseisomology, Solar System [studies], exoplanets, [star] clusters, stellar activity, binary stars, extragalactic [studies], etc. There’s a little something for everybody.”

The study of supernovae is expected to take center stage as Kepler’s K2 progresses, which will hopefully allow astronomers to gain important insights to the explosive processes that drive these cosmic fireworks, as well as the stellar alchemy that takes place in the cores of these stars, processes which maintain the recycling of stellar material and possibly even the continuation of life in other parts of the Cosmos. “All heavy elements in the Universe come from supernova explosions,” comments Steve Howell, project scientist for the Kepler and K2 missions at NASA’s Ames Research Center in California. “For example, all the silver, nickel, and copper in the earth and even in our bodies came from the explosive death throes of stars. Life exists because of supernovae.”

Kepler’s latest results underscore a fact that has been showcased time and time again in the history of space exploration: The study of the Universe is also a study of ourselves and no space mission is too small or insignificant in this regard. Important insights often come from paths that often seem inconsequential. In the end, that’s all the more reason to commit more strongly as a species to a vigorous program of space exploration.

A computer animation of a supernova explosion with its accompanying shockwave breakout, based on the observations of KSN 2011a and d, conducted with the Kepler space telescope. Video Credit: NASA Ames, STScI/G. Bacon

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## Triple-alpha or red-supergiant phase

The star’s core still is not in equilibrium and continues to contract until it hits the limit imposed by the QM exclusion principle, which keeps its electrons from occupying the same QM state. Within the core, the helium resulting from the preceding hydrogen core fusion begins to fuse into heavier elements, 8 Be, 12 C (and 16 O, in still heavier stars). Since 12 C formation requires three alpha particles ( 4 He nuclei), the process is called the triple-alpha process and the star is in the core helium fusion phase. The star now has two layers, the inner one fusing to heavier elements than the outer one.

For the Sun, this stage will last less than about 1 billion years, during which it will “burn” 4 He into 12 C and 16 O. A star of about this size will live for a total of around 10 Gy 5 Most authors would say 10 billion years, but in order to be consistent with later chapters, we will say 10 Gy . Since the Sun has already lived for almost 5 Gy (as we shall see in the geology chapter), it has (and we have) about 5 Gy left.

The core helium is used up faster than its hydrogen was and when this occurs, the core contracts again. The increased temperature causes a thin shell of remaining helium just inside the hyrdogen-fusion shell to go on fusing. The core now has three layers: an inner layer of carbon and oxygen, a middle layer of helium fusion and an outer shell of hydrogen fusion. The star expands again and becomes a red supergiant.

## 1. You Should Know — Neutron Star Terminology & Information

Neutron & Pulsar Stars | Protostar | Brown Dwarf | White Dwarf | Red Giant | Variable Star | Neutron Star | Supergiants | Quasar | Cepheid

Cluster M4 Crab Pulsar RX J1856.5-3754 IC443 Vela Star-Jet Binary Neutrons SN & Neutron Centaurus X-3 Cir X-1 The Crab Neutron

In This Section You Should Know

Definitions from the Astronomical Science of Neutron Stars

» Neutron - An electrically neutral subatomic particle in the baryon family, having a mass 1,839 times that of the electron, stable when bound in an atomic nucleus, and having a mean lifetime of approximately 1.0 x seconds as a free particle. It and the proton form nearly the entire mass of atomic nuclei and as such, make up most of the mass of the visible matter in the universe.
» Pulsar - a neutron star which emits beams of radiation that sweep through the earth's line of sight. The term pulsar is an abbreviation for pulsating radio star or rapidly pulsating radio sources. Like a black hole, it is an endpoint to stellar evolution. The "pulses" of high-energy radiation we see from a pulsar are due to a misalignment of the neutron star's rotation axis and its magnetic axis. Pulsars pulse because the rotation of the neutron star causes the radiation generated within the magnetic field to sweep in and out of our line of sight with a regular period. [1]
» Neutron Star - Neutrons were discovered in 1932 and very shortly afterward (in 1934) a suggestion was made by Walter Baade and Fritz Zwicky that neutron stars were formed in supernovae. Neutron stars were proven to exist when in 1967 Jocelyn Bell, working on radio observations of quasars, found the radio emissions of pulsar CP 1919. It was quickly thereafter determined that the source was a highly rotating neutron star. [2]
» Jocelyn Bell Burnell - discovered the first radio pulsars in 1967 Ph.D. in radio astronomy from Cambridge University in 1968. For a short bibliography and interview see the StarChild website page entitled Jocelyn Bell Burnell
» Crab Pulsar - a much-studied supernova remnant containing a neutron star. For additional information on the Crab Pulsar see Wikipedia & the National Radio Astronomy Observatory The Crab Pulsar and Nebula
» Solar Mass - The solar mass (M), 1.98892 x 1030 kg, is a standard way to express mass in astronomy, used to describe the masses of other stars and galaxies. It is equal to the mass of the Sun, about two nonillion kilograms or about 332,950 times the mass of the Earth or 1,048 times the mass of Jupiter. For additional information on soalr mass see Wikipedia
» Density - measured in grams per cubic centimeter (or kilograms per liter): the density of water is 1.0 iron is 7.9 lead is 11.3. A typical neutron star has a mass between 1.4 and 5 times that of the Sun. It cannot be more massive than this or gravity will overwhelm it and it will become a black hole! The radius of a neutron star may be between 10 and 20 kilometers. [3] The surface and core density of a neutron star has often been stated via anology — a teaspoon full of it's core material would weigh between a billion and ten billion tonnes it's surface material would weigh a million tonnes plus.
» Gravitational Collapse - is ( for purposes of this section ) the implosion of a star or other stellar object under the influence of its own gravity. The resulting object is many times smaller and denser than the original body from which it was formed.
» Implosion - in a star, implosion is the result caused by the sudden stop of it's fusion process thereby causing a violent collapse inward of it's layers towards the iron core.
» The Chandrasekhar limit - is a law which sets the maximum limit possible for a nonrotating mass which can be supported against gravitational collapse by electron degeneracy pressure. It was named after Subrahmanyan Chandrasekhar, shared winner of the 1983 Nobel Prize in physics for his work on the theory of white dwarf stars. For additional reference see Chandrasekhar limit at the Free Dictionary website. » Electron Degeneracy Pressure - Electron degeneracy pressure is a consequence of the Pauli exclusion principle, which states that two fermions cannot occupy the same quantum state at the same time. The force provided by this pressure sets a limit on how much matter can be squeezed together without it collapsing into a neutron star or black hole. It is an important factor in stellar physics because it is responsible for the existence of white dwarfs. For additional reference see Wikipedia
» Type II, Type Ib or Type Ic Supernova - also called core-collapse supernovas. A star is formed of layers of different elements with the outer layers of hydrogen, helium, carbon, and silicon burning around an iron core, building it up. Eventually, the massive iron core succumbs to gravity and it collapses to form a neutron star. The outer layers of the star fall in and bounce off the neutron core which creates a shock wave that blows the outer layer outward. This is the supernova explosion. [4]
» Black Hole - an area of space-time with a gravitational field so intense that its escape velocity is equal to or exceeds the speed of light. For additional study of black holes see Black Hole at the High Energy Astrophysics Science Archive Research Center (HEASARC) which is the data archive repository for extremely energetic phenomena, from black holes to the Big Bang.
» Spin, Rotation & Effects - a neutron's high axial or off-axial spin is based on the law of conservation of momentum. The neutron star, prior to it's supernova demise, rotates at X times per cycle. After it loses it layers, only the core survives, a much small object, and now that same rotation cycle is much reduced , the rate of spin is much increased as a result ( e.g. If a solar-size star with a 100 day spin period collapses into a neutron star, its spin period will become about 1 ms ). The effects: as most young neutron stars are spinning very fast, the strong magnetic fields combined with rapid rotation create an awesome generator that can produce electric potential differences of quadrillions of volts. Such voltages, which are 30 million times greater than those of lightning bolts, create deadly blizzards of high-energy particles. [5]
» Types, Classification - types of neutron stars include: Radio-quiet neutron stars & radio loud neutron stars, Rotation-powered pulsar, Magnetar, Soft gamma repeater, Anomalous X-ray pulsar, Binary pulsars, Low-mass X-ray binaries ( LMXB ), Intermediate-mass X-ray binaries ( IMXB ), High-mass X-ray binaries ( HMXB ), Accretion-powered pulsar ( "X-ray pulsar" ), X-ray burster ( a neutron star with a low mass binary companion from which matter is accreted resulting in irregular bursts of energy from the surface of the neutron star ) and Millisecond pulsar ( MSP ). This list excludes all hypothetical types for which no evidence of their existence is currently ( 2009 ) at hand. For more details on this listing of neutron star types see Wikipedia
NEW ! Starting with this edition, the following "You Should Know" category is introduced:

» Astronomers, Astrophysicists & Researchers - the following are just a few of the individuals historically associated with the stellar object under study and include professional or amateur astronomers, scientists, astrophysicists and researchers the world over.
Jocelyn Bell Burnell Fritz Zwicky * Walter Baade * Rudolf Minkowski ** William Parsons

* The Columbia Encyclopedia, Sixth Edition. 2008. Encyclopedia.com. 13 May. 2009.
** The Bruce Medalists by Joseph S. Tenn Photo of Rudolf Minkowski courtesy Astronomical Society of the Pacific.

## Fact or Fiction?: The Explosive Death of Eta Carinae Will Cause a Mass Extinction

When we think about &ldquoexistential&rdquo threats, things that could potentially end the lives of everyone on Earth, most of the possibilities come from right here on our own planet&mdashclimate change, global pandemics and atomic warfare. Turning a paranoid gaze to the skies, we typically worry about asteroid strikes or perhaps some perilously massive burp from our sun.

But if you trust everything you read on the fringe regions of the internet, you may think the most fearsome heavenly threat may not only be extraterrestrial, but also extrasolar. Some 7,500 light-years away in the constellation of Carina a star called Eta Carinae, at least a hundred times more massive than our own sun, is approaching the point where it will detonate as a supernova. Simply put, Eta Carinae is a supermassive stellar powder keg nearing the end of its fuse. It could, in fact, already have met its doom, and the light bearing news of its cataclysmic death could be streaming toward us even now. Whenever that luminal death rattle arrives, tomorrow or tens of thousands of years in the future, there are two general sets of opinions about what would happen next.

The first opinion, held by various online alarmists I shall not indulge by linking here, holds that there would be a global mass extinction. This idea plays on fears that Eta Carinae&rsquos supernova could unleash a gamma-ray burst (GRB), one of the brightest explosions in the universe. When a very massive star dies in a supernova, its core collapses in on itself, typically forming a stellar remnant, a neutron star or a black hole. If the core is spinning very fast, the stellar remnant will be spinning even faster, whipping a disk of material around its edges at nearly light-speed. Through processes still not fully understood this superheated and magnetized whirling disk then forms a pair of jets, like lighthouse beams, that blast out from its poles at relativistic speeds. The highly focused, extremely energetic emission from those jets is what we see as a GRB.

Over the years GRBs have been proposed as one of the reasons why we seem to be so alone in the universe&mdashsooner or later, the thinking goes, most any inhabited planet will be struck by a GRB, blasting any biosphere practically to oblivion. And some researchers have speculated that one might have already hit Earth, at the end of the Ordovician period nearly 450 million years ago. Whatever did happen way back then, it managed to exterminate more than an estimated 80 percent of all species living at the time. It could be that even more GRBs hit our planet far earlier in its life, stifling the emergence of Earth&rsquos biosphere until their cosmic prevalence fell below some critical threshold.

According to a somewhat plausible worst-case scenario, a direct hit by an extremely bright GRB generated by Eta Carinae could devastate our planet in a manner similar to but far worse than full-scale thermonuclear war. For several searing seconds, the planetary hemisphere facing the faraway star would be bathed in intense high-frequency radiation. The skies would fill with light much brighter than the sun, bright enough to ignite enormous continent-scouring wildfires on half the globe. The energetic burst of light would kick off atmospheric showers of highly penetrating radioactive subatomic particles called muons, which would stream down to poison life on the surface as well as that some distance underground and underwater. Even the far side of the planet facing away from Eta Carinae would not be spared, as the GRB&rsquos intense energy would destroy the entire ozone layer while also sending superstorms rippling around the world. In the aftermath blackened, soot-filled skies would unleash torrents of acid rain, clearing only to soak the surface with damaging ultraviolet radiation. In a literal flash the Earth would become a planetary charnel house, and the shattered biosphere would require millions of years to piece itself back together.

The second opinion, held by most astrophysicists, is that Eta Carinae won&rsquot produce a GRB at all&mdashand if it did, it wouldn&rsquot hit Earth. And even in a scenario where our planet did find itself in the crosshairs of a GRB from Eta Carinae, if the burst was of average brightness, its light would be too attenuated across 7,500 light-years to seriously harm the biosphere. In this scenario Eta Carinae&rsquos demise would manifest as scarcely more than the star brightening to approach the luminosity of the full moon before gradually fading in the sky.

To understand how this stark divergence in opinion can exist it helps to know a bit more about Eta Carinae. Since first being catalogued by Edmond Halley in 1677, the star has fluctuated wildly in brightness, peaking in 1843 to become the second-brightest star in the sky for some two decades. Astronomers now consider that event a &ldquosupernova impostor&rdquo&mdashinstead of blowing apart the star ejected perhaps 10 percent of its total mass as two huge clouds of gas and dust, which is now known as the Homunculus Nebula. Glowing remnants of even earlier near-death experiences still wreathe the star. Viewed through a large telescope today, the total effect makes Eta Carinae look a bit like a peanut roasting in a fire.

Eta Carinae is shining so brightly that it is eroding itself, generating outward radiation pressure so intense that it almost counteracts the inward pull of gravity, sending its outer layers slowly streaming away on powerful stellar winds. Deep inside the star, below a thick outer envelope of hydrogen, fusion reactions are &ldquoburning&rdquo a variety of nuclear fuels in layers akin to those inside an onion. Eta Carinae&rsquos past outbursts and pulsations are probably linked to instabilities between its inner layers created when it exhausted one nuclear fuel and transitioned to another.

Alex Filippenko, an astrophysicist at the University of California, Berkeley, says Eta Carinae&rsquos massive envelope of hydrogen and strong stellar winds both reduce the likelihood of the star producing a GRB. &ldquoA thick hydrogen shell makes it difficult for a relativistic jet to pummel its way out of the star,&rdquo Filippenko says. &ldquoBut if Eta Carinae doesn&rsquot explode until quite a long time from now, there would be enough time to get rid of the outer shell, and it would then be more likely to become a GRB.&rdquo Except, he adds, once the outer shell is gone, the stellar winds would likely increase in strength, dissipating much of the angular momentum that would be required to spin up a GRB when Eta Carinae&rsquos core collapses. &ldquoAll this makes a GRB less likely, but not impossible,&rdquo Filippenko says. &ldquoAnd even if it gets rid of its hydrogen shell prior to exploding and does become a GRB, [Eta Carinae] is probably not pointing at us right now.&rdquo The twin lobes of Eta Carinae&rsquos Homunculus Nebula are tilted away from us at an angle of about 40 degrees whereas Filippenko says a GRB emerging from a collapsing star&rsquos polar axis would have a spread of about 10 degrees or less. So if the Homunculus Nebula is aligned with Eta Carinae&rsquos polar axis, an emitted GRB would miss our solar system by a very wide margin.

Unfortunately, there is one major complication to this picture: Astronomers discovered in 2005 that Eta Carinae is actually a binary system with a relatively small companion of &ldquoonly&rdquo 30 times the mass of our sun in an approximately five-year orbit around the 100-solar-mass star. If the smaller companion doesn&rsquot orbit in alignment with the more massive star&rsquos rotational axis, then the Homunculus Nebula might not be aligned with the massive star&rsquos poles. And, conceivably, gravitational interactions between the two stars, or with another passing star, could shift the orientation of the more massive star&rsquos axis, potentially aiming it right at us. Finally, the presence of the companion star could also alter how the more massive star evolves, throwing more uncertainty into the timing and mechanics of any eventual supernova.

Piled one atop the other, all those variables are in large part why Eta Carinae is &ldquoour biggest embarrassment today,&rdquo says Stan Woosley, an astrophysicist at the University of California, Santa Cruz, who specializes in modeling the evolution and death of stars. &ldquoNo one knows just what&rsquos going on there&hellipIt could die tomorrow or a long time from now.&rdquo

Some of what happens next depends on which nuclear fuel is currently dominant inside Eta Carinae. If it is fusing elements such as oxygen or carbon in or near its core, it may only have years to live, centuries at most, and could soon eject its outer envelope of hydrogen. If its core is instead fusing helium, the star could potentially shine on for a few hundred thousand years more. Alternatively, helium fusion could cause Eta Carinae to swell up like a balloon to become a supergiant star, in which case its smaller companion star might enter and disrupt the outer hydrogen envelope, hastening the supergiant&rsquos explosive death.

Once the star dies, Woosley says, its core will likely collapse to form a black hole, although one rotating too slowly to make a relativistic disk and a GRB. Without the creation of such a disk the death of Eta Carinae could be &ldquoparticularly unspectacular,&rdquo failing to even produce a supernova as the star&rsquos remnants simply slip behind the black hole&rsquos event horizon.

&ldquoSometimes I wonder if Eta Carinae already has,&rdquo Woosley says. &ldquoBut people tell me they can still see the star.&rdquo

## Collapse and Explosion

When the collapse of a high-mass star’s core is stopped by degenerate neutrons, the core is saved from further destruction, but it turns out that the rest of the star is literally blown apart. Here’s how it happens.

The collapse that takes place when electrons are absorbed into the nuclei is very rapid. In less than a second, a core with a mass of about 1 MSun, which originally was approximately the size of Earth, collapses to a diameter of less than 20 kilometers. The speed with which material falls inward reaches one-fourth the speed of light. The collapse halts only when the density of the core exceeds the density of an atomic nucleus (which is the densest form of matter we know). A typical neutron star is so compressed that to duplicate its density, we would have to squeeze all the people in the world into a single sugar cube! This would give us one sugar cube’s worth (one cubic centimeter’s worth) of a neutron star.

The neutron degenerate core strongly resists further compression, abruptly halting the collapse. The shock of the sudden jolt initiates a shock wave that starts to propagate outward. However, this shock alone is not enough to create a star explosion. The energy produced by the outflowing matter is quickly absorbed by atomic nuclei in the dense, overlying layers of gas, where it breaks up the nuclei into individual neutrons and protons.

Our understanding of nuclear processes indicates (as we mentioned above) that each time an electron and a proton in the star’s core merge to make a neutron, the merger releases a neutrino. These ghostly subatomic particles, introduced in The Sun: A Nuclear Powerhouse, carry away some of the nuclear energy. It is their presence that launches the final disastrous explosion of the star. The total energy contained in the neutrino s is huge. In the initial second of the star’s explosion, the power carried by the neutrinos (10 46 watts) is greater than the power put out by all the stars in over a billion galaxies.

While neutrinos ordinarily do not interact very much with ordinary matter (we earlier accused them of being downright antisocial), matter near the center of a collapsing star is so dense that the neutrinos do interact with it to some degree. They deposit some of this energy in the layers of the star just outside the core. This huge, sudden input of energy reverses the infall of these layers and drives them explosively outward. Most of the mass of the star (apart from that which went into the neutron star in the core) is then ejected outward into space. As we saw earlier, such an explosion requires a star of at least 8 MSun, and the neutron star can have a mass of at most 3 MSun. Consequently, at least five times the mass of our Sun is ejected into space in each such explosive event!

The resulting explosion is called a supernova ([link]). When these explosions happen close by, they can be among the most spectacular celestial events, as we will discuss in the next section. (Actually, there are at least two different types of supernova explosions: the kind we have been describing, which is the collapse of a massive star, is called, for historical reasons, a type II supernova. We will describe how the types differ later in this chapter).

Figure 2. The arrows in the top row of images point to the supernovae. The bottom row shows the host galaxies before or after the stars exploded. Each of these supernovae exploded between 3.5 and 10 billion years ago. Note that the supernovae when they first explode can be as bright as an entire galaxy. (credit: modification of work by NASA, ESA, and A. Riess (STScI))

[link] summarizes the discussion so far about what happens to stars and substellar objects of different initial masses at the ends of their lives. Like so much of our scientific understanding, this list represents a progress report: it is the best we can do with our present models and observations. The mass limits corresponding to various outcomes may change somewhat as models are improved. There is much we do not yet understand about the details of what happens when stars die.

The Ultimate Fate of Stars and Substellar Objects with Different Masses
Initial Mass (Mass of Sun = 1) 1 Final State at the End of Its Life
< 0.01 Planet
0.01 to 0.08 Brown dwarf
0.08 to 0.25 White dwarf made mostly of helium
0.25 to 8 White dwarf made mostly of carbon and oxygen
8 to 10 White dwarf made of oxygen, neon, and magnesium
10 to 40 Supernova explosion that leaves a neutron star
> 40 Supernova explosion that leaves a black hole

## Betelgeuse and biological time: Who’s afraid of a supernova!?

What if a second “Sun,” suddenly ignited in the afternoon sky? Or an intrusive light-source, started vying with the gibbous Moon – flaring out, from where a familiar red star once twinkled? That would be a supernova explosion—a seminal cosmic event—erupting in Orion constellation, where the bright star Betelgeuse is being blasted into oblivion.

Dazzling Orion the Hunter, with its three brilliant—and strikingly slanted—“Belt Stars,” is the most easily recognizable of the International Astronomical Union’s 88 official constellations (star-groups with distinctive patterns). Betelgeuse beams leftward, just above the Belt.

Orion’s iconic “shoulder,” has been getting lots of attention. A “Betelgeuse Workshop” was held in Paris, as far back as 2012. But global excitement reached a fever pitch last December, when astronomers found that the fabled red supergiant was dimming lopsidedly and shrinking.

Betelgeuse has now stopped dimming: And could reclaim its status, as the night sky’s 10th brightest star! Even so, scientists insist that it must blow eventually: Any time, from this minute to the next 100,000 years. When it does, they predict one hell-of-a light show—viewable worldwide, day and night, and lasting for months!

The problem though, is that such a dramatic celestial display, might unsettle many ill-informed Nigerians. In some cases, feelings of fear and dread could, conceivably, morph into a dangerous “End-Time” delusion: An apocalyptic and potentially suicidal mindset.

“How you react,” stresses Dr. Ahmad Shaba, Director, Strategic Space Applications, at the National Space Research and Development Agency (NASRDA), “has a lot to do with your personal history—with what you’ve read, the sermons and lectures you’ve heard and the movies you’ve seen.”

In short, he counsels, coping with the eruption would depend “on whether you’ve learned enough astronomy to avoid a mental meltdown. One should also be able to fend off material and emotional predators. Otherwise, you could lose lots of money—and possibly your life.”

Shaba is adding NASRDA’s voice, to a swelling cautionary chorus, insisting that Betelgeuse’s plight is not “spiritual”. Nor are supernovae, as such, rare. Thousands have been recorded, since 1885. “Today,” Wikipedia reports, “amateur and professional astronomers are finding several hundred every year…”

“What then,” you are bound to ask, “is the big deal about Betelgeuse?” Well, unlike other recent progenitors, it belongs to the Milky Way Galaxy (our Sun’s star group), which averages only two per century—and is, as Dieter Hartmann put it to Sky & Telescope, “long overdue for its next supernova”.

The last naked-eye burst in our galaxy, dates back to 1680. Another, the historic SN 1987A, occurred 33 years ago, in the Large Magellanic Cloud—a satellite galaxy of the Milky Way, 168,000 lightyears out. A Betelgeuse blast would be comparatively close and, for the first time, astronomers could watch, from start to finish.

Why study stellar explosions? The universe (all the energy, matter, space and time that exists) is a vast, and complex, system of interrelated units—which includes you, me and the planet we inhabit! So, the more we learn about the universe, the better we understand ourselves.

Look around! Everything you see, is made of atoms—most of which, stars bequeathed. As Michigan State University Physicists, Artemis Spyrou and Hendrik Schatz aver, in The Conversation, “Element by element nuclear processes in stars take…hydrogen atoms and build heavier elements.”

Big stars, are munificent chemical donors—since they can fuse elements heavier than helium. They’ve richly endowed Earth, with industrially useful and biogenic substances. The latter, not incidentally, consist of chemical components, for the bio-contrivance that winks and smiles at you in the mirror!

The American Association of Variable Star Observers notes that visible light makes up only 13 per cent of the radiation Betelgeuse emits. Yet it is seeable enough, to have been mythicized as the “shoulder” of Orion in Europe, a “fierce lion” among the Xhosa of South Africa and a “fiery club” in native Australian lore.

This high celestial profile, belies the 700 lightyears that separate us from Betelgeuse —keeping in mind, that just one lightyear (the distance light travels in 12 months, at 300,000 km per second) amounts to roughly 10 trillion km!

The upshot, is that Alpha Orionis (to use its Latin appellation), has awesome bulk properties. More mundanely: Betelgeuse is a bee-ig mammy-tappy! An illustration of the Atacama Large Millimeter Array (ALMA), depicts a behemoth that would—if it replaced the Sun—engulf all the planets, out to Jupiter!

ALMA’s diagram, visualizes jaw-dropping stats: Betelgeuse has 20 times the Sun’s mass (i.e., 20 “solar masses”) and 1,500 times its diameter (the Sun itself, being 1.4 million km wide!). Thanks to this huge radiating surface, its peak brightness is 14,000 solar luminosities!

Betelgeuse originated as a contracting clump of gas, in the Orion Molecular Cloud, which (like countless similar clumps) heated up to 10 million kelvins (K)—and began nuclear fusion. It was born into an age-group of big blue stars—the Orion OB1 Association—with surface temperatures of up to 30,000 K.

But, Wikipedia recounts, Betelgeuse was later ousted from the Association, under pressure from exploding supernovae. It is now a “runaway star,” racing through the interstellar medium (the space between stars), at 30 km per second.

As it careers through the cosmos, a melodrama is unfurling in Betelgeuse’s core. Two physical forces—Heat and Gravity—are locked in a back-and-forth tussle, a “You push me, I push you” scenario. “Heat” heaves stellar mass outward, against “gravity’s” ceaseless tug towards the centre.

“The simple model,” explains an outreach posting of the Australian Telescope National Facility, “…is of a dense gas/fluid in a state of hydrostatic equilibrium. The inward acting force, gravity, is balanced by outward acting forces of gas pressure and the radiation pressure”.

Gravity will, in time, prevail—collapsing Betelgeuse into a stellar corpse, called a neutron star. Yet the alluring reds, yellows, oranges, blues and whites of the dark sky, indicate that heat is, heroically, holding its own, at least in the interim.

Otherwise, we wouldn’t see a firmament of colours. The myriad tints and hues of stars, signify varying intensities of heat being radiated from their surfaces. Stellar colours, send the same quantitative message, as the flames in your cooker: Blue is hottest and red coolest, with gradations in between.

But whereas your cooker sources its fuel externally, Betelgeuse generates its energy internally, through nucleosynthesis. In the hydrogen-fuel phase, four protons (hydrogen nuclei) are fused into a helium nucleus (alpha particle). The reaction transforms 0.7 per cent of the protons’ mass into energy.

Hydrogen consumption, is the longest period in stellar life. The famous Hertzsprung-Russel (H-R) diagram thus depicts this phase, graphically, as the “main sequence” —an oblique reference to the habit stars have, of fusing progressively heavier nuclei, in rapid succession, after a sustained diet of hydrogen.

Big stars burn their fuel faster than small ones. Hence Betelgeuse is in crisis, after just 10 million years—while our Sun (a dwarf star) has five billion years remaining, in a lifespan of 10 billion. Betelgeuse left the main sequence one million years ago, theory holds, and has been a red supergiant for 40,000 years.

Gone are the “O-B” days. Bloated, and radiating at a clement 3,950 K., Betelgeuse is now relegated to spectral class “M”, on a scale that ranks stars, from hot to cool, “O,” “B,” “A,” “F,” “G,” “K” or “M”. (You can use this mnemonic, to remember the scale: “Oh Be A Fine Girl (or Guy) Kiss Me!”)

Yet as early as 1923, the famous Cambridge astronomer, Arthur S. Eddington, apprised readers of Scientific Monthly that radiation from the surface of Betelgeuse, “is just the marginal temperature of the furnace, affording us no idea of the terrific heat within.”

A NASA table (Imagine The Universe), delineates enormous fusion temperatures: It takes 0.8 billion K to forge neon and magnesium from carbon 1.5 billion K to fuse neon into oxygen and magnesium 2.0 billion K for Oxygen to yield silicon and sulfur and 3.3 billion K to meld silicon nuclei into iron.

“Iron,” is the endgame in stellar evolution—beyond which, the fusion of lighter nuclei into heavier ones, stalls. That’s because, in the accepted model, iron nucleons are bound so tightly, that fusing them consumes, rather than releases, energy.

Betelgeuse’s fate, is therein foreshadowed. No fusion, no heat—and no outward push, again. So, “gravity” gets a walkover win: And, in celebration, sends all the progenitor’s mass crashing inward, onto its core, at a quarter of the speed of light!

“As a result,” says NASA, “an explosive shock wave travels out from the core…and accelerates the surrounding layers. In addition, …energy from… neutrinos (nearly massless elementary particles) cause (most) of the star’s mass to be blown off into space…. Astronomers refer to this as a Type II supernova.”

The visual effect, may evoke “End-Time” fears in some. But, in reality, supernovae kickstart biological time! They not only blast heavy atoms (iron, gold, tin, lead, etc.) into space, but also carbon, oxygen, nitrogen, sulfur and phosphorus (biogenic elements): To be recycled, into new planetary systems.

Thus, Priscilla Long’s panegyric, in The American Scholar, aptly strokes supernovae: “From where did we get the iron in our blood, the carbon in our cells, the oxygen in our lungs? From exploding stars, that’s where. We may or may not be starry-eyed, but we are all part star”.

Obatala is Nigeria’s best-known amateur astronomer—having written a weekly column, in The Guardian, for 16 years (until 2017). He gives public lectures on the subject, and is an External Advisor to the National Space Research and Development Agency (NASRDA).