Tuesday, October 18, 2016

Supernovae PART 4: What Happens to Super-Massive Stars?

For Supernovae PART 1: Introduction, click here.
For Supernovae PART 2: Low Mass Stars, click here.
For Supernovae PART 3: Massive Stars, click here.


As we've seen in PART 3, stars with 10 - 50 solar masses tend to end their lives much more violently than less massive faint supernova stars do, but the process is essentially the same. In the cores of these stars, there is enough gravitational pressure for fusion to smoothly continue as neon, oxygen and eventually silicon fuse. If a core temperature of about 3 GK is reached, silicon and sulphur fuse with alpha particles released by the photodisintegration of these and other elements, eventually creating, in a step-wise nucleus-building fusion process, nickel-56. Of all the elements, iron-58 and nickel-62 have the highest binding energy per nucleus. Fusion into the slightly smaller nucleus of nickel-56, produced when iron-52 fuses with an alpha particle, is the first reaction that consumes energy rather than releases it. This means that fusion stops at nickel-56. It is stellar ash. Nuclei larger than nickel-62 release energy when they split apart (nuclear fission) rather than when they fuse. Although the onion diagram we saw in PART 3 shows an innermost core of iron, nickel-56 is the last fusion product in any stellar core, regardless of stellar mass. Any further fusion consumes energy so fusion abruptly stops and the core collapses. Meteorites and rocky planets contain significant iron-56. This is the radioactive decay product of unstable nickel-56.

In 10 - 50 M stars, like all stars, the outward pressure of nuclear fusion initially keeps matter in its ordinary atomic state (the main-sequence phase). As fuel is consumed and fusion eventually slows down, matter is squeezed into an electron degenerate state. Then, electron degeneracy pressure can no longer hold up the core, which exceeds the Chandrasekhar limit, a core mass of approximately 1.4 M. By this point, the star is on its way to an iron-core collapse. Electron degeneracy is overcome and the inner core implodes explosively, over a course of just a few seconds. The outer core follows inward, reaching a velocity of almost  the speed of light. The core temperature spikes and electron capture proceeds rapidly, transforming the inner core into a neutron star. Neutron degeneracy pressure acts like a wall, as mentioned, creating a shock wave that redirects the implosion outward. In stars in this range the shockwave is more intense, resulting in a typical Type II-P supernova. Most of this energy comes from a 10-second blast of thermal neutrinos. These particles are much more abundant than the electron capture neutrinos formed earlier. Thermal neutrinos are formed at about 100 billion K as part of the pair production of neutrino/antineutrino pairs (in all flavours). With a whopping combined energy of about 1046 joules, about 10% of the star's mass is carried off in a thermal neutrino blast. This is the explosive energy of the supernova. To get an idea of just how much energy 1044 joules is, check out this energy scale page and keep scrolling down. Of course, neutrinos are invisible. Through some process that isn't yet understood, most of the blast energy (1044 joules) must be reabsorbed into the core somehow to produce the intensely bright EM explosion (a lot of which is in the visible range). During the brief microseconds before pressure and temperature dissipate, a cornucopia of elements heavier than iron, including heavy neutron-rich radioactive elements, are fused and become part of the expanding cloud.

Origins of the elements are shown in the helpful periodic table below.

For stars between 40 M and 90 M, some outer material might fall back onto the newly formed neutron star. If the star's core grows massive enough, estimated at between 2 and 3 M, it will overcome neutron degeneracy pressure (the TOV limit) and collapse into a black hole. The inward fallback of matter can reduce the outward kinetic energy of the explosion, so much so that in some cases there is no supernova at all. In some cases, the in-falling matter (particularly if it is rapidly spinning) generates two opposing jets of matter traveling outward at close to light speed, a brief event called a gamma ray burst (GRB) that can last from a few milliseconds to hours, or an exceptionally bright supernova (called a hypernova) can result, or both.

A massive star's mass, along with its metallicity, determines its eventual fate whether it will become a neutron star, black hole or leave behind nothing at all. Some stars will even collapse into a black hole with no accompanying supernova. The screen shot below from Wikipedia's Supernova entry lists possible stellar fates.

This chart showcases the myriad ways in which massive stars can die. How a star dies depends primarily on two factors: how massive it is and of what it's made. Low-metal stars contain very few atoms more massive than helium as they start out on the main sequence of their lives; high-metal stars have a larger but still tiny percentage of heavier atomic nuclei. An enormous range of stars from 8 M up to 250 M can explode, and that explosion can be faint, typical or tremendously bright.

A Note on Gamma Ray Bursts

A gamma ray burst (GRB) is the most energetic event in the universe outside of the Big Bang itself. The most powerful one ever detected was the 2008 GRB (called GRB 080916C). It was 2.5 million times brighter than the brightest supernova ever detected. To offer perspective, a typical Type II-P supernova usually outshines an entire galaxy. For a few seconds in 2008, its light, from 7.8 billion light-years away, was visible even to the naked eye.

Gamma ray bursts appear to accompany the collapses of very massive rapidly spinning stars. They were first discovered in the 1960's when the U.S. detected what they suspected were flashes of radiation from secret Soviet nuclear tests. Researchers began to realize that these randomly located, powerful transient bursts did not accompany any obvious objects or stars in the Milky Way. In fact, none have ever been detected in our galaxy, though they may have occurred in its past. Through a process not well understood, much of the energy (and a significant amount of the mass) of a collapsing massive star converts into gamma radiation released in two tightly focused bilateral jets that can travel across the universe. The 2008 GRB described had an estimated energy output of 9 x 1047 J, about five times more energy than the equivalent of the entire Sun's mass.

Sometimes described as a shot heard clear across the universe, these extremely energetic photons start out as gamma rays and stretch as the universe expands while they travel. Billions of years later, they are detected as much longer, less energetic, but still intense X-ray, radio or even infrared bursts. Although GRB's are fairly frequent events in the universe at large (about one per day is detected), Earth's detectors don't pick up the vast majority of them because we must be lined up with the narrow jet of the GRB in order to detect it.


Pair Instability Supernova

Very massive stars (over 140 M) may be completely destroyed by their powerful supernovae. A process called pair instability leaves nothing, not even a black hole, behind. In the core of such a star, collisions between atomic nuclei and gamma photons are so violent that the new gamma photons (shown as white squiggly lines below) created from those collisions produce new matter/antimatter pairs - electrons and positrons (black and white spheres below) - via a process called pair production. The energy of the gamma photon must be extreme, equivalent to the rest mass of the particle pair (0.511 MeV x 2) in order for this to happen. Each particle pair created drains significant energy from the core as energy is converted into matter. As energy decreases, outward pressure drops and the core begins to collapse.

NASA/CXC/M. Weiss;Wikipedia
The temperature in the massive core is so high by the time it starts this final contraction that runaway fusion reactions generate enough energy to blow entire star into space, sometimes accompanied with a particularly bright GRB. Only a nebula of ionized gases and heavy elements mark the original location of the star.

Pair production reenacts matter creation that occurred just after the Big Bang. At a threshold temperature of about 10 GK, gamma photons convert back and forth into electron/positron pairs in an equilibrium state between energy and matter. Temperature is the average kinetic energy of particles so some gamma rays will be more energetic than average and initiate pair production well before this temperature is reached in the core. There is tremendous core energy in all extremely massive stars, but only stars within a specific stellar mass and metallicity range can undergo pair instability.

Stars below 100 M aren't massive enough to trigger core pair production. They follow one of explosion pathways typical of stars between 40 M and 90 M. Stars between 100 and 140 M are large enough to trigger some pair production but it isn't energetic enough. There aren't enough pairs produced and photons taken out of the core to reduce the outward pressure enough to trigger runaway fusion. Instead the fusion rate is increased just enough to return the star to equilibrium. These stars go through several increased fusion/pair production cycles in a series of pulses, each time losing stellar gas, until the star mass falls below 100 M. Stars between about 140 and 250 M are true pair instability stars. In these stars, more thermonuclear energy is released than the entire star's gravitational binding energy. The entire star is ripped apart in what can be an exceptionally powerful supernova (a hypernova). Not all of its matter is converted into radiation, however. Up to dozens of solar masses of unstable nickel-56 can be blown away from the core, which decays into cobalt-56 and then into stable iron-56, contributing to an iron-rich stellar nebula. Solar material can also be sprayed across the universe in a powerful GRB as well. The radiation contributed by various heavy nuclei decay reactions could make such a supernova exceptionally luminous and long-lived as well.

Stars Over 300 M

The most massive known star is R136a1. Shown in the center of the 2010 near-infrared image below, Hubble Space Telescope resolved it from the R136 concentration of stars, located 156,000 light years away, in 1992.

ESO/P.Crowther/C.J. Evans;Wikipedia
There are many massive stars in this cluster. R136a1 is the most massive, hottest and most luminous of any known star. It would occupy the extreme top left corner of the Hertzsprung-Russell diagram. Formed from an initial mass of 325 M, it has a current mass of 315 M. Less than a million years old, intense EM radiation from this very hot star creates powerful solar winds that strip away its mass at a rate a billion times faster than the Sun. This star is close to the Eddington limit. Calculated using hydrogen plasma, stellar winds from a star any more luminous would quickly strip away its mass before it could evolve further. GRB's and supernovae, by the way, greatly surpass the Eddington limit and that is why they are so brief and mass loss is so intense. R136a1 is still fusing hydrogen in its core, using the CNO cycle because its core is very hot. Significant levels of helium and nitrogen in its spectrum reveal that it is also strongly convective.

Expected to stay on the main sequence for only 1.7 million years, stars such as this live hot and die young. By the time it uses up its hydrogen and evolves into a luminous blue variable star, it will have only about 80 M left. As carbon and oxygen accumulate in its helium core, core temperature will continue to rise and mass loss will increase further. After several hundreds of thousands of years the helium will be used up. Heavier element fusion will last only a few thousand years. Though estimates vary greatly its massive core will collapse and likely trigger a supernova that may or may not leave behind a black hole. Its supernova spectrum will likely classify it as a Type 1c explosion because by the time it explodes, its hydrogen and helium will have long been blown away by stellar winds.

It is expected to have a metallicity similar to the Large Magellanic Cloud in which it exists, about 1/4 that of the Sun. The Large Magellanic Cloud is a gas-rich metal-poor neighbor galaxy full of star-forming nebulae and young populations of stars. Below, enormous R136a1 (bright medium blue) is compared a 0.1 M red dwarf, the Sun (shown incorrectly yellow) and an 8 M blue star (pale blue), all in the main-sequence phase of their lives.

ESO/M. Kornmesser;Wikipedia
Massive Metal-free Stars

Despite the confirmation of R136a, stars over 300 M remain a bit of a mystery. Estimates of how these stars die vary greatly depending on the model used. While it is possible that they could be totally destroyed in pair-instability supernovae like slightly less massive stars, it is also possible that they could end as photodisintegration, rather than pair production, triggers core collapse. This process can be modeled with a massive metal-free star, consisting of hydrogen, helium and a tiny percentage of lithium and beryllium - precisely the kind of first stars to form from the pristine gases and dust left over from the Big Bang. These stars fused the first heavier elements to exist in the universe.

Unlike R136a1, no zero-metallicity stars, stars formed strictly from primordial material left over form the Big Bang, have been directly detected, but theory points to their once existence. Because we look back in time as we peer across the universe, these stars, if they exist, will be located extremely far away at the extreme edge of the visible universe. They may have shone just as the first light from the Big Bang itself was able to stream through space, and space itself would have been just 1/30th of its present size. This would result in extremely red-shifted extremely dim light from once exceptionally bright bluish white behemoths.

This might be what those stars look like today. Not quite what one might expect at first thought.

This infrared image taken by the Spitzer Space Telescope in 2005 has all the stars, galaxies and artifacts greyed out and the background enhanced to reveal a glow not attributed to present stars or galaxies and not attributed to cosmic background radiation. Researchers are uncertain but it might be the extremely red-shifted light from the first stars to shine in the universe.

Below, an artist's impression shows what those first stars may have looked like at the time, just 400 million years after the Big Bang. Their light would have been the first light to shine from any object ever.

NASA/WMAP Science team
It is possible that such distinctly metal-free stars would not be visible at all to us today. If these stars were convective, they could dredge up their fusion products from their cores to their surfaces, making them indistinguishable from regular low-metal stars. These stars could have lived their short lives before EM radiation could escape (during the so-called the "dark ages" prior to the recombination period). Interstellar space then was hot dense plasma consisting  of photons, electrons and protons. A process called Thomson scattering made it opaque to all EM radiation. Until the universe expanded and cooled enough for protons and electrons to recombine into neutral atoms, no light, including that from any embedded stars, could shine outward to be detected by us.

Unlike the catastrophic runaway fusion reactions triggered during pair-instability, photodisintegration is an endothermic process for nuclei up to iron (the most tightly bound nucleus). It absorbs energy. And unlike the incomplete photodisintegration process in less massive stars, which knocks off one or two nuclear protons or neutrons, the disintegration in this case should be complete, down to alpha particles and protons. Based on computer modeling, this process should start at about 6 x 109 K.

Most, but not all, modeling suggests that these no-metal stars could have been very massive, perhaps up to 1000 M. Star-forming molecular clouds then would have been much warmer, up to 800 K (525°C) as opposed to cold molecular clouds just a few degrees above absolute zero today. This means that in such an energetic environment a much larger minimum mass would be required to form a star and a much higher Eddington limit could be achieved. These stars would have appeared much like R136a1 - very massive, very hot, very bright and very short lived, burning through their fuel in just a few million years.

Because these stars have no or almost no carbon in them to start with, there would be no carbon to trigger the CNO (carbon/nitrogen/oxygen) fusion cycle. The only fusion reaction available during the main-sequence phase would be the p-p chain reaction of hydrogen fusion. This reaction is much less temperature sensitive than CNO fusion, and this means that it doesn't serve as the built-in thermostat that CNO fusion does. The star's core could therefore get much hotter than it would in stars with metallicity. Eventually it would get hot enough to trigger the triple alpha fusion process and from there fusion would proceed, as carbon is fused from beryllium and helium. Once sufficient carbon is present, the CNO cycle would be triggered as well.

Though fusion would not be runaway, it would be at such a high rate under such enormous gravitational pressure that it would eventually trigger photodisintegration. Gamma photons would be powerful enough to rip apart even the smallest most tightly bound nuclei. In a matter of perhaps minutes, millions of years of core fusion would come undone as gamma photons, more energetic than those in pair instability cores, are absorbed by the core nuclei, causing them to split into alpha particles, free neutrons and free protons. The sudden absorption of free energy (the gamma photons) would trigger catastrophic complete collapse before the core had any opportunity to re-ignite fusion. The core, as a result, would continue to collapse into a massive black hole, with no counter-process to stop it.

If massive low to no-metal stars ended their lives because of photodisintegration, there should be a number of massive black holes left from their destruction. If little or no explosion occurred after the stars collapsed, the black holes should have masses close to the original stars. Unexpectedly bright galaxies (suspected to contain clumps of very massive hot stars) from the period when light first began to shine in the universe are now being detected using a variety of telescopes. Some individual star candidates between 250 and 1000 M have also been indirectly detected. This leaves the mystery of how the young universe seeded itself with nuclei larger than beryllium. A near complete collapse into a massive black hole takes any metals formed right along with it. It is possible that the first stars spewed out their matter in powerful GRB's as they collapsed into black holes (and these most ancient GRB's should be detectable). It is also possible that the first stars were slightly less massive than 250 M, and they might have blown up completely as extremely powerful supernovae instead, spewing out lots of fusion products and leaving behind no trace other than a large radioactive cloud destined to become a star nursery for stars with metallicity.

Despite gas-rich low-metal regions that are still actively making huge R136a-like stars, ancient massive, luminous, hot, metal-free stars could represent the ancient beginnings of a global evolutionary trend toward decreasing stellar mass, as star nurseries in general cool and grow richer in massive elements. The most abundant type of star today is the red dwarf. These stars are so long-lived that if they formed early on in the universe they should still be burning. However, no metal-free red dwarf stars have been observed (although they are dim and small so they would be very difficult to detect at such a great distance). All observed red dwarfs have metal content and almost all of them contain a significant percentage of larger elements, indicating that they were formed in molecular clouds thoroughly seeded with the residues of many previous supernovae. This, along with the fact they are found only in the spiral arms of young galaxies like our own, indicates that red dwarfs tend to be more recently formed than distant and long-dead metal-poor massive stars.


Stars dotting the night sky might seem eternal, but if we could live for millions of years we would bear witness to countless dramatic life and death events. After lives that last anywhere from millions to trillions of years, stars die. Some die peacefully and gradually while others blow up with unimaginable force. Understanding what happens to matter under such atom-ripping conditions is a huge challenge because these forces cannot be replicated on Earth. Gravity crushes atoms in the cores of massive stars into mysteriously dense physical states where the rules of atomic behaviour no longer apply. During some supernovae, matter is crushed completely into a black hole where even the rules of space and time no longer apply. In both cases, new larger atomic nuclei are also created. Scientists are just now able to fuse the largest of these, such as ununoctium (which decays within a millisecond), in the most powerful particle accelerators. It is a job requiring extreme energy, one that is done naturally in microseconds during a supernova.

The universe burst into being almost 14 billion years ago, laced with only the lightest elements, mostly hydrogen along with helium and a tiny amount of lithium that were fused before expansion and cooling halted the process. Gravity shaped that primordial material into stellar fusion reactors that supply the conditions to fuse a far greater variety of elements, up to iron. The synthesis of even heavier elements requires enough energy to overcome a highly endothermic process that absorbs energy rather than releases it. Only in the very brief, chaotic and extremely energetic environment of a supernova can elements such zinc, silver, gold, mercury and lead, as well as heavy unstable elements such as radium, uranium and plutonium come into existence. Furthermore, the explosive process blasts out not only those elements fused in the supernova but those fused earlier inside the star as well. That debris cools into molecular clouds that later become new stars and their planets. Rocky planets like Earth can only form from molecular clouds laced with elements forged in the bellies of massive stars and forged in their violent deaths.