How do the most massive stars die: supernova, hypernova or direct collapse?

Illustration of the process of a supernova explosion observed from Earth in the 17th century in the constellation Cassiopeia. The material surrounding it and the constant emission of electromagnetic radiation played a role in the continuous illumination of the remnants of a star.

Create a massive enough star, and it will not end its days quietly - just as our Sun faces, which will first burn smoothly for billions and billions of years, and then shrink to a white dwarf. Instead, its core will collapse, and trigger an uncontrolled fusion reaction, which disperses the outer layers of the star in a supernova explosion, and squeezes the internal parts into a neutron star or black hole. At least, so considered. But if you take a fairly massive star, the supernova may fail. Instead, there is another possibility - a direct collapse, in which the entire star simply disappears, turning into a black hole. And another possibility is known as hypernova - it is much more energetic and bright than supernova, and does not leave behind the remnants of the nucleus. How do the most massive stars end their lives? This is what science says about it.


A nebula from the remnants of the supernova W49B , still visible in the X-ray range, as well as on radio and infrared waves. A star must exceed the Sun in mass at least by a factor of 8-10 to generate a supernova and create the heavy elements necessary for the appearance of such planets as the Earth in the Universe.

Each star immediately after birth synthesizes helium from hydrogen in its core. Stars like the Sun, red dwarfs, just a few times larger than Jupiter, and supermassive stars that are tens and hundreds of times larger than ours all pass through this first stage of nuclear reactions. The more massive the star, the higher the temperature of its core, and the faster it burns nuclear fuel. When hydrogen runs out in the core of the star, it shrinks and heats up, after which - if it reaches the desired density and temperature - it can begin the synthesis of heavier elements. The sun-like stars will be able to warm up enough after the hydrogen fuel runs out, and start the synthesis of carbon from helium, but this stage will be the last for our Sun. To go to the next level, the synthesis of carbon, the star must exceed the sun by mass of 8 (or more) times.


The ultramassive star WR 124 (a Wolf-Rayet star ) with its surrounding nebula is one of thousands of Milky Way stars that can become the next supernova. It is also much larger and more massive than those stars that can be created in the Universe containing only hydrogen and helium, and can already be at the stage of burning carbon.

If the star is so massive, then it is waiting for a real space fireworks. Unlike the sun-like stars, gently tearing off their upper layers, from which a planetary nebula is formed, and contracting to a white dwarf rich in carbon and oxygen, or to a red dwarf that will never reach the stage of burning helium, and simply squeezing to a white dwarf rich in helium , the most massive stars prepared a real cataclysm. Most often, especially in stars with not the largest mass (≈ 20 solar masses and less), the core temperature continues to rise until the synthesis process moves to heavier elements: from carbon to oxygen and / or neon, and then, according to the periodic table , to magnesium, silicon, sulfur, coming as a result to iron, cobalt and nickel. The synthesis of further elements would require more energy than is released during the reaction, so the core collapses and a supernova appears.


Anatomy of a supermassive star during its life, ending with a type II supernova

This is a very bright and colorful ending, overtaking many massive stars in the universe. Of all the stars that have appeared in it, only 1% gain enough mass to reach such a state. As the mass increases, the number of stars that reach it decreases. About 80% of all stars in the universe are red dwarfs; the mass of 40% of them does not exceed the mass of the Sun. At the same time, the Sun is more massive than 95% of stars in the Universe. The night sky is full of very bright stars: those that are easiest to see for a person. But beyond the threshold of the lower limit for the emergence of a supernova, there are stars that exceed the sun in mass by tens and even hundreds of times. They are very rare, but very important for space - all because massive stars can end their existence not only in the form of a supernova.


The Bubble Nebula is located on the outskirts of the remains of a supernova that appeared thousands of years ago. If remote supernovae are in a dustier environment than their modern counterparts, this will require a correction to our current understanding of dark energy.

First, many massive stars have flowing streams and ejected material. Over time, when they are approaching either the end of their life, or the end of one of the stages of the synthesis, something causes the nucleus to shrink for a short time, due to which it heats up. When the core becomes hotter, the speed of all types of nuclear reactions increases, which leads to a rapid increase in the amount of energy created in the core of the star. This increase in energy can emit a large amount of mass, giving rise to a phenomenon known as pseudo-supernovae : a flash occurs brighter than any normal star, and the mass is lost in the amount of up to ten solar. Eta Carina's star (below) became a pseudo-supernova in the 19th century, but inside the nebula she created she still burns, waiting for the final fate.


The 19th century pseudo supernova revealed itself in the form of a gigantic explosion, throwing material from several suns into the interstellar space from Eta Carina. Such stars of large mass in metal- rich galaxies (like ours, for example) emit a significant fraction of their mass, which is different from stars in smaller galaxies containing less metals.

So what is the ultimate fate of stars weighing more than 20 times the size of our Sun? They have three possibilities, and we are not yet completely sure what conditions exactly lead to the development of each of the three. One of them is supernovae, which we have already discussed. Any ultramassive star losing a lot of its mass can turn into a supernova if its mass suddenly falls within the correct limits. But there are two more gaps of the masses - and again, we do not know exactly which masses are exactly - allowing two other events to occur. Both of these events definitely exist - we have already seen them.


Photographs in visible and near-infrared light from Hubble show a massive star, about 25 times the Sun in mass, suddenly disappeared, and left no supernova or any other explanation. The only reasonable explanation would be a direct collapse.

Black holes direct collapse. When a star turns into a supernova, its core collapses and can become either a neutron star or a black hole, depending on the mass. But only last year, for the first time, astronomers observed how a star with a mass of 25 solar simply disappeared. The stars do not disappear without a trace, but a physical explanation exists for what could have happened: the core of the star ceased to create sufficient radiation pressure to balance the gravitational contraction. If the central region becomes sufficiently dense, that is, if a sufficiently large mass is compressed into a sufficiently small volume, an event horizon is formed and a black hole arises. And after the appearance of a black hole, everything else just gets pulled inside.


One of the many clusters in this region is highlighted by massive, short-lived blue stars. In just 10 million years, most of the most massive stars will explode, becoming Type II supernovae - or simply experience a direct collapse.

The theoretical possibility of direct collapse was predicted for very massive stars, more than 200-250 solar masses. But the recent disappearance of a star of such a relatively small mass has put the theory in question. Perhaps we are not so well aware of the internal processes of stellar nuclei, as we thought, and perhaps the star has several ways to simply collapse completely and disappear, without dumping some tangible amount of mass. In this case, the formation of black holes through direct collapse can be a much more frequent phenomenon than was thought, and this can be a very convenient way for the Universe to create supermassive black holes at the earliest stages of development. But there is another result, quite the opposite: a light show, much more colorful than a supernova.


Under certain conditions, the star may explode so that it does not leave anything behind!

Hypernova explosion. Also known as super-bright supernova. Such events are much brighter and give completely different light curves (a sequence of raising and lowering brightness) than any supernovae. The leading explanation for the phenomenon is known as the " pairwise unstable supernova ". When a large mass - hundreds, thousands and even many millions of times more than the mass of our entire planet - collapses into a small volume, a huge amount of energy is released. Theoretically, if the star is massive enough, of the order of 100 solar masses, the energy released by it will be so large that individual photons can begin to turn into electron-positron pairs. Everything is clear with electrons, but the positrons are their counterparts from antimatter, and they have their own characteristics.


The diagram shows the process of producing steam, which, according to astronomers, led to the emergence of the hypernova SN 2006gy . When photons of sufficiently high energy appear, electron-positron pairs will appear, which will cause the pressure to drop and an uncontrollable reaction will begin, destroying the star.

If there are a large number of positrons, they will begin to collide with any available electrons. These collisions will lead to their annihilation and the appearance of two photons of gamma radiation of a certain, high energy. If the rate of appearance of positrons (and, therefore, gamma rays) is low enough, the core of the star remains stable. But if the speed increases strongly enough, these photons, with energies greater than 511 keV, will warm up the nucleus. That is, if you start the production of electron-positron pairs in a collapsing core, their production rate will grow faster and faster, which will warm up the core even more! Infinitely, this cannot continue - as a result, it will lead to the appearance of the most spectacular supernova of all: a pair-unstable supernova, in which the entire star explodes in a mass of more than 100 suns!



This means that for a supermassive star there are four options for the development of events:

• Low-mass supernovae produce a neutron star and gas.
• Higher mass supernovae generate a black hole and gas.
• As a result of direct collapse, massive stars produce a massive black hole without any other remnants.
• After the explosion of a hypernova, only gas remains.

On the left, the artist’s illustration of the viscera of a massive star that burns silicon and is in the final stages preceding a supernova. On the right, an image from the Chandra telescope showing remnants of the supernova Cassiopea A shows the presence of such elements as iron (blue), sulfur (green) and magnesium (red). But this result was not necessarily inevitable.

When studying a very massive star, it is tempting to assume that it will become a supernova, after which a black hole or neutron star will remain. But in fact, there are two more possible scenarios that have already been observed, and which occur quite often by cosmic standards. Scientists are still working to understand when and under what conditions each of these events occurs, but they actually occur. Next time, considering a star many times larger than the Sun in mass and size, do not think that a supernova will become the inevitable result. In such objects there is still a lot of life, and many options for their death. We know that our observable Universe began with an explosion. In the case of the most massive stars, we are still not sure whether they will end their lives with an explosion, destroying themselves entirely, or with a quiet collapse, fully collapsing into the gravitational abyss of emptiness.