The ultimate fate of a star depends on its mass, and various stages in the deaths of stars of various masses are important in the production of the elements, other than hydrogen and helium, that form the parts of the universe interesting to us. Also, since most stars are members of multiple-star systems, the interaction between dying stars in the same system is extremely important in the formation of various nuclei. The most exotic remnants of dead stars are the black hole and the neutron star. Neutron stars are tough to describe in detail because we do not currently know enough about the nuclear equation of state to get a detailed understanding of their composition. Important processes include nucleosynthesis during the first 3 minutes of the universe, nucleosynthesis during the normal life cycle of stars, explosive nucleosynthesis, neutron star mergers, possibly Black Hole accretion disk nucleosynthesis, and cosmic ray spallation.
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When the entire universe was about a millionth of a second old, it had a temperature and density similar to what we call a quark-gluon plasma, greater than the matter density inside protons or neutrons! As the universe further cooled and expanded, individual protons and neutrons condensed out of the plasma, like drops of water condensing out of very humid air. But free neutrons are not stable, having an average lifetime of only 15 minutes. Furthermore, the universe was expanding and cooling at a rapid rate. So there was a very brief time interval, about 3 minutes long, in which nuclear fusion could produce helium, and a few light elements. This allows us to understand the otherwise stunning fact that 75% of the universe is still protons, with the remaining 25% being almost entirely helium! Direct observation of the last scattering surface confirms these predictions, which just use basic nuclear physics.
The two most mysterious and glamorous objects which wind up as end stages of stellar evolution, and also play a role in the production of nuclei, are the neutron star and the black hole. The existence of neutron stars was first predicted by Fritz Zwicky in 1933 (he was also the co-discoverer of dark matter and invented the name "supernova"). The first solution to Einstein's gravitational field equations for a black hole was found by Karl Schwarzschild in 1916. However it was not until the late 1960s that astronomers were sure that various objects they were observing were neutron stars, and it was not until 1971 that the first object clearly identifiable as a black hole was seen.
  Spinning Black Hole |
  Neutron Star |
Neutron stars have masses in the range of 1.4 to 2.9 solar masses, and radii ranging from 10 to 15 km. Their precise composition is unknown at present, due to our lack of knowledge of possible states of very compressed nuclear matter. The masses of black holes that formed as an end-stage of stellar evolution (as observed) range from about 3 times the mass of our sun to about 50 times the mass of our sun. However, at the centers of galaxies are super-massive black holes, which formed by collisions of countless numbers of stellar-mass black holes, and their observed masses range from 50,000 solar masses to many billions of solar masses. There is probably no upper limit to the possible black hole mass. Observations with the Webb Space Telescope are now hinting that very massive black holes could also have formed by extreme density fluctuations in the very early universe, a possibility first suggested by Steven Hawking.
Briefly, apart from the hydrogen, the other atoms in our bodies were made in dying low mass stars, and exploding high mass stars, although exploding white dwarf stars make a significant contribution. Our planet earth is made mainly of silicon, oxygen, nickel and iron, which in turn were made in exploding high mass stars and exploding white dwarf stars. All familiar things are made of the stuff of dying stars. Observations of supernovae in other galaxies suggest they probably occur in the Milky Way on average about three times every century. But only 5 certain supernovae have been seen by earth observers in the last thousand years.
Black hole collisions just create a new, more massive black hole. But most black holes are surrounded by large accretion disks of matter orbiting the hole, and when two black holes collide, the accretion disks collide as well. It is quite possible that when we learn more about the composition of typical such disks, we will discover that some nuclei are commonly formed in this way.
Recently observed: a supermassive black hole at the center of a galaxy, which is being orbited by an intermediate black hole, which periodically punches holes in the accretion disk that surrounds the SM black hole, with brilliant bursts of all forms of radiation created every time the orbiting BH goes through the disk! No telling what processes like this can "manufacture."
