LATE STAGES OF STELLAR EVOLUTION

If M is less than 1.44 solar masses, the star becomes a white dwarf and gradually goes out. If M is between 1.44 and about 2.5 solar masses, a neutron star is left after a supernova explosion. If M is greater than about 3 - 4 solar masses, a black hole is formed. The most massive neutron star detected so far, PSR J0952–0607, is estimated to be 2.35 ± 0.17 M. The smallest black hole known, XTE J1650-500, has 3.8 M.  Both the neutron star and the supernova are of interest from the standpoint of nuclear and particle physics. If we knew more about the nuclear equation of state,we might have some idea of what kinds of things can form between roughly 2.5 and 3.5 solar masses...  nothing well-into this range has been identified to date.

Type Ia supernovae occur when a white dwarf abruptly accumulates mass from its surroundings, almost always a companion star, to the point of initiating carbon fusion. The very distinctive light curve of this type of supernova, and the fact that all such supernovae have the same absolute luminosity, has proven vital in cosmology, and for example led to the discovery of Dark or Vacuum Energy. Type II supernovae occur at the end of the evolution of a massive, layered supergiant star as the core collapses. All types of supernovae emit neutrinos, but they have been detected so far only from supernova 1987a.







A single supernova has much more luminosity than the entire galaxy of hundreds of billions of stars it is contained in, and so supernovae can be seen even in the most distant observable galaxies, providing a reliable cosmic distance scale.




Neutron stars used to be pictured basically as balls of neutrons 12 to 13 km in radius, with essentially nuclear density... a nucleus so large it is bound by gravity. But as usual, reality is far more complex, and neutron stars must exhibit a layered structure with a surface that's a metallic (iron?) lattice, an inner layer of neutrons and neutron-rich nuclei, an innermost layer of protons and neutrons as close together as they can be squeezed, and a core that might be a quark-gluon plasma.





Ordinary nuclear matter has a density of about 2.3 × 10−28 kg/fm3 or 2.3 × 1017 kg/m3. If we compress nuclear matter until the nucleon probability distributions are just about to overlap significantly, the density can be roughly 3.5 times as great, around 7.8 × 10−28 kg/fm3 or 7.8 × 1017 kg/m3, called ρ0 in the topmost neutron star diagram. Beyond that, it's anybody's guess.  For example the nucleon boundaries could dissolve leaving a Fermi liquid of three kinds of quarks, or a quark-gluon plasma. However you visualize it, these things should not be called “neutron stars.” It is now realized that such objects play a major role in nucleosynthesis, provided they are part of what was originally a double-star system.


While we don't normally think of black holes as playing any part at all in the formation of the elements, it has very recently been suggested that individual neutron stars could actually collide with a free-floating black hole (mass 10−14 M < MPBH < 10−8 M), presumably left over from the Big Bang, with a resulting catastrophic eating-up of the neutron star from within, which could easily synthesize a variety of nuclei in the cast-off material.  Such primordial black holes may indeed exist, based on the very massive black holes being detected very early in the history of the universe by the Webb telescope; more interesting is what would happen if an ordinary black hole and a neutron star collided.  At least two such collisions have now been observed! The neutron star is basically disassembled by tidal forces. Even more interesting, in regard to synthesis of the chemical elements, are several binary neutron star collisions that have been observed. We will discuss these in more detail later.




Is there any practical approach to interstellar travel?


Unexpected results for neutron star radii
MORE ON NEUTRON STARS

"Traditional" Nucleosynthesis!