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.
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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.
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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 seem unlikely to
exist; 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!
