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 has 3.3 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 2.4 and 3.3 solar masses... nothing
well-into this range has been identified to date.
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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. An alternate mechanism
is direct collision of two white dwarf stars. 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. However there are some severe
observational problems associated with these supernovae used as
a distance scale!
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.
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The primary source
of neutrinos in a core‑collapse supernova is found in the
collapse and rebound of the star’s iron core, which produces an
enormous burst of neutrinos and antineutrinos of all flavors.
When a massive star (≥ ~8 M☉) exhausts its fusion
possibilities, it can no longer support itself against gravity.
The iron core, once it exceeds the Chandrasekhar limit (~1.4 M☉),
collapses in less than a second. This collapse: (1) Compresses
protons and electrons into neutrons via electron capture,
producing electron neutrinos. (2) Releases gravitational binding
energy — about 99% of the total energy of the supernova — mainly
as a burst of neutrinos and antineutrinos. And, (3) Heats the
newly formed neutron star to extreme temperatures, causing it to
radiate neutrinos of all three flavors for several
seconds. In our own galaxy there are roughly only 1 to 3
supernovae per century, so virtually all of the supernova
neutrinos we hope to see stream in from other galaxies.
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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, hopefully 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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What would happen if a
black hole and a neutron star collided? That is
probably far from uncommon. 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.
Neutron star + neutron star
collision (left), and neutron star + black hole collision
(right)
Unexpected results for neutron
star radii
MORE ON NEUTRON STARS
"Traditional" Nucleosynthesis!
