So what, if anything, remains of the core of the original star? Unlike
in smaller stars, where the core becomes essentially all carbon and
stable, the intense pressure inside the supergiant causes the electrons to
be forced inside of (or combined with) the protons, forming neutrons. In
fact, the whole core of the star becomes nothing but a dense ball of
neutrons. It is possible that this core will remain intact after the
supernova, and be called a neutron star. However, if the original star was
very massive (say 15 or more times the mass of our Sun), even the neutrons
will not be able to survive the core collapse and a black hole will
IV. More about the Stellar Endpoints
A. White/Black Dwarfs
A star like our Sun will become a white dwarf when it has
exhausted its nuclear fuel. Near the end of its nuclear burning stage,
such a star expels most of its outer material (creating a planetary
nebula) until only the hot (T > 100,000 K) core remains, which then
settles down to become a young white dwarf. A typical white dwarf is half
as massive as the Sun, yet only slightly bigger than the Earth. This makes
white dwarfs one of the densest forms of matter, surpassed only by neutron
White dwarfs have no way to keep themselves hot (unless they accrete
matter from other closeby stars); therefore, they cool down over the
course of many billions of years. Eventually, such stars cool completely
and become black dwarfs. Black dwarfs do not radiate at all.
Many nearby, young white dwarfs have been detected as sources of soft
X-rays (i.e. lower-energy X-rays); soft X-ray and extreme ultraviolet
observations enable astronomers to study the composition and structure of
the thin atmospheres of these stars.
B. Neutron Stars
Neutron stars are typically about ten miles in diameter, have
about 1.4 times the mass of our Sun, and spin very rapidly (one revolution
takes mere seconds!). Neutron stars are fascinating because they are the
densest objects known. Due to its small size and high density, a neutron
star possesses a surface gravitational field about 300,000 times that of
Neutron stars also have very intense magnetic fields - about
1,000,000,000,000 times stronger than Earth's. Neutron stars may "pulse"
due to electrons accelerated near the magnetic poles, which are not
aligned with the rotation axis of the star. These electrons travel outward
from the neutron star, until they reach the point at which they would be
forced to travel faster than the speed of light in order to still
co-rotate with the star. At this radius, the electrons must stop, and they
release some of their kinetic energy in the form of X-rays and gamma-rays.
External viewers see these pulses of radiation whenever the magnetic pole
is visible. The pulses come at the same rate as the rotation of the
neutron star, and thus, appear periodic. Neutron stars which emit such
pulses are called pulsars.
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