).There are three primary species of compact objects. These are white dwarfs, neutron stars, and black holes. These objects are formed when normal stars die. A normal star dies when its nuclear fuel runs out.
All of these objects exist in an extremely dense state of matter as there is not enough pressure outward from nuclear reactions to prevent gravitation from causing them to collapse in on themselves. It is believed that these astrophysical objects are essentially gravitationally stable.
Compact objects form when a star dies, and
the specific type of compact object that
a star becomes is determined by the mass
of the star. The following table relates
the initial mass of the star (in solar masses,
).
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Where 
.
White dwarfs are a class of supernova remnants characterized by matter that is so dense that individual atomic electrons can no longer tell which atom they are a part of. Such matter is called electron-degenerate. It is this degenerate matter that causes the pressure to prevent further gravitational collapse.
Neutron stars are a class of supernova remnants characterized by matter that is so dense that individual nuclear neutrons can no longer tell which atomic nucleus they are a part of. Such matter is called neutron-degenerate. It is the pressure of these degenerate neutrons that prevents further gravitational collapse.
A pulsar is a rapidly rotating neutron star that emits a radio pulse.
Black holes are a bizarre class of supernova remnants characterized by mass-energy densities so high that not even light can escape their gravitational pull. Black holes are completely collapsed objects.
Active galactic nuclei (AGN) are a class of objects at the center of galaxies emitting radio and x-ray signals of great intensity. It is now believed that AGNs are powered by supermassive black holes.
The intense gravitational fields of compact objects interact with any companion stars by cannabalizing their companions. Gas, plasmas, and stellar winds are drawn into the compact object through a process called accretion. As matter accretes it goes into a Keplerian, or near-Keplerian orbit that spreads out through a combination of friction and angular momentum conservation, forming a disk.
Gamma ray bursts (GRB) are extemely intense events that emit high energy gamma rays from discrete sources throughout the sky. Their exact origin is still a mystery, and thus they are not true compact objects; though the means of studying them are the same as those for studying compact objects. It is almost certain that at least some of these objects are at cosmological distances.
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