Formation of black holes
According to general relativity, a black hole can form when a
massive star runs out of nuclear fuel and is crushed by its own
gravitational force. While a star burns fuel, it creates an outward
push that counters the inward pull of gravity. When no fuel remains,
the star can no longer support its own weight. As a result, the
core of the star collapses. If the mass of the core is three or
more solar masses, the core collapses into a singularity in a
fraction of a second.
Galactic black holes
Most astronomers believe that the Milky Way Galaxy -- the galaxy
in which our solar system is located -- contains millions of black
holes. Scientists have found a number of black holes in the Milky
Way. These objects are in binary stars that give off X rays. A
binary star is a pair of stars that orbit each other.
In a binary system containing a black hole, that object and a
normal, visible star orbit one another closely. As a result, the
black hole strips gas from the normal star, and the gas falls
violently toward the black hole. Friction between the gas atoms
heats the gas near the event horizon to several million degrees.
Consequently, energy radiates from the gas as X rays. Astronomers
have detected this radiation with X-ray telescopes.
Astronomers believe that a number of binary star systems contain
black holes for two reasons: (1) Each system is a source of intense
and variable X rays. The existence of these rays proves that the
system contains a compact star -- either a black hole or a less
compact object called a neutron star. (2) The visible star orbits
the compact object at such a high velocity that the object must
be more massive than three solar masses.
Super massive black holes
Scientists believe that most galaxies have a super massive black
hole at the center. The mass of each of those objects is thought
to be between 1 million and 1 billion solar masses. Astronomers
suspect that super massive black holes formed several billion
years ago from gas that accumulated in the centers of the galaxies.
There is strong evidence that a super massive black hole lies
at the center of the Milky Way. Astronomers believe this black
hole is a radio-wave source known as Sagittarius A* (SgrA*). The
clearest indication that SgrA* is a super massive black hole is
the rapid movement of stars around it.
The fastest of these stars appears to orbit SgrA* every 15.2 years
at speeds that reach about 3,100 miles (5,000 kilometers) per
second. The star's motion has led astronomers to conclude that
an object several million times as massive as the sun must lie
inside the star's orbit. The only known object that could be that
massive and fit inside the star's orbit is a black hole.