Black holes are
the evolutionary endpoints of stars at least 10 to 15 times as massive as the
Sun. If a star
that massive or larger undergoes a
supernova explosion, it may
leave behind a fairly massive burned out stellar remnant. With no outward
forces to oppose
gravitational forces, the remnant will collapse in on itself.
The star eventually collapses to the point of zero volume and infinite density,
creating what is known as a “singularity ". As the density increases, the
path of light rays emitted from the star are bent and eventually wrapped
irrevocably around the star. Any emitted photons are trapped into an orbit by
the intense gravitational field; they will never leave it. Because no light
escapes after the star reaches this infinite density, it is called a black
hole.
But contrary to popular myth, a black hole is not a cosmic vacuum cleaner.
If our Sun was suddenly replaced with a black hole of the same mass, the
earth''s orbit around the Sun would be unchanged. (Of course the Earth''s
temperature would change, and there would be no solar wind or solar magnetic
storms affecting us.) To be "sucked" into a black hole, one has to
cross inside the Schwarzschild radius. At this radius, the escape speed is
equal to the speed of light, and once light passes through, even it cannot
escape.