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Black Hole
In astronomy, an area in space formed by
a collapsed massive star into which matter has contracted so that it
vanishes except for an intense gravitational effect.
This gravity is so powerful that not even light
can escape from it (the escape velocity exceeds the velocity of light), making
impossible to see a black hole directly; like their name says, they are black,
and exist in the universe in large numbers.
The material inside a black hole is
concentrated into a singularity: a single point of infinitely high density
where space and time are infinitely distorted. Distant objects can escape
from a black holes gravitational pull, but objects inside the so-called
event horizon, the surface of the region surrounding the black hole where
gravity is inescapable, inevitably fall toward the center. Such objects would have to
move faster than light to escape, which is impossible according to the laws
of physics.
Two types of black holes are found in the
universe: stellar-mass black holes and super-massive black holes. They are
characterized by different masses and formation mechanisms.
Stellar-mass black holes form when a
heavy star collapses under its own weight in a supernova explosion. This
happens after the nuclear fuel, which makes the star shine for millions of
years, is exhausted.
Super-massive black holes are found in
the centers of galaxies that contain billions of stars. They may exist in
most galaxies and probably formed at the same time as the galaxies
themselves.
See
Exobiology
and Universe, Time
and Space.
Sources: (1)
NASA;
(2) Lerner, K. Lee and Brenda Wilmoth (editors),
The Gale Encyclopedia of
Space Sciences,
Thomson Gale; (3) Matzner, Richard A. (editor),
Dictionary of Geophysics, Astrophysics, and Astronomy,
CRC.
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