From: Gina Miller (echoz@hotmail.com)
Date: Wed Apr 28 1999 - 23:24:43 MDT
black hole
in astronomy, theoretically predicted final stage in the life history
of certain stars (see stellar evolution). A star in the last phases
of gravitational collapse is often referred to as a “black hole.” The
collapse begins when a star has depleted its steady sources of
nuclear energy and can no longer produce the expansive force, a
result of normal gas pressure, that supports the star against the
compressive force of its own gravitation. As the star shrinks in size
(and increases in density), it may assume one of several forms
depending upon its mass. A less massive star may become a white
dwarf, while a more massive one would become a supernova. If the mass
is less than 3 times that of the sun, it will form a neutron star.
However, if the final mass of the remaining stellar core is more than
3 solar masses, nothing remains to prevent the star from collapsing
without limit to an indefinitely small size. At this point, the
effects of Einstein's general theory of relativity become paramount.
According to this theory, space becomes curved in the vicinity of
matter; the greater the concentration of matter, the greater the
curvature. When the star (or supernova remnant) shrinks below a
certain size determined by its mass, the extreme curvature of space
seals off contact with the outside world. The place beyond which no
radiation can escape is called the event horizon. For a star with a
mass equal to that of the sun, this limit is a diameter of only 1.8
mi (3 km). Even light cannot escape the black hole but is turned back
by the enormous pull of gravitation. Because light and other forms of
energy and matter are permanently trapped inside the black hole, it
can never be observed directly. However, a black hole could be
detected if it is orbited by a visible star or during the collapse
while it was forming. Four possible black holes have been detected in
our galaxy, the Milky Way. The first discovered (1971) is Cygnus X-1,
an X-ray source in the constellation Cygnus. In the 1980s, one of the
strongest cases for a black hole was identified in the constellation
Monoceros and labeled A0620–00. V404 Cygni, close to Cygnus X-1, and
Nova Muscae 1991 in the constellation Musca were discovered more
recently.
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Gina "Nanogirl" Miller
Nanotechnology Industries
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