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BLACK HOLE

A black hole is a theorized celestial body whose surface
gravity is so strong that nothing can escape from within it.
Although black holes have been of intense scientific interest
only in the later 20th century, the concept goes back to the
French mathematician Pierre Simon de LAPLACE.  In a 1798
treatise Laplace agreed with Newton that light is composed of
particles.  He reasoned that if enough mass were added to a
star like the Sun, the gravitational force of the star
eventually would become so great that its escape velocity would
equal the velocity of light.  At that point, light particles
would not be able to leave the surface of the star, and it
would "blink out" and become an invisible black star.

More than a century later, Einstein, developing his special
theory of relativity, argued that nothing can move faster than
light.  This means that Laplace's black stars are also black
holes, because if light cannot escape, all other matter must be
trapped as well.  The surface of a black hole thus acts like a
one-way membrane:  material may fall into a black hole, but no
information or energy can come out of a black hole.  The
detailed properties of black holes are studied by using
Einstein's general theory of relativity and gravitation.

In 1917 a German astrophysicist, Karl SCHWARZSCHILD, used
Einstein's theory to calculate that if a star of any given mass
were compressed to a size smaller than a critical radius, now
called the SCHWARZSCHILD RADIUS in his honor, the density would
become so high and the gravitational force so great that the
star would become a black hole.  The spherical surface about
the star at the Schwarzschild radius is called the "event
horizon" and marks the outer surface of the black hole at which
the escape velocity just equals the velocity of light.  Further
calculations revealed to Schwarzschild that the critical radius
of a black hole is proportional to its mass.  For the Sun, this
radius is about 3 km (2 mi).  To find the Schwarzschild radius
of any other object, one need only divide the mass of that
object by the Sun's mass and then multiply by 3 km.

If a star more massive than the Sun undergoes gravitational
collapse at the end of its evolution, it will form either a
WHITE DWARF, a NEUTRON STAR, or a black hole, depending
primarily on its mass (see STELLAR EVOLUTION).  If this
collapse process is nonspherical, perhaps because the star is
rotating and flattened at the poles, then theoretical
GRAVITATIONAL WAVES could be given off just before the black
hole is formed.  Attempts are being made to detect such waves.

The only other way to identify a black hole would be through
its interactions with other matter.  For example, if the black
hole is formed in a BINARY STAR system, gas from the normal
star may later flow toward the black hole.  As the gas falls
toward the hole, its molecules increase in speed and approach
the speed of light.  The molecules begin to bunch up and
collide, heating them to temperatures at which X rays are
emitted.  Such X rays have been detected in eclipsing binary
star systems in which the X-ray source is not visible.  The
binary star Cygnus X1 may include such a companion;  other
candidates are an X-ray source in the nearby Large Magellanic
Cloud galaxy, and X-ray novas in the constellations Monoceros
and Vulpecula.  No black hole has yet been positively
identified.

Black holes, if they exist, could come in an extreme range of
sizes.  The English physicist Stephen HAWKING has speculated
that tiny black holes with masses no larger than that of a
large mountain are possible.  Such black holes, in the size
range of elementary particles, would have been formed only
under the extreme conditions that COSMOLOGY theories indicate
existed in the very first moments of the universe (see
INFLATIONARY THEORY).  On the other hand, gigantic black holes
may lie at the center of galaxies.  Some astronomers suggest
that such black holes may be linked to the differences that
exist between galaxies, ranging from normal ones such as our
own to the highly active galaxies called radio galaxies and
quasars (see EXTRAGALACTIC SYSTEMS).

Various speculations have been made as to the permanence or
instability of black holes themselves.  Laws of physics suggest
that black holes would emit particles and shrink with time.
According to Hawking, their temperatures would rise as they
shrank, and they might fully evaporate with an enormous burst
of energy.  Other theorists suggest that certain factors would
halt this process along the way.  Some theorists have even
suggested the existence of so-called white holes in which
matter would flow through a completely collapsed black hole--a
singularity, or body of zero radius and infinite density--into
another universe.  This notion, however, is extremely
speculative.

A popular misconception is that black holes act like cosmic
vacuum cleaners, sucking up everything within reach.  In fact
the gravitational attraction of a black hole would be no
stronger than that of a normal star of the same mass.  For
instance, if the Sun could collapse to form a black hole (it
cannot, however, and will instead end its life as a white
dwarf), the Earth would continue to orbit just as it does now.

Bibliography:  Chaisson, Eric, Relatively Speaking (1988);
Greenstein, George, Frozen Star (1984);  Hawking, Stephen, A
Brief History of Time (1988);  McClintock, Jeffrey, "Do Black
Holes Exist?" Sky and Telescope, January 1988;  Novikov, Igor,
Black Holes and the Universe, translated by Vitaly Kisin
(1991);  Parker, Barry, "In and Around Black Holes," Astronomy,
October 1986;  Price, R.  H., and Thorne, K.  S., "The Membrane
Paradigm for Black Holes," Scientific American, April 1988;
Sullivan, Walter, Black Holes:  The Edge of Space, the End of
Time (1980).

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