| I | INTRODUCTION |
Black
Hole, an extremely dense celestial body that has been theorized to exist
in the universe. The gravitational field of a black hole is so strong that, if
the body is large enough, nothing, including electromagnetic radiation, can
escape from its vicinity. The body is surrounded by a spherical boundary, called
a horizon, through which light can enter but not escape; it therefore appears
totally black.
| II | PROPERTIES |
The black-hole concept was developed by the
German astronomer Karl Schwarzschild in 1916 on the basis of physicist Albert
Einstein’s general theory of relativity. The radius of the horizon of a
Schwarzschild black hole depends only on the mass of the body, being 2.95 km
(1.83 mi) times the mass of the body in solar units (the mass of the body
divided by the mass of the Sun). If a body is electrically charged or rotating,
Schwarzschild’s results are modified. An “ergosphere” forms outside the horizon,
within which matter is forced to rotate with the black hole; in principle,
energy can be emitted from the ergosphere.
According to general relativity, gravitation
severely modifies space and time near a black hole. As the horizon is approached
from outside, time slows down relative to that of distant observers, stopping
completely on the horizon. Once a body has contracted within its Schwarzschild
radius, it would theoretically collapse to a singularity—that is, a
dimensionless object of infinite density.
| III | FORMATION |
Black holes are thought to form during the
course of stellar evolution. As nuclear fuels are exhausted in the core of a
star, the pressure associated with their energy production is no longer
available to resist contraction of the core to ever-higher densities. Two new
types of pressure, electron and neutron pressure, arise at densities a million
and a million billion times that of water, respectively, and a compact white
dwarf or a neutron star may form. If the star is more than about five times as
massive as the Sun, however, neither electron nor neutron pressure is sufficient
to prevent collapse to a black hole.
In 1994 astronomers used the Hubble Space
Telescope (HST) to uncover the first convincing evidence that a black hole
exists. They detected an accretion disk (disk of hot, gaseous material)
circling the center of the galaxy M87 with an acceleration that indicated the
presence of an object 2.5 to 3.5 billion times the mass of the Sun. By 2000,
astronomers had detected supermassive black holes in the centers of dozens of
galaxies and had found that the masses of the black holes were correlated with
the masses of the parent galaxies. More massive galaxies tend to have more
massive black holes at their centers. Learning more about galactic black holes
will help astronomers learn about the evolution of galaxies and the relationship
between galaxies, black holes, and quasars.
The English physicist Stephen Hawking has
suggested that many black holes may have formed in the early universe. If this
were so, many of these black holes could be too far from other matter to form
detectable accretion disks, and they could even compose a significant fraction
of the total mass of the universe. For black holes of sufficiently small mass it
is possible for only one member of an electron-positron pair near the horizon to
fall into the black hole, the other escaping (see X Ray: Pair
Production). The resulting radiation carries off energy, in a sense evaporating
the black hole. Any primordial black holes weighing less than a few thousand
million metric tons would have already evaporated, but heavier ones may
remain.
The American astronomer Kip Thorne of
California Institute of Technology in Pasadena, California, has evaluated the
chance that black holes can collapse to form 'wormholes,' connections between
otherwise distant parts of the universe. He concludes that an unknown form of
'exotic matter' would be necessary for such wormholes to survive.
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