Black holes
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.
Properties
The black-hole concept was developed by the German
astronomer Karl Schwarzschild in 1916 on the basis of
German-American physicist Albert Einstein's theory of
general 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
"ergo sphere" forms outside the horizon, within which
matter is forced to rotate with the black hole; in
principle, energy can be emitted from the ergo sphere.
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.
Formation
Black holes may form during the course of stellar
evolution. As nuclear fuels are exhausted in the core of
a star, the pressure associated with their heat is no
longer available to resist contraction of the core to
ever higher densities. Two new types of 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 core mass exceeds about
1.7 solar masses, 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 measured the acceleration of
gases around the center of the galaxy M87 and found that
an object of 2.5 billion to 3.5 billion solar masses
must be present. A second potential black hole at the
center of the galaxy NGC 4258 was discovered in 1995 by
astronomers using the Very Long Baseline Array (VBLA),
an array of radio telescopes that spans a large
geographical area. Also in 1995, astronomers used the
HST to discover a third black hole near, but slightly
displaced from, the center of the galaxy NGC 4261. In
both of the 1995 discoveries, astronomers detected
accretion disks, or disks of hot, gaseous material,
circling the centers of the galaxies with accelerations
that indicated the presence of a very massive object.
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. In reaction
to the concept of singularities, Hawking has also
proposed that black holes do not collapse in such a
manner but instead form so-called "worm holes" to other
universes besides our own.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 . 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.
[9 major planets]
[Sun]
[Comets]
[Mercury]
[Venus]
[Earth]
[Mars]
[Jupiter]
[Saturn]
[Uranus]
[Neptune]
[Pluto]
[Go Home!!]
By TNN & Co.