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Black hole is a gigantic dense body that has been exist in the universe. Because of it's strong gravitational field, if the body is large enough, nothing, including electromagnetic radiation, can escape from its vicinity. The Black Hole is surrounded by some kind of spherical boundary, called a horizon. �Light can travel into it, but it couldn't escape!
The concept of black-hole was developed by the German astronomer Karl Schwarzschild in 1916 on the basis of German-American physicist Albert Einstein's general theory of relativity. The radius of the horizon depends only on the mass of the body; if 2.95km times the mass of the body in solar units. If a body is electrically charged or rotating, Schwarzschild's results are modified. An ergoshere will forms at the outside of the horizon, within which matter is forced to rotate with the black hole; which energy can be emitted from the ergosphere. According to relativity, gravitation modifies space and time near the black hole severely. �When the horizon is approached from outside, it slows time down relative to that of distant observers, stopping on the horizon.
Black holes form during the course of star evolution. When nuclear fuels are exhausted in the center of a star,(which is the core) the pressure that relative with their heat is no longer available to resist contraction of the core to higher densities. Two new types of pressure has formed at densities a million and a million billion times that of water, a compact white dwarf or a neutron star may form. However if the core mass is above 1.7 solar masses, neither electron nor neutron pressure will beable to prevent collapse to a black hole.
In 1994 astronomer used the Hubble Telescope to uncover the first evidence that a black hole exists. They measured the acceleration of gases around the center of the galaxy M87 and that there must be an object of 2.5 billion to 3.5 billion solar masses present. In 1997 astronomers unsing the Hubble Telescope and grounded-based telescopes in Hawaii announced that a census of nearby, ordinary galaxies shows that almost each galaxy may contain a massive black hole in its center.
For black holes of sufficiently small mass would be possible for only one member of an electron-positron pair near the horizon to fall into the black hole, the other electron may escaping.
The radiation carries off energy, in a sense evaporating the black hole. Some of the original black holes weighing less than a few thousand million metric tons might have been already evaporated, but the heavier ones may still remain. ?
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This page was created by Ben Hsieh in the Summer Science Astronomy Class/Broward in July, 1998 |
