A black hole is a theoretical concept in the study of gravitation. A black hole is an extremely dense body which has a gravitational field that 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 an event horizon, through which light can enter but not escape; it therefore appears totally black. Many people believe that black holes "suck" everything within a million miles. That is not true. First of all, black holes don't "suck" things in; they pull them in. Second, only if something gets too close to the event horizon will it get pulled inward, but it won't get drawn in just then. It may orbit around the black hole for a couple of years and then get pulled in. The matter inside a black hole is already crushed beyond comprehension (at least for star-sized black-holes - for galaxy-sized black holes things might not be quite so crushed) so there is plenty of room inside for all that totally squashed matter.

The first person to develope the idea of black holes was the French mathematician Pierre Simon de Laplace in 1798. Laplace agreed with Isaac Newton that light is made up of particles. He theorized that if enough mass were added to a star, the gravitational force would become so great that its escape velocity would equal the speed of light. Then, light particles wouldn't be able to leave the star, and it would "blink out" and become a black star. When Einstein developed his special theory of relativity, he argued that nothing can move faster than light. This means that Laplace's black stars are also black holes, because if light can't escape, all other matter would be trapped also. The black hole concept was expanded upon by the German astronomer Karl Schwarzschild in 1916 on the basis of Albert Einstein's theory of general relativity.

Black holes arise in general relativity, a classical theory of gravity. Roger Penrose and Stephen Hawking showed thirty years ago that, according to general relativity, any object that collapses to form a black hole will go on to collapse to a singularity (a dimensionless object of infinite density) inside the black hole. The presence of a singularity in the classical theory also means that once we go sufficiently far into the black hole, we can no longer predict what will happen. Physics as we know it no longer applies. Also, 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.

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, and a compact white dwarf or a neutron star may form. If the core mass exceeds about 1.7 solar masses (the mass of the body divided by the mass of the sun), however, neither electron nor neutron pressure is sufficient to prevent collapse to a black hole. To show how compressed an object must be to become a black hole, here's an example. Let's say we want to make our favorite politician into a black hole. We would need to compress him/her to 10-29 of a meter (Moore and Nicolson 59). A good way to rid the world of some unwanted politicians, but it's unrealistic. Such microscopic black holes could only form during the first few seconds of the formation of the universe. In contrast, Hawking has proposed that black holes do not collapse in such a manner but instead form �worm holes� to other universes besides our own.

Ordinary black holes are relatively small objects. A black hole with a mass equal that of the sun, for example, would have a diameter of only about three miles across. In contrast, the sun has a diameter of about 860,000 miles. Hawking has speculated that tiny black holes with masses no larger than large mountains are possible. Such black holes would have been formed only under extreme conditions that cosmology theories indicate existed in the first moments of the universe. More dramatic are the "super-massive black holes" that are now believed to reside at the cores of most galaxies in the universe. These may harbor debris from millions to billions of stars that is all tied together at a single point. One of the signatures of these 'cosmic vacuum cleaners' is that they spray jets of gas millions of light years into space.

Laws of physics suggest that black holes would emit particles over time and shrink slowly. This is known as "Hawking radiation", named after Stephen Hawking. For black holes of sufficiently small mass it is possible for one member of an electron-positron pair near the event horizon to fall into the black hole, the other escaping. The resulting radiation carries off energy, in a sense evaporating the black hole. According to Hawking, their temperatures would get higher as they shrank, and may totally evaporate with an enormous burst of energy. Any primordial black holes weighing less than a few thousand million metric tons would have already evaporated, but heavier ones may remain.

This is an extremely startling discovery; classically, no radiation can escape from a black hole, but scientists, using quantum mechanics, have determined that there is steady flux of radiation coming out of the black hole! This outgoing radiation decreases the mass of the black holes, so eventually the black hole will disappear. The temperature goes up as the black hole gets smaller (unlike most things, which cool off as they lose energy), so the black hole will disappear abruptly, in a final flash of radiation.

There are two ways to identify black holes. One is to detect theoretical gravitational waves that could be given off just before the black hole is formed. The other is through its interactions with other matter. For example, if a black hole is formed in a binary star system, gas from a star may flow toward the black hole. As it flows toward the black hole, its molecules increase in speed and approach the speed of light. They begin to bunch up and collide, heating them up to temperatures at which X-rays are emitted, which is about one million Kelvins. Such X-rays have been detected in binary star systems where the source is not visible. No black hole has yet been positively identified.

By the late 1980s, many astrophysicists were conjecturing that many, if not all, galaxies of substantial size may contain a black hole in their center. The astrophysical evidence for black holes has become so overwhelming in recent years that astronomers now treat these once-fanciful objects as normal components of the universe. Recent evidence indicates a staggering number of black holes exist in the universe. They include the millions of ordinary black holes believed to be peppering each of the estimated 50 billion to 100 billion galaxies in the universe up to giant black holes in the centers of most galaxies with masses that are billions of times greater than the sun's.

Astronomers have discovered X-ray emissions from a binary star system, Cygnus X-1, in which the primary is a normal star of about 30 solar masses. Doppler shifts in its spectrum show that a companion object of 10 to 15 solar masses must be in orbit around it; evidence exists that the X rays originate near the companion. Normally such X rays are produced by an �accretion disk,� a dense, hot disk of gas that forms as the gas from a normal star spirals into a compact object. The companion in Cygnus X-1, because of its massiveness, is thought likely to be a black hole rather than a white dwarf or neutron star. Other potential candidates for a black hole are an X-ray source in a neighboring galaxy, the Large Magellanic Cloud, another X-ray source located in the constellation Monoceros, and yet another in the galaxy M-87 (seen on front cover, the picture shows the intense X- rays being emitted from M-87 - picture courtesy of NASA's Hubble Space Telescope). The strongest evidence yet is immense clouds containing water vapor that have been observed circling in the center of the galaxy NGC 4258. By tracking the motion of the water-vapor clouds with powerful radio telescopes, astrophysicists can chart the gravitational field of the black hole at the center.

These telescope observations allow us to see clouds of water vapor tracing out a disk that is similar in some respects to the disk of our solar system. We can plot the speed of particular clouds and find that they move faster and faster the closer they are to the center, signaling the existence of a black hole.

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.

Even in this day in age, with all of our high-tech "gizmos", black holes remain a mystery to us. Centuries from now they may still be as mysterious to us as they are today. Why? Because there's no way to study these objects. Even if we could detect one and sent a probe to it, the probe would be ripped to shreds on the atomic level by the gravity before it could collect any useful information. Millions of dollars worth of equipment would be wasted. Wouldn't the tax payers be happy with NASA? Black holes may be the greatest mystery of all time, and possibly, one that was never meant to be solved.

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