what is a black hole?

Answer 1

Black hole
One of the end points of gravitational collapse, in which the collapsing matter fades from view, leaving only a center of gravitational attraction behind. General relativity predicts that if a star of more than about 3 solar masses has completely burned its nuclear fuel, it should collapse to a configuration known as a black hole. The resulting object is independent of the properties of the matter that produced it and can be completely described by stating its mass, spin, and charge. The most striking feature of this object is the existence of a surface, called the horizon, which completely encloses the collapsed matter. The horizon is an ideal one-way membrane: that is, particles and light can go inward through the surface, but none can go outward. As a result, the object is dark, that is, black, and hides from view a finite region of space (a hole). See also Gravitational collapse; Relativity.

The possible formation of black holes depends critically on what other end points of stellar evolution are possible. There can always be chunks of cold matter which are stable, but their mass must be considerably less than that of the Sun. For masses on the order of a solar mass, only two stable configurations are known for cold, evolved matter. The first, the white dwarf, is supported against gravitational collapse by the same quantum forces that keep atoms from collapsing. However, these forces cannot support a star which has a mass in excess of about 1.2 solar masses. The second stable configuration, the neutron star, is supported against gravitational collapse by the same forces that keep the nucleus of an atom from collapsing. There is also a maximum mass for a neutron star, estimated to be between 1 and 3 solar masses.

It would appear from the theory that if a collapsing star of over 3 solar masses does not eject matter, it has no choice but to become a black hole. There are, of course, many stars with mass larger than 3 solar masses, and it is expected that a significant number of them will reach the collapse stage without having ejected sufficient matter to take them below the 3-solar-mass limit. Further, more massive stars evolve more rapidly, enhancing the rate of formation of black holes. It seems reasonable to conclude that a considerable number of black holes should exist in the universe.

The black hole solutions of general relativity, ignoring quantum-mechanical effects, are completely stable. Once massive black holes form, they will remain forever; and subsequent processes, for example, the accumulation of matter, only increase their size. Steven Hawking showed that when quantum effects are property taken into account, a black hole should emit thermal radiation, composed of all particles and quanta of radiation which exist. Since a radiating system loses energy and therefore loses mass, a black hole can shrink and decay if it is radiating faster than it is accumulating matter. However, for black holes formed from the collapse of stars, the ambient radiation incident on the black hole from other stars, and from the big bang itself, is much larger than the thermal radiation emitted by the black hole, implying that the black hole would not shrink. Even if the ambient radiation is shielded from the black hole, the time for the black hole to decay is much longer than the age of the universe, so that, in practice, black holes formed from collapse of a star are essentially as stable as they were thought to be before the Hawking radiation was predicted.

Because black holes themselves are unobservable, their existence must be inferred from their effect on other matter. Such is the case with the binary x-ray star system Cygnus X-l. There are a number of binary x-ray systems known. The model which best explains the data is one in which a fairly normal star is in mutual orbit about a very compact object. Because these two are so close, mass flows from the star onto an accreting disk about the compact object. As the mass in the disk spirals inward, it heats up by frictional forces. Because the central body is so compact, the matter heats to a temperature at which thermal x-rays are produced. The only compact objects known that could accomplish this are neutron stars and black holes. The existence of very short-time bursts of radiation also points to an object of small diameter, that is, compact. In some of these binary x-ray systems, there is also a regular pulsed component to the x-rays, indicating a rotating neutron star (by reasoning similar to that given for pulsars). In these systems, the compact object could not be a black hole because that would imply a more complicated structure than a black hole would allow. In other systems, however, there are only irregular pulsations or fluctuations; they are candidates for possible black holes.

The crucial evidence comes from the mass determination of the compact object. Because the inclination of the orbit is not known, a range of masses is found; however, there will be a typical mass obtained by assuming that the orbit is not in an extreme orientation. For three x-ray binaries, Cygnus X-1, LMC X-3, and A0620-00, the typical mass of the compact body is about 10 solar masses, much larger than the maximum mass of a neutron star. In fact, the compact objects in the first and third binary systems are more massive than the maximum mass of a neutron star, no matter what orientation the orbit is assumed to have. Assuming that general relativity is the correct theory of gravitation (and this assumption is now supported very well experimentally), there can be no compact objects of such a mass other than a black hole. In this sense it can now be said that black holes exist.

While the evidence is less direct and more model-dependent, there is growing acceptance of the idea that supermassive black holes exist at the cores of nuclei of active galaxies, including quasars and radio galaxies. Here, the black hole is assumed to interact with accreting matter in such a way as to provide a source of energy to power these ultraluminous objects.

Black holes are thought to exist in the nuclei of other galaxies as well, their presence not giving rise to amounts of radiation as spectacular as for active galactic nuclei only because of differing conditions near the black hole. In the Milky Way Galaxy, observations of the proper motions of stars within a fraction of a parsec of the galactic center demonstrate unambiguously that a central mass concentration of 2 × 106 solar masses is present in a region so compact that no explanation other than that of a central black hole is feasible. Similar, although less convincing, observations of the presence of central black holes have been made for several nearby galaxies. The existence of supermassive black holes is virtually certain.

Answer 2

Black holes, those mysterious, invisible gravity whirlpools, have been in the news as one of the most exciting scientific ideas of this century. Many people envision a black hole as some tremendous whirling current traveling through space, devouring any hapless planets or stars in its path. But at present, black holes are still merely theoretical objects. However, astronomers are carefully observing places in space where these objects may exist.
A black hole is "black" or invisible because according to theory, its intense gravity pulls everything into itself. Its gravity is so strong that even light cannot escape. Without light, we can't see an object or take a picture of it. In fact, any object whose gravity is so tremendous that its escape velocity exceeds the speed of light is a black hole. (Escape velocity is the velocity required for an object to leave the surface of a body and avoid being drawn back down to it. For example, the escape velocity for Earth is about 39,600 kilometers per hour. Thus, if you threw a baseball up into the sky, you would have to throw it at least at 39,600 km/hr for the ball to keep going out into space and not fall back down. Our Sun's escape velocity is 2,160,000 km/hr. By contrast, the escape velocity of a black hole must begreater than the speed of light, which is about 1,079,252,848.8 kilometers per hour.) If some force could squeeze our Sun into a ball just 6 kilometers in diameter, it would be a black hole. The more massive an object is, the more gravity it has. But it takes a very great amount of mass compressed into a small area to create a gravitational field strong enough to keep light from escaping. The escape velocity for an object, keeping its mass constant, will be greater if the object's radius is smaller. Thus, this same mass squeezed into a smaller radius has a stronger gravity than it did before, and thus a higher escape velocity.

Answer 3

It's just a gravity thing.

Some clouds of gas don't even have enough gravity to pull together, or if they do, they just stay a clump of gas. These are nebulae.

Some have enough mass that the force of gravity and friction can create conditions favourable to fusion of hydrogen into helium. Welcome to star country.

But no star has a limitless supply of hydrogen. Eventually it runs out or is outcompeted by other atoms that get in the way. Then either fusion stops and the star explodes in a huge nova OR if there's enough mass it moves onto something heavier to fuse.

And so on and so on. Some stars, however, have TRULY massive gravities, and so they can start to do things really bizarre when they run out of normal fuel:

Neutron stars have enough gravity to force electrons and protons together... effectively making the star one single atomic nucleus. This is no mean feat, when you consider that the electric force is some 10^36 times more powerful than the gravitic force. A neuton star is also so small as to be practically invisible in stellar terms - if our sun were a neutron star, it would be the size of Manhattan Island instead 1.3 million times the size of Earth. Neutron stars also emit no light. But we can tell the're there because they still exert the influence of gravity and they're so pifflingly small in size.

Black holes are larger yet. A black hole has so much mass that it can distort space around it in such a way that nothing gets out. Even a neutron star will flare up and emit light as it passes through a cloud of new hydrogen as it fuses and rips the atoms to shreds. A black hole will not... things that hit a black hole just seem to disappear.

Thus we have a distinction between black holes and neutron stars. And we HAVE observed things that MUST be black holes, based on this difference. There are also any number of things that MAY be black holes around, but because they are surrounded by so much clutter or because our angle to them is not exactly right, we can't be sure if they are or are not.

Likewise, because black holes and neutron stars (as well is burned out husks of former stars called brown dwarfs) emit no light, it's difficult to be sure exactly how many of them are around, but not near enough to anything else to give signs of their presence.

Hope that helps!

Answer 4

"A black hole is defined to be a region of space-time where escape to the outside universe is impossible. " -- From Wikipedia, the free encyclopedia

However, my answer is a black hole is simply a gateway just like wormholes. I believe they lead to something that we as humans are not familiar with. And if we were to step inside it, we could possibly step out of it with some advanced knowledge that is not yet known on Earth.

Answer 5

Well, your answer has already been answered by several people. However, I think I have one in my house; it's where many things go to hide to make me crazy looking for them. Once, my new bifocals got lost in the B/H for almost a year then mysteriously re-appeared. Currently, my address book (has 40 years of entries) is hiding in my own personal black hole. Seriously, though, I think the existence of such Cosmic phenomenon is truly fascinating! TTIOT !

Answer 6

it is a very deep hole and is black

Answer 7

An unstable time warp or space warp which is not man-made.

Answer 8

No one knows for sure. They only know what some one else has said.