Stars can turn into varieties of things as they collapse, including white dwarfs, nuetron stars... A black hole is suggested to be the end product of a large star that is collapsing into itself. Due to the fact that gravitational acceleration is calculated by the formula :
where mB is the mass of the black hole, as the radius (r) of the star decreases, the gravitational field on it's surface increases. This causes a chain reaction in which a greater force is put on the star to collapse, thus decreases in size even further, and the gravity of it's surface increases. It is suggested that a star would have to have a mass equivalent to three times that of our sun to become a black hole. If though a star with an equivalent mass to the Earth were to collapse into a black hole, the space that all of the matter would take up would have a radius of less than 9mm. It is easy to see that the density of this would be huge-thus demonstrating why it would have such noticeable effects.The gravitational field created would have important effects to it's surrounding environment, producing signs for astronomers to observe when looking for a black hole.
Einstein's theory of general relativity, suggest that close to the star itself, strong distortions occur in the structure of space. He found that the acceleration was equal when caused by changing motion, compared to when changed by gravitational fields. From this we deduce that at the point of a gravitational field, space itself is curved such that moving particles follow the same path as they would if they were being accelerated. This has applications towards photons of light as well as any other particle.
The effect of this gravitational field produces a enhancement of the curvature of space, in terms of a phonton of light projected from the surface of the star that is not directly along the path of the normal. It becomes deflected, causing an increased angle compared to the angle that it was projected at. Similarly, light that 'grazes' the surface of a strong gravitational sphere is deflected in the same way. The stronger the gravitational field is (i.e.the denser that the star is), the greater the angle of deflection and the greater the velocity of the wave that has to be projected to escape the field. As the density increases, the field's pull is so great that the photon of light directed horizontally at the field is deflected into the orbit of the star.
The star's light may be projected from the surface of the star to escape it's gravitational field. When the projection's angle is equal to that of the normal, the light is projected radially, escaping deformation, yet when the light is projected at any other angle, it is deflected away from the normal. The stronger the gravitational field, the greater the deflection, and the smaller the angle becomes that light is allowed to project away from the surface at without being pulled into orbit. Thus as the star becomes more dense, it's gravitational field strength is increased, until eventually the angle at which light is allowed to project away from the star is 0 degrees. As light has the greatest velocity of any known thing, and is said to go at the natural speed limit (approx 300,000,000m per second), as soon as light cannot escape from the boundary of the decaying star, neither can anything else. At this point light from both from the star itself, and that hitting the field from other sources cannot escape, thus a black hole is born.
Black holes were first understood by Kurt Schwarzchild well over 60 years ago. He proposed the properties that he expected the outer limit of the black hole to exhibit. He gave his name to the radius in which a star has a strong enough gravitational field to trap photons of light. This Schwarzchild radius, as it became known, was only dependant on the mass of the star in question, and was proportional to it. For instance if a star had a mass of 5 times that of our sun, it's Schwarzchild radius would be 15 km. As soon as the collapsing star has shrunk beyond it's Schwarzchild radius, it is said to have passed it's event horizon, as no outside observations can be made into it. The photon-sphere however is the point when light is forced to orbit the star, but is not pulled into the event horizon.
The point at which the star's mass is centred is called the singularity. This in his equations lay at the very centre of the black hole, and is considered the centre of it's gravitational field. The singularity is infinitesimally small because mathematically it is found to be a single point.
Penrose diagrams of a Schwarchild black hole show an additional singularity. This is said to be of a white hole, which supposedly demonstrates the opposite feature to that of a black hole. Instead of the huge attractive force of gravity, it actually repels matter. It is suggested that it is the missing link between our universe and another, releasing the matter that for us is lost into the black hole. As this phenomena has never been observed, the theory is mainly by-passed.
As all stars are known to rotate, it is almost impossible that we would be able to find an example of the Schwarzchild black hole in nature. The relative equations to this fact were only discovered in 1963 by an Australian mathematician named Roy P. Kerr. He found them accidently whilst working on another problem, and found that although the spinning black hole held resemblances to the Schwarzchild model, there were also distinct differences. In this new type of black hole, a body that enters it would be forced to move in a spinning motion down towards the singularity, like water in a plughole. The limit at which light can still escape this dragging force, is known as the stationary limit. The momentum of the spin decreases the size of the event horizon, the limit between this and the stationary limit being the ergosphere. On a theoretical level, a body travelling faster than the speed of light within the ergosphere could escape it, yet there is no escape from being dragged around when still within it. The ergosphere is thought to produce an oval shape, being in contact with the poles of the event horizon, while on the equator having double the diameter of the event horizon.
It is also mathmatically possible that the speed of the spin of a black hole could cause the shrinking of the event horizon such that it disappears, and the singularity is left on view. This would cause a naked singularity. This would not display the usual gravitational traits of a black hole, and would be possible to blunder into without any previous warning. It also carries the implication that we could potentially travel freely in and out of the singularity, as the event horizon is no longer present. If this were the case, by going into the orbit of a naked singularity, time travel into the past could occur. In general this is conceived to be an impossible situation, as black hole properties are assumed by the size of the mass alone, the charge and spin having little effect.
These Kerr Black Holes would have a singularity that takes the form of a ring. It's singularity is not space-like, as demonstrated in the other model, but time-like instead. Only objects that enter the event horizon on it's equator would be subject to destruction via the singularity. The interior of the singularity is a area of negative space-time, implying the reversal of the force of gravity at this point. Another possible concept is that of objects within this plain having a negative radius, but no-one has yet been able to fathom out this idea rationally.
It has also been suggested that other black holes were created when the Big Bang occured. These black holes were tiny, some as light as 0.0000001kg. We know that the density of matter as it crosses the event horizon varies inversely to the mass of the black hole, such that black holes of this miniscule nature much have had enormous pressures applied to create them. These pressures were only thought to exist during the creation of the universe as we know it. There is no evidence of their existence, except for in the laws of quantum mechanics. It has been put forward by Hawking that these black holes could have evaporated.
It is known that the componants of particles can be split to particles and anti-particles. When this occurs, and the pair remeets, they annihilate each other, and energy is created. Similarly, energy can be converted into pairs of partcles. This is known as pair production, and only works because mass and energy are equivilant. Taking this idea further, matter can be created from nothing for very breif periods of time. As it occurs almost simultaneously, it does not violate the conservation laws. If this occured near to a black hole, and half of the pair were to fall into it, the inevitable annihilation could not occur, The other half of the pair would be able to escape; energy is created. This energy has to have a notable source, as energy cannot be created or lost. The source of such energy is the black hole itself. As it is robbed of energy, it is also robbed of it's equivilant, mass, thus the black hole evaporates due to pair production. This event would only have a noticable consequence on the very smallest of the black holes. If this process did occur, we would expect to see occaisional bursts of gamma radiation being emmited from these mini black holes.
As we obviously cannot see black holes, the only things we can do to assertain their existence are apply theoretical knowledge, and observe the things that we suspect they cause. Detection of black holes is most likely to occur when we find an invisible object that has a mass which could only possibly demonstrate one. Even then we are working on the assumption that white dwarfs and neutron stars are unable to survive at such a mass. One way of calculating the mass of an object we cannot see (thus cannot gauge luminoscity or magnitude), is to follow the orbit around it of a companion star. If this star is found to be part of a binary system,with an invisible partner, then the mass of the companion can be calculate via spectral and visual analysis. If this mass is found to be in excess of 3 solar masses, then a black hole is presumed to have been found.Another way is by examing the matter that they pull towards themselves. This matter forms an accretion disk,which due to the forces acting upon it becomes hot enough to emit X-rays. These in turn can be detected, and provide us with information on the fields acting upon them.
A black hole is said to encompass the four dimensions of space and time, thus as a body approaches the event horizon, time is distorted due to the force of the acceleration, and force of the field. To an outside observer it would slow gradually, and along with it, wavelengths, although maintaining velocity, are red-shifted. As the body becomes even closer to the event horizon, time appears to stop. Strong tidal forces would cause the body to be ripped apart. Upon reaching the event horizon, the body would never be seen again, and is thought by scientists to race irreversibly towards the singularity, and become infinitely more dense.
The bizarre nature of Einstein's equations suggest that black holes should theoretically lead to parallel univeses, i.e. one that is seperate from our own. There may be many different ones of these, each slightly different to the one we are presently existing in. This however is still very much only a hyperthetical situation.
Although black holes have never been seen as such, their effect on the surroundings is clear to see. Thus by a principle called Occam's Razor, i.e. that 'the explanation of any phenomenon that requires the fewest arbitrary assumptions is the most likely to be the correct one', we assume that black holes exist, and continue to make their own indivdual mark in the universe we live in.
By Anne-Marie Cumberlidge, Keele University- 1997.
'The Dynamic Universe' by Theodore P. Snow
'Exploration of the Universe' by Abell, Morrison, and Wolff
STIS image from HST public information.