Black hole

A black hole is an (almost invisible) body in space, created most likely from a collapsed red super giant star, that is so dense that neither light nor matter can escape its gravitational pull.

Inside a star there is a constant battle between inward pressure from gravity, and outward pressure from heat. If you were to throw an unopened can of soda into a fire, the beverage would expand from the heat and explode. This is the same principle at work when a star is burning, its heat is generating great outward pressure but this constant explosion is matched by gravity that is equally strong, thus a star maintains its shape and size.

When a star nears the end of its life it cools off slowly and the outwards pressure grows weaker and weaker as the temperature of the star drops. When the outward pressure from the heat is nearly gone, the inward pressure of gravity still remains and is determined by the size of the star. It is theorized that when a star roughly ten times the size of our sun nears the end of its life, it shrinks as its own gravity slowly pulls it in, but as it becomes more and more dense the gravity becomes stronger.

The gravity becomes so intense that not even light can escape it. If you have ever watched water swirling down a drain, then you have a pretty good idea what happens as a black hole pulls things in. As matter and light approach the vicinity of a black hole they are slowly drawn in. If they are not headed straight for the spacial anomaly then they are taken into a violent and unstable orbit around the black hole until finally the orbit falls apart and it is sucked down by the immense gravity.

The size of the black hole is determined by the mass of the collapsed star. The critical radius of a non-rotating black hole is called the Schwarzschild radius, called after the German astronomer Karl Schwarzschild (1873-1916) who investigated the problem in 1916 on the basis of Einstein's theory of general relativity. According to general relativity the gravitation of a black hole bends space and time to such an extent where they broken down into a dimensionless body of infinite density.

The boundary around the collapsed star having this radius is referred to as the 'event horizon'. Anything, whether it be light or matter passing this boundary will be forever lost within the black hole with no chance of escape. What happens beyond the event horizon nobody can tell, because all the laws of physics break down and no longer apply. There are many theories but little proof to support them.

Black holes can't be seen, as they do not emit any electromagnetic radiation^*. But they can be detected because of their affects on the surrounding stars.

In a binary star system, Cygnus X-1, (where the primary is a normal star of approximately 30 solar masses) due to Doppler shifts from the system it is believed that there is a companion of approximately 10 to 15 solar masses orbiting the primary. There are X-ray emissions from the system usually associated with an 'accretion disk' (a hot, dense disk of gas from the primary star spiraling down into the compact object orbiting the primary). There is evidence indicating that the X-rays are being emitted from the orbiting companion. Due to the mass of the companion object it is thought that it is a black hole.

Evidence of black holes is mounting, and it is now believed that most galaxies of a large enough size and possibly our own have a black hole at their centre.

* It is now known that black holes emit what is called Hawking Radiation, this is a complex process but for those who are interested, here is a brief explanation. Virtual particle pairs are constantly being created near the horizon of the black hole, as they are everywhere. Normally, they are created as a particle-antiparticle pair and they quickly annihilate each other. But near the horizon of a black hole, it's possible for one to fall in before the annihilation can happen, in which case the other one escapes as Hawking radiation.