Ken Tapping, June 9 2010

When Isaac Newton came up with the idea that every bit of stuff in the universe attracts every other bit of stuff with a force we now call "gravity", he had come up with something fundamental to astronomy. These days, almost everything we do in astronomy has gravity buried in there somewhere. 

Every particle in a lump of something attracts every other particle. For small things this force is very weak. Providing nothing else is disturbing them, bodies bigger than a few kilometres in size generate enough gravity to hold themselves together. However, the force is not big enough to pull them into a spherical shape. That is why small asteroids and the lumps of dusty ice hidden in the heads of comets are almost always shaped like potatoes or peanuts.

Lumps a hundred kilometres in size and larger have stronger gravitational attractions, enough to pull them into spheres. That is why the Moon, Earth, other planets and the Sun are spherical. As the bodies get more massive, the weight of the overlying material pressing down on the cores produces higher and higher pressures. For objects the size of the Earth or Moon, this is interesting but not hugely exciting. However, we need to take into account another important factor... .heat. 

The planets, stars and other objects we see around us were formed from the collapse of great clouds of cosmic gas and dust. As all these bits crashed together, often at speeds of many kilometres a second, the impacts released a lot of heat. Our Earth started some 4.5 billion years ago as a great lump of molten rock. By the time the Earth started to solidify, the heavy elements like iron and nickel had accumulated mainly in the middle, iron and magnesium minerals formed a layer around the nickel-iron core, and silicon and aluminium minerals formed a crust on top. The interior of our planet is still molten, and, according to recent discoveries, Venus has a molten interior too. We expected this, because Venus is about the same size as the Earth, was born around the same time. 

As bodies get more and more massive, the pressures in the centres get higher and higher. Bodies with masses 100 or more times higher than the planet Jupiter generate pressures and temperatures in their cores high enough for nuclear fusion to take place. The energy released is enough to make the body shine; making it a star. 

If we pile on more mass, or compress the core with a shock wave from an explosion, we can reach a point where the pressure in the core is so high that the atoms collapse. The electrons get jammed intro the protons and we end up with a lump of neutrons. This is a neutron star. If we did that to the Sun it would be compressed down to about 10km across. 

We can compress things even further, and then something really strange happens. The gravitational force compressing the body exceeds the pressure resisting compression and the shrinkage goes to a point where time and space become distorted. The body becomes surrounded by an "event horizon", trapping even light. It has become a black hole.