A quick guide to black holes

Astronomers have announced the discovery of the two biggest black holes ever seen, each one around 300 million light years from Earth and with a combined mass equivalent to more than 30 billion Suns.

These objects are some of the strangest in our known universe, where the laws of physics seem to break down and space gets very strange. One thing we know is that getting close to one is a bad idea.

Black holes begin as giant stars (at least six times the mass of our Sun) and, after billions of years they collapse in on themselves into a point smaller than the full-stop at the end of this sentence. Nothing nearby can escape the resulting gravity pull.

Even at some distance outside the edge, it would take all the effort in the universe to resist getting pulled into orbit around the hole. Closer still, because of the sharp rate of increase of the forces, if your head was nearer the hole than your feet, the atoms in your hair would feel a stronger force than those in your toes. The difference would tear you apart, turning you into a spaghetti-like line of atoms.

But a black hole would not need to suck the Earth in to cause us trouble. If one wandered within two billion km of our solar system, it could knock the Earth into a dangerous elliptical path around the Sun, where winters would drop to -50°C and summers would reach hundreds of degrees Celsius. Or, if one knocked us out of the solar system, our planet would wander through deep space. Without our Sun, life on Earth would freeze to death within months. There are probably more than 10m dead stars in the Milky Way that could be candidates for black holes, but the chances of our solar system running into one of them is small, because space is vast. Who knows how many planets have been destroyed in the vicious maw of a black hole, but it is safe to say that they were the extremely unlucky ones. — © Guardian Newspapers Limited, 2011