Alisha Vara, of Rangi Ruru Girls School, asks :-

Which black hole is closest to Earth?

Alan Gilmore, an astronomer at the University of Canterbury, responded.

We can't give a definite answer to that question. Black holes reveal themselves only by their gravity. A black hole exactly on our line of sight to a star will reveal itself by 'gravitational lensing', bending the light of the star in a particular way. Or the black hole might have some obvious effect on a star or gas orbiting close to it.

The closest black holes we know of are orbiting around visible stars. We know of several big stars with close companions that are small, dark and massive. These stars also give off x-rays as gas drawn from the star spirals into the black hole. These black holes are more than three times the mass of the sun. The nearest black hole that we know about to date is V4641 Sagittarius which is only 1600 light years from us, towards the galactic center. (Light travels 300 thousand kilometres in a second so a light year is nearly 10 million million kilometres.)

However, these are just the black holes with companion stars. We believe that many massive stars become black holes after they have used up the last of their energy. So there may be nearer `solitary' black holes that we don't know about.

The really massive black holes, those millions of times the mass of the sun, are found in the centres of galaxies. There is one in the centre of the Milky way, 27,000 light years away. It is around four million times heavier than the sun.