Natalie Ford, of Villa Maria College, asks :-

Dylan Greenfield, of Ridgway School, asked:-

What is a black hole?

Bruce Liley, a theoretical physicist at Waikato University, responded.

Stars much larger and brighter than our sun burn up much more quickly than the sun. Having burnt all their fuel they then begin to collapse. As they collapse their gravity increases and eventually becomes so large that the velocity needed for escape from the star exceeds the speed of light so nothing, not even light, can escape, while anything that is dropped on to them disappears inside forever. Hence they become black-holes, black since nothing ever comes out again.

However, in the neighbourhood of such a star, gases which are falling in towards the hole become very hot and radiate energy, in particular X-rays. Satellites with X-ray cameras are therefore used to try and detect the X-rays and hence presence of such holes. So far, in this way, certain stars have been observed which could be black holes, but many more observations are needed before this can be confirmed, even though theory predicts that there should be very large numbers of such holes in our own Milky-Way Galaxy as well as in other Galaxies within the Universe.