Black holes in depth.
A black hole is a region of space that contains at its centre some matter squeezed into a point of infinitesimal size and infinite density. This concentration of matter is called singularity. In a spherical region around the singularity, the inwards gravitational pull is so great that nothing not even light, can escape – that’s why a black hole looks so black.
Origins of black holes
There are two main types of black hole.
A stellar black hole forms from the death of a very large star, which occurs when the star runs out of fuel for its energy producing purposes and explodes in a supernova. During this explosion, part of the star implodes inwards and just keeps on collapsing until it concentrates matter in a single point.
Supermassive black holes are found at the centre of galaxies. These may be a by-product of the process of galaxy formation.
Detection of black holes
Because no matter or light comes out of black holes they are difficult to detect by normal means. A black hole would look like a tiny, black circular disc, very difficult to see against the black background of space, however, a hole should be detectable from the behaviour of the matter such as, gas, dust and stars close to the black hole, as this matter would be gravitationally attracted towards it. Several objects in our galaxy have been believed to be black holes because the gas from stars is being attracted towards these areas and then whirling around into them. Astronomers are now certain that there is a supermassive black hole in the center of the Milky Way galaxy. The evidence for this is based on the behavior of stars – their positions show they are orbiting something that is compact and has a very high mass but also seems to be invisible (see this video version of ESO’s full dome show Journey to the Centre of the Milky Way).
Education Links about Black Holes are here.