‘Black holes’ are truly black. When an object gets within a certain distance from a black hole, it will get swallowed forever with no chance to escape. That includes light, which means that black holes do not shine.How do astronomers detect black holes if they are unable to see them? Well, to be precise, astronomers do not detect black holes. But they do detect the phenomena that can only be explained by the existence nearby of objects that match the description of black holes! The strong gravitational attraction of a black hole affects the motion of nearby objects. When astronomers see a star circling around something, but they cannot see what that something is, they may suspect it is a black hole, or a neutron star - the ultra-dense ‘corpse’ of a star.

Astronomers can even infer the mass of a black hole by measuring the mass of the star and its speed. The same kind of calculation can be done whith supermassive black holes that lurk at the centre of many galaxies, including our own galaxy, the Milky Way.

In the Milky Way, observations have revealed the existence of stars and gas moving very fast near the centre, a behaviour that can only be explained if a mass of several million times that of the Sun is at the centre of the galaxy.
Such mass has to be concentrated within a radius of only 10 light-days - roughly 40 times times the distance from the Sun to Pluto - and is most likely to be a black hole.

In fact, at the very centre of our galaxy, radio and X-ray telescopes have detected a powerful source called ‘Sagittarius A’, identified as the candidate to be this massive black hole.


View: Full Article | Source: ESA