Black Holes

What is a black hole?

The idea of black holes goes back to the eighteenth century when scientists first realised they might exist.
A black hole is defined by the escape velocity that would have to be attained to escape from the gravitational pull exerted upon an object.
The escape velocity of an object depends on the ratio of its mass to radius.
The point at the centre of a black hole is called a singularity.
Within a certain distance of a black hole, not even light can escape it.

A common type of black hole is the type produced by some dying stars.
When a very massive star exhausts its nuclear fuel it explodes as a supernova.
Smaller star cool and die peacefully as a white dwarf.
Stars and planets at a safe distance will circle around the black hole, much like the motion of the planets around the Sun.
It is suggested that a star would have to have a mass equivalent to three time the mass of our Sun to become a black hole.

There are two types of black holes: Schwarzschild - Non-rotating black hole and Kerr - Rotating black hole.
The Schwarzschild radius can be calculated using the equation for escape speed.
Black holes can shrink until they vanish entirely.

Since black holes do not shine, they are difficult to detect.
Detection of black holes is most likely to occur when we find an invisible object that has a mass which could only possibly demonstrate one.
Black holes emit radiation and scientists use this to identify them.
Even though we cannot see a black hole, it does have three properties that can or could be measured: Mass, Electric charge, Rate of rotation (angular momentum).
Astronomers have found evidence for black holes in several galaxies including the M87 galaxy.
This black hole was estimated to be 3 billion times the mass of our Sun.

Binary X-ray sources are places to find strong black hole candidates.
The matter around a stellar black hole is composed of gas and dust.
Matter around a supermassive black hole is also made up of stars.
An instrument aboard the Hubble Space Telescope, called the STIS, was installed in 1997.
STIS is the space telescopes main black hole hunter.
A spectrograph splits the incoming light into its rainbow pattern. Each spectrum tells scientists how fast they are going.
The faster the stars go, the more massive the central object must be.