
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.
How is a stellar black hole created?
- 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.
What does a black hole look like?
- 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.
What evidence do we have for the existence of black holes?
- 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.
How does the Hubble Space Telescope search for black holes?
- 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.
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