Black+hole

Black holes are believed to form from massive stars (about 15 - 20 Solar masses) at the end of their life times. The gravitational pull in a black hole is so great that nothing can escape from it, not even light.

Most neutron stars have a mass between 1.4 - 3 times that of the sun. Remember this is the star's mass **after** a supernova, so a lot of the star's material has exploded off into space.

If the mass of the supernova's core remnant is doubled (>3), an entirely new phenomena occurs. For bigger cores the remaining matter continues to collapse on itself. A black hole collapses intself into one point because gravity is so strong. This single, incredibly dense point is called a **singularity**. The singularity is a difficult concept that can best be explained as the point mass at the centre of the hole. These are called black holes.

Black holes are so dense that nothing entering its gravitational field ever escapes, **not even light**! At the edge of a black hole, the point of no return if you will, is referred to as event horizon. Once event horizon is reached a body **cannot** escape.

A black hole doesn't emit light of any kind (UV, gamma, x rays, visible etc...) so how on Earth do we detect them?
 * __So how do we know they exist?__**

If a black hole passes through a cloud of interstellar matter or is close to a normal star, the black hole can pull some of that matter into itself. As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. The heating ionizes the atoms, and when the atoms reach a few million degrees Kelvin, they emit X-rays. The X-rays are sent off into space before the matter crosses the Schwarzschild radius and crashes into the singularity. Thus we can see this X-ray emission.

__**Myth**__ But contrary to popular belief, a black hole is not a cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the same mass, the Earth's orbit around the Sun would be **unchanged**. Of course the Earth's temperature would change, things would get much colder!

To be sucked into a black hole the Earth would have to cross inside the Schwarzschild radius. At this distance from the singularity, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape.

If the Sun was replaced with a black hole that had the same mass as the Sun the Schwarzschild radius would be 3 km. Hence the Earth would have to get very close to get sucked into a black hole at the centre of our solar system.

__**Existence**__ Astrophysicists generally agree that black holes exist. There is good observational evidence from X-ray observations and from the Hubble Space Telescope that there are massive black holes.

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