What is a Black Hole?


 A black Hole is a region in space where gravity is so strong that not even light can escape from it. Black holes in our galaxy are thought to be formed when stars more than ten times as massive as our Sun end their lives in a supernova explosion. There is also evidence indicating that super massive black holes (more massive than ten billion Suns) exist in the centres of some galaxies.

            How Big Is A Black Hole?

There are two different ways to describe how big something is. We can say how much mass it has, or we can say how much space it takes up…

There is no limit to how much or how little mass a black hole can have. Any amount of mass at all can in principle be made to form a black hole if you compress it to a high enough density.

It is suspected that most of the black holes that are actually out there were produced in the deaths of massive stars, and so we expect those black holes to weigh about as much as a massive star. Astronomers also suspect that other galaxies may harbour extremely massive black holes at their centres. These are thought to weigh about a million times as much as the Sun, or 1x1036 kilograms.

The more massive a black hole is the more space it takes up. In fact, the if one black hole weighs ten times as much as another, its radius is ten times as large. A black hole with a mass equal to that of the Sun would have a radius of 3 kilometres. So a typical 10-solar-mass black hole would have a radius of 30 kilometres, and a million-solar-mass black hole at the centre of a galaxy would have a radius of 3 million kilometres. Three million kilometres may sound like a lot, but it's actually not so big by astronomical standards. The Sun, for example, has a radius of about 700,000 kilometres, and so that super massive black hole has a radius only about four times bigger than the Sun.
If The Earth were a black hole then it would be only about 9m radius roughly the size of my bedroom
 

If a black hole existed, would it eventually suck up all the matter in the Universe?

Hell, no. A black hole has a "horizon," which means a region from which you can't escape. If you cross the horizon, you're doomed to eventually hit the singularity (middle). But as long as you stay outside of the horizon, you can avoid getting sucked in. In fact, to someone well outside of the horizon, the gravitational field surrounding a black hole is no different from the field surrounding any other object of the same mass. In other words, a one-solar-mass black hole is no better than any other one-solar-mass object (such as, for example, the Sun) at "sucking in" distant objects.

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What happens if Luke falls into a black hole?

Let's suppose that Luke gets into his spaceship and points it straight towards the million-solar-mass black hole in the centre of our galaxy. (Actually, there's some debate about whether our galaxy actually contains this central black hole, but let's assume it does for the moment.) Starting from a long way away from the black hole, he just turns off his rockets and coasts in. What happens?

At first, he doesn’t feel any gravitational forces at all. Since he is in free fall, every part of his body and his spaceship ...

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