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Black Holes

There are two types of black hole. The supermassive black holes that lurk at the centre of elliptical and spiral galaxies (like our very own Milky Way) and the black holes left by massive stars when they eventually die.
A black hole is not black. Its lit up by the light from the nearby star it is feeding from. It ventures through space looking for helpless victims that it can guzzle up.
The supermassive black hole situated in the heart of our Galaxy weighs as much as four million Suns. Scientists believe that the formation of this kind of black hole occurs at the same time as the formation of a galaxy. All the stars in a galaxy orbit this massive stellar mass. Our own Sun orbits the Galactic core once every 225 million years or so. Its very likely it,and in fact any member of our solar system could come into contact with a black hole of the other kind...

Stars that are more than twice the size than our own Sun will contract into a black hole after they have died. This has a gravity so strong that not even light can escape.
When a massive star exhausts its nuclear fuel, it will lose heat and contract. The warping of spacetime will become so intense that a black hole will form. Not even light can escape. Time will come to an end inside of it.
The escape speed from Earth is a velocity of 25,000 miles per hour. That's 11km per second. If you can imagine an object with the mass of the Sun and diameter of the Earth,the escape speed would be about 6500km per second. That's about 588 times faster than Earth's true escape speed.
Still keeping the diameter fixed and after two thousand solar masses squeezed inside of this Earth-sized sphere, the escape velocity exceeds the speed of light (300,000km per second). If the escape velocity is greater than the speed of light then nothing can escape. Not even light. This sphere will become a black hole.
Its only appears black because it emits no light and space itself is black.
If the Earth was compacted down to form a black hole, it'd be about the size of a marble.
The closer you get to a black hole, the stronger its gravity pulls. When the escape velocity exceeds the speed of light, this point of no return is called the Schwarzchild radius (named after the German physicist, Karl Schwarzchild).
The Schwarzchild radius of a one-solar-mass black hole is a few km.
The Schwarzchild radius increases in proportion to the mass of a black hole. A million-solar-mass black hole has a radius of about four times bigger than our Sun.
The singularity at the centre of a black hole is called the event horizon. Nothing can escape from inside it. The size of the event horizon is the Schwarzchild radius.
As a black hole leaks its mass energy away, its rate of heat loss gradually mounts, a black hole can't last forever. It is destined to evaporate into nothingness. The process of black hole evaporation is known as "Hawking Radiation" named after Stephen Hawking, who discovered it.

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