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An
artist's drawing a black hole named Cygnus X-1. It formed when a large
star caved in. This black hole pulls matter from blue star beside it.
Image Credit: NASA/CXC/M.Weiss
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Because no light can get out, we cannot see black holes. They are invisible.
How do black holes form?
A black hole forms when a massive star collapses due to its own gravity to an almost
infinitely small point.
How big
is a black hole?
All
matter in a black hole is squeezed into a region of infinitely small volume,
called the central singularity. The event horizon is an imaginary sphere that
measures how close to the singularity you can safely get. Once you have passed
the event horizon, it becomes impossible to escape: you will be drawn in by the
black hole's gravitational pull and squashed into the singularity.
The size of the event horizon (called the Schwarzschild radius, after the German physicist who discovered it while fighting in the first World War) is proportional to the mass of the black hole. Astronomers have found black holes with event horizons ranging from 6 miles to the size of our solar system. But in principle, black holes can exist with even smaller or larger horizons. By comparison, the Schwarzschild radius of the Earth is about the size of a marble. This is how much you would have to compress the Earth to turn it into a black hole. A black hole doesn't have to be very massive, but it does need to be very compact!
What
types of black holes are there?
Black
holes often look very different from each other. But this is because of variety
in what happens in their surroundings. The black holes themselves are all
identical, except for three characteristic properties: the mass of the black
hole (how much stuff it is made of), its spin (whether and how fast it rotates
around an axis), and its electric charge. Amazingly, black holes completely
erase all of the other complex properties of the objects that they swallow.
Astronomers can measure the mass of black holes by studying the material that orbits around them. So far, we have found two types of black holes: stellar-mass (just a few times heavier than our Sun) or supermassive (about as heavy as a small galaxy). But black holes might exist in other mass ranges as well. For example, recent observations suggest there may be black holes with masses between stellar-mass and supermassive black holes.
Black holes can spin around an axis, although the rotation speed cannot exceed some limit. Astronomers think that many black hole in the Universe probably do spin, because the objects from which black holes form (stars for example) generally rotate as well. Observations are starting to shed some light on this issue, but no consensus has so far emerged. Black holes could also be electrically charged. However, they would then rapidly neutralize that charge by attracting and swallowing material of opposite polarity. So astronomers believe that all black holes in the Universe are uncharged.
There are fourth basic types of black holes:
1. Stellar black hole. It forms when a massive star runs out of nuclear fuel and collapses under its own weight (supernova).
2. Supermassive black hole. These holes hide in the cores of galaxies (including our own galaxy, the Milky Way). They have mass of millions of Suns. These supermassive black holes originate at the time of galaxy formation or formed when smaller black holes merge together. Because of their location in the centers of galaxies, close to many tightly packed stars and gas clouds, supermassive black holes continue to grow on a steady diet of matter.
3. An Intermediate-mass black hole. Mass of such hole varies from a few tens to a few millions of the mass of the Sun. It is not clear how such a black hole would form.
4. Mini black hole. These have the mass of an asteroid or less. None has ever been detected, but they could form under the extreme conditions.
How to find a Black hole?
Either by looking for signs of light bent round them – light that passes close enough to be affected by the black hole’s gravity, but not so close that it gets sucked into it, or by studying the gravity affects on nearby stars and gas around black hole.
Once you spot bent lights, you can
confirm the black hole is there by looking for X-rays. This is because
things that are being sucked into the hole heat up as they spin round
it, and when they get hot enough, they start to radiate X-rays. So if
you spot a weird light-bending pattern in Space, and it’s also emitting
X-rays, there’s a good chance it’s a black hole.
Also when a black hole and a star are close together, high-energy light is made. This kind of light can not be seen with human eyes. Scientists use satellites and telescopes in space to see the high-energy light.
Also when a black hole and a star are close together, high-energy light is made. This kind of light can not be seen with human eyes. Scientists use satellites and telescopes in space to see the high-energy light.
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