Black hole
From Wikipedia, a free encyclopedia written in simple English for easy reading.
A black hole is an object in the Universe that has such a strong pull of gravity, even light is unable to escape it. Until recently, many astronomers did not even know if they existed, but through using telescopes and observing the Universe, they found objects with such a strong gravity, they could be nothing else except a black hole.
[edit] History
In 1784, an English geologist named John Michell wrote that it might be possible for something to be so big and heavy that gravity would pull even light toward it. Gravity gets stronger the bigger and more massive (heavier) something is. For a small thing, like a rocket, to escape from a larger thing, like Earth, it has to go upward very fast or it will fall back down. The speed that it must travel upward to get away from Earth's gravity is called escape velocity. Bigger planets (like Jupiter) and stars have much stronger gravity than Earth, so the escape velocity is much faster. John Mitchell thought it was possible for something to be so big that the escape velocity would be faster than the speed of light, so even light could not escape.
Some scientists thought Mitchell might be right, but others thought that light had no mass and would not be pulled by gravity. His theory was forgotten.
In 1915, Albert Einstein wrote an explanation of gravity called general relativity. It is a very complicated theory, but there are two important things about it:
- Mass causes space (and spacetime) to bend, or curve. Moving things "fall along" or follow the curves in space. This is what we call gravity.
- Light always travels at the same speed, and is affected by gravity. If it seems to change speed, it is really travelling along a curve in spacetime.
A few months later, another German physicist named Karl Schwarzschild calculated that a black hole could exist.
In 1930, Subrahmanyan Chandrasekhar predicted that stars heavier than the sun could collapse when they ran out of hydrogen to burn and died. In 1939, Robert Oppenheimer and H. Snyder calculated that a star would have to be at least three times as massive as the sun to form a black hole.
In 1967, John Wheeler gave black holes the name "black hole" for the first time. Before that, they were called "dark stars."
In 1970, Stephen Hawking and Roger Penrose proved that black holes must exist. Although the black holes are invisible, the matter that is falling into them is very bright.
[edit] How do black holes form?
Most black holes are made when a giant star, at least three times bigger than our own Sun, dies. Stars die when they run out of hydrogen or other fuel to burn and start to cool off.
A big star's death is called a supernova. Stars are usually in equilibrium, meaning they are making enough energy to push their mass outward against the force of gravity. When the star runs out of energy, gravity takes over. Gravity pulls the center of the star inward very quickly (so quickly that it would have to be repeated several thousand times before it took up a single second), and it collapses into a little ball. The collapse is so fast and violent that it makes a shock wave, and that causes the rest of the star to explode outward.
The ball in the center is so dense (a lot of mass in a small space, or volume), that if you could somehow scoop only one teaspoon of material and bring it to Earth, it would be the heaviest single object on Earth. This densely packed ball is called a singularity.
Even without a supernova, a black hole will form any time there is a lot of matter in a small space, without enough energy to act against gravity and stop it from collapsing.
[edit] What do black holes look like?
The singularity in the middle of a black hole cannot be seen, because light cannot escape its gravity. Around the tiny singularity, there is a large area which no light can escape. The boundary of this area is called the event horizon. The gravity of the black hole gets weaker at a distance. The event horizon is farthest point where it is still strong enough to trap light.
Even farther away, light and matter will be pulled toward the black hole. If a black hole is surrounded by matter, the matter will form an "accretion disk" (accretion means "gathering"). An accretion disk looks something like the rings of Saturn. The disk is very hot and shoots X-ray radiation into space.
Most black holes are too far away and small to see the accretion disk and jet, though. The best way to know one is there is by seeing how stars, gas, and other things behave around it. With a black hole nearby, even objects as big as a star move in a different way, and a lot faster than they would if the black hole was not there. Also, if a black hole passes between us and a source of light very far away, the light will become quite distorted, much like a fun-house mirror in a circus, until the black hole moves out of the way. The light can also be magnified, like a magnifying glass, allowing scientists to see things farther away.
Black holes have also been found in the middle of every major galaxy in the Universe. These are called supermassive black holes, and are the biggest black holes of all. They formed when the Universe was very young, and also helped to form all the galaxies.
Some black holes are also responsible for making quasars. When astronomers first found quasars, they thought they had found objects close to us, but after using a measuring technique called red shift, they discovered these quasars were actually very far away in the Universe. A quasar occurs when a black hole consumes all the gas surrounding it. As the gas gets close to the black hole itself, it heats up from a process called friction, and glows so brightly that this light can be seen on the other side of the Universe. It is often brighter than the whole galaxy the quasar is in.
A lot of science fiction writers use black holes in their stories, and many scientists wish to find one relatively close to Earth to study one better.
