According to the general theory of relativity, a black hole is a region of space from which nothing, including light, can escape. It is the result of the denting of spacetime
caused by a very compact mass. Around a black hole there is an
undetectable surface which marks the point of no return, called an event horizon. It is called "black" because it absorbs all the light that hits it, reflecting nothing, just like a perfect black body in thermodynamics. Under the theory of quantum mechanics black holes possess a temperature and emit Hawking radiation through slow dissipation by anti-protons.
Despite its undetectable interior, a black hole can be observed through its interaction with matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. Alternatively, when gas falls into a stellar black hole from a companion star or nebula, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and Earth-orbiting telescopes.
Astronomers have identified numerous stellar black hole candidates, and have also found evidence of supermassive black holes at the center of every galaxy. After observing the motion of nearby stars for 16 years, in 2008 astronomers found compelling evidence that a supermassive black hole of more than 4 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy.
In 1783, an English geologist named John Mitchell wrote that it might be possible for something to be so big and heavy that the escape speed from its gravity is equal to the speed of light. Gravity gets stronger as something gets bigger or more massive. For a small thing, like a rocket, to escape from a larger thing, like Earth, it has to escape the pull of our gravity or it will fall back. 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 more mass, so have 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 1916, Albert Einstein wrote an explanation of gravity called general relativity. It is a very complicated theory, but there are two important things about it:
In 1930, Subrahmanyan Chandrasekhar predicted that stars heavier than the sun could collapse when they ran out of hydrogen or other nuclear fuels 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 (they cannot be seen), some of the matter that is falling into them is very bright.
Despite its undetectable interior, a black hole can be observed through its interaction with matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. Alternatively, when gas falls into a stellar black hole from a companion star or nebula, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and Earth-orbiting telescopes.
Astronomers have identified numerous stellar black hole candidates, and have also found evidence of supermassive black holes at the center of every galaxy. After observing the motion of nearby stars for 16 years, in 2008 astronomers found compelling evidence that a supermassive black hole of more than 4 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy.
In 1783, an English geologist named John Mitchell wrote that it might be possible for something to be so big and heavy that the escape speed from its gravity is equal to the speed of light. Gravity gets stronger as something gets bigger or more massive. For a small thing, like a rocket, to escape from a larger thing, like Earth, it has to escape the pull of our gravity or it will fall back. 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 more mass, so have 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 1916, 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 traveling along a curve in spacetime.
In 1930, Subrahmanyan Chandrasekhar predicted that stars heavier than the sun could collapse when they ran out of hydrogen or other nuclear fuels 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 (they cannot be seen), some of the matter that is falling into them is very bright.
Formation of black holes
Most black holes are made when a giant star, called a supergiant, at least twenty times bigger than our own Sun dies, and leaves behind a mass that is at least one solar mass. Stars die when they run out of hydrogen
or other nuclear fuel to burn and iron is produced. Iron does not give
off energy and therefore the star has no fuel and in a short amount of
time the star collapses.
A supergiant star's death is called a supernova. Stars are usually in equilibrium, which means they are making enough energy to push their mass outward against the force of gravity. When the star runs out of fuel to make 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. As the gravity pushes the star inward, the pressure in the center of star reaches to such an extreme level that it enables heavier molecules like iron and carbon to interact to release nuclear energy. The release of the energy from the star during a very short period of time (about one hour) is with such a high rate that it outshines an entire galaxy.
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 sink to the core of the planet. If the original star was large enough the densely packed ball is called a singularity, the core of a black hole, but if it was not it would become either a neutron star or a dwarf star.
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.
If supernovas are so bright, why do we not see them often? Actually, there are usually hundreds of years between naked-eye super nova sightings. It is because the period of being a super nova in a star life cycle is only a few hours out of the billions of years in a star's life span. The probability (chance) of looking at a star in sky and that being in super nova state is equal to the ratio of an hour over several billion years.
It is worth mentioning that all of the heavier materials like carbon, oxygen, all the metals, etc., that make the life on the earth possible and are ingredients of all living creatures, can only form in the extreme pressure at the center of a super nova. So we are all a remnant ash from one exploding star several billion years ago.
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 things called quasars. 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. When astronomers first found quasars, they thought they had found objects close to us. After using a measuring technique called red shift, they discovered these quasars were actually very far away in the universe.
At the middle of a black hole, there is a really small thing called a singularity, but it is impossible to see it because light gets sucked into it, and not reflected. Around the tiny singularity, there is a large area where light which would normally pass by gets sucked in as well. The edge of this area is called the event horizon. The gravity of the black hole gets weaker at a distance. The event horizon is the place farthest away from the middle where the gravity is still strong enough to trap light. The singularity is like the pipe under a sink, while the event horizon is like the edge of the drain where water always gets sucked in.
Outside the event horizon, light and matter will still be pulled toward the black hole. If a black hole is surrounded by matter, the matter will form an "accretion disk" (accretion means "gathering") around the black hole. An accretion disk looks something like the rings of Saturn. As it gets sucked in, the matter gets very hot and shoots x-ray radiation into space. Think of this as the water spinning around the hole before it falls in.
Most black holes are too far away and small to see the accretion disk and jet. The best way to know a black hole 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, usually faster than they would if the black hole was not there.
Also, because black holes can bend light passing by, if a black hole passes between us and a source of light very far away, the light will become quite distorted, like a fun-house mirror at 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 (this is called gravity lensing).
We cannot actually see black holes; one way of detecting them is to look at the sky when a black hole passes between us and a source of light, the light bends around the black hole creating a mirror image, so when astronomers see patches of sky that are identical, they may have found a black hole.
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.
Scientists also think black holes might cause wormholes, theoretical "portals" through space.
A supergiant star's death is called a supernova. Stars are usually in equilibrium, which means they are making enough energy to push their mass outward against the force of gravity. When the star runs out of fuel to make 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. As the gravity pushes the star inward, the pressure in the center of star reaches to such an extreme level that it enables heavier molecules like iron and carbon to interact to release nuclear energy. The release of the energy from the star during a very short period of time (about one hour) is with such a high rate that it outshines an entire galaxy.
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 sink to the core of the planet. If the original star was large enough the densely packed ball is called a singularity, the core of a black hole, but if it was not it would become either a neutron star or a dwarf star.
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.
If supernovas are so bright, why do we not see them often? Actually, there are usually hundreds of years between naked-eye super nova sightings. It is because the period of being a super nova in a star life cycle is only a few hours out of the billions of years in a star's life span. The probability (chance) of looking at a star in sky and that being in super nova state is equal to the ratio of an hour over several billion years.
It is worth mentioning that all of the heavier materials like carbon, oxygen, all the metals, etc., that make the life on the earth possible and are ingredients of all living creatures, can only form in the extreme pressure at the center of a super nova. So we are all a remnant ash from one exploding star several billion years ago.
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 things called quasars. 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. When astronomers first found quasars, they thought they had found objects close to us. After using a measuring technique called red shift, they discovered these quasars were actually very far away in the universe.
At the middle of a black hole, there is a really small thing called a singularity, but it is impossible to see it because light gets sucked into it, and not reflected. Around the tiny singularity, there is a large area where light which would normally pass by gets sucked in as well. The edge of this area is called the event horizon. The gravity of the black hole gets weaker at a distance. The event horizon is the place farthest away from the middle where the gravity is still strong enough to trap light. The singularity is like the pipe under a sink, while the event horizon is like the edge of the drain where water always gets sucked in.
Outside the event horizon, light and matter will still be pulled toward the black hole. If a black hole is surrounded by matter, the matter will form an "accretion disk" (accretion means "gathering") around the black hole. An accretion disk looks something like the rings of Saturn. As it gets sucked in, the matter gets very hot and shoots x-ray radiation into space. Think of this as the water spinning around the hole before it falls in.
Most black holes are too far away and small to see the accretion disk and jet. The best way to know a black hole 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, usually faster than they would if the black hole was not there.
Also, because black holes can bend light passing by, if a black hole passes between us and a source of light very far away, the light will become quite distorted, like a fun-house mirror at 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 (this is called gravity lensing).
We cannot actually see black holes; one way of detecting them is to look at the sky when a black hole passes between us and a source of light, the light bends around the black hole creating a mirror image, so when astronomers see patches of sky that are identical, they may have found a black hole.
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.
Scientists also think black holes might cause wormholes, theoretical "portals" through space.
No comments:
Post a Comment