Black hole
A mass of matter is called a black hole if its gravitational field is strong enough to completely bend space-time around itself so that nothing, not even light, can escape. A small amount of matter compressed to a very high density (such as the Earth compressed to the size of a pea), or an extremely large blob of lower density material (such as a few million times the mass of the Sun spread over a diameter as large as the solar system) This situation can occur in a ball (roughly having the density of water).
The first person to propose that there might be a 'black hole' with a gravity so strong that light cannot escape was John, a special member of the Royal Society. Mitchell, who stated this idea to the Royal Society in 1783. Mitchell's calculations were based on Newton's gravity theory and the particle theory of light. The former was the best gravity theory at the time. The latter imagined light as being like a small A cannonball's stream of tiny particles (now called photons). Mitchell postulated that these light particles should be affected by gravity like any other object. Since Ole Romer accurately determined the speed of light more than 100 years ago .So Mitchell was able to calculate how big an object with the density of the Sun would have to be to have an escape velocity greater than the speed of light.
If such objects existed, light could not escape them, so they should be black. Sun The escape velocity on the surface is only 0.2% of the speed of light, but if you imagine a series of increasingly larger objects with the same density as the sun, the escape velocity increases rapidly. Mitchell pointed out that such an object with a diameter of 500 times the diameter of the sun (similar to The size of the solar system is similar), its escape velocity should exceed the speed of light.
Pierre Laplace independently reached and published the same conclusion in 1796. Mitchell once A particularly prescient comment pointed out that although such objects are invisible, 'if any other luminous objects happened to orbit them, it might still be possible to infer the existence of the central object from the motion of these orbiting objects. In other words, Mitchell believed that black holes would be most easily discovered if they existed in binary stars. But this idea of ??black holes was forgotten in the 19th century until astronomers realized that black holes can be created in another way, in It was brought up again when studying Albert Einstein's general theory of relativity.
Karl Schwarzschild, an astronomer who served on the Eastern Front during World War I, was the first to pay attention to love. One of the people who analyzed the conclusions of Einstein's theory. General relativity explains gravity as the result of the curvature of space-time near matter. Schwarzschild calculated a rigorous mathematical model of the geometric properties of space-time around a spherical object and sent his calculations to Einstein , which submitted them to the Prussian Academy of Sciences in early 1916. These calculations showed that for 'any' mass there is a critical radius, now called the Schwarzschild radius, which corresponds to an extreme deformation of space-time such that if the mass is squeezed Pressed within a critical radius, space will curve around the object and isolate it from the rest of the universe. It effectively becomes a separate universe of its own, and nothing (not even light) can escape it.< /p>
For the Sun the Schwarzschild radius is kilometers. For the Earth it is equal to 0.88 centimeters. This does not mean that the center of the Sun or Earth has a suitably sized black hole now known as a black hole (a term first coined by John Whew in 1967). Something that is used in this sense exists. At this distance from the center of the celestial body, there are no anomalies in space-time. What Schwarzschild’s calculations show is that if the sun is squeezed into a sphere with a radius of 2.9 kilometers, or if the earth Squeezed into a ball with a radius of only 0.88 centimeters, they will always be inside a black hole and isolated from the outside universe. Matter can still fall into such a black hole, but nothing can escape.
These conclusions It was considered a purely mathematical curio for decades because no one thought that real, physical objects could collapse to the extreme densities required to form a black hole. White dwarfs began to be understood in the 1920s, but even white dwarfs have roughly the same mass as the sun but are about the same size as the Earth, with a radius much greater than 3 kilometers. People also failed to realize in time that if there was a large amount of matter of general density, a black hole could be created that was essentially the same as what Mitchell and Laplace had imagined. The Schwarzschild radius corresponding to any mass M is given by the formula 2GM/c2, where G is the gravitational constant. c is the speed of light.
In the 1930s, Subrahmanyan Chandrasekhar showed that even a white dwarf star is stable only if its mass is less than 1.4 times the mass of the sun, and any death If the star is heavier than this, it will collapse further. Some researchers have thought about the possibility that this might lead to the formation of neutron stars. The typical radius of a neutron star is only about 1/700 of a white dwarf, which is a few kilometers in size. But this idea was not widely accepted until the discovery of pulsars in the mid-1960s, which proved that neutron stars did exist.
This rekindled interest in black hole theory, because neutron stars are on the verge of becoming black holes.
Although it is difficult to imagine compressing the sun to a radius of less than 2.9 kilometers, it is now known that there are neutron stars with a mass equal to that of the sun and a radius of less than 10 kilometers. From neutron stars to black holes, it is only one step away.
Theoretical studies show that the behavior of a black hole is dictated by just three of its properties - its mass, its charge and its rotation (angular momentum). Black holes with no charge and no rotation are described by the Schwarzschild solution of Einstein's equations; black holes with charge and no rotation are described by the Reisner-Nordstrom solution; black holes with no charge and rotation are described by Kerr Solution description; a black hole with electric charge and rotation is described by Kerr-Newman solution. Black holes have no other properties, which is summed up by the famous saying 'black holes have no hair'. A realistic black hole should probably spin and have no charge, so the Kerr solution is the most interesting.
It is now believed that both black holes and neutron stars are produced in the death struggle of an E-mass star when it explodes as a supernova. Calculations show that any dense supernova remnant with a mass roughly less than 3 times the mass of the sun (Oppenheimer-Folkov limit) can form a stable neutron star, but any dense advancing and receding nova remnant with a mass greater than this limit will collapse into a black hole , its contents will be pressed into the singularity at the center of the black hole, which is exactly the mirror inversion of the Big Bang singularity from which the universe was born. If such an object happened to be in orbit around an ordinary star, it would strip the companion star of material, forming an accretion disk of hot material funneling toward the black hole. The temperature in the accretion disk can rise so high that it radiates X-rays, making the black hole detectable.
In the early 1970s, Mitchell's prediction had repercussions: such an object was discovered in a binary star system. An X-ray source called Cygnus X-1 was identified as the star HDE226868. The orbital dynamics of this system indicate that the source's X-rays come from an object smaller than Earth in orbit around a visible star, but the source's mass is greater than the Oppenheimer-Folkov limit. This could only be a black hole. Since then, a handful of other black holes have been identified using the same method. In 1994, the system V404 Cygnus became the best black hole 'candidate' so far. This is a system with a star with a mass of 70% of the sun's mass orbiting an X-ray source of about 12 times the mass of the sun. However, these recognized black hole identifications are probably just the tip of the iceberg.
Such 'stellar-mass' black holes, as Mitchell realized, can only be detected if they are in a binary star system. An isolated black hole lives up to its name—it is dark and undetectable. However, according to astrophysics theory, many stars should end their lives as neutron stars or black holes. Observers have actually detected almost as many suitable black hole candidates in binary systems as they have discovered pulsars, which means that the number of isolated stellar-mass black holes should be the same as the number of isolated pulsars, a conjecture supported by theoretical calculations. support. About 500 active pulsars are now known in our galaxy. But theory shows that the active period of a pulsar as a radio source is very short, and it quickly decays into an undetectable quiet state. So, accordingly, there should be more ‘dead’ pulsars (quiet neutron stars) around us. Our Milky Way contains 100 billion bright stars and has been around for billions of years. The best estimates are that our galaxy contains 400 million dead pulsars today, and even conservative estimates of the number of stellar-mass black holes are in the low 100 million range. If there are really so many black holes, and black holes are scattered randomly in the Milky Way, the nearest black hole is only 15 light-years away from us. Since there's nothing unique about our galaxy, every other galaxy in the universe should contain just as many black holes. Ic
Galaxies may also contain some kind of object much like the 'black star' originally envisioned by Mitchell's Laplace. Such objects, now called 'supermassive black holes', are thought to exist at the centers of active galaxies and quasars, and the gravitational energy they provide may account for the enormous energy source of these objects. A black hole the size of the solar system and millions of times the mass of the sun can eat the material of one or two stars from its surroundings every year. In this process, a large part of the star's mass will be converted into energy according to Einstein's division of labor E=mc2. Quiet supermassive black holes may exist at the centers of all galaxies, including our own.
In 1994, using the Hubble Space Telescope, a hot material disk of about 150,000 parsecs in size was discovered in the galaxy M87, which is 15 million parsecs away from our Milky Way. It is moving around the center of the galaxy. , the speed reaches about 2 million kilometers per hour (about 5*10-7 5 times 10 to the power of 7, centimeters per second, almost 0.2% of the speed of light). A gas jet with a length of more than 1 kiloparsec is shot from the central 'engine' of the M87. The orbital velocity in the central accretion disk of M87 conclusively proves that it is under the gravitational control of a supermassive black hole with a mass of 3 billion times that of the sun. The jet can be explained as energy emerging from one of the polar regions of the accretion system.
Also in 1994, astronomers from Oxford University and Keele University identified a stellar-mass black hole in a binary star system called V404 Cygnus.
We have already pointed out that the orbital parameters of this system allowed them to accurately 'weigh' the black hole, which concluded that the mass of the black hole is about 12 times that of the sun, while ordinary stars orbiting it only have about 70% of the mass of the sun. This is the most accurate measurement of the mass of a 'black star' so far, and therefore it is also the best and unique proof of the existence of black holes.
Some people speculate that a large number of micro black holes may have been produced in the big bang. Or primordial black holes, which provide a significant portion of the mass of the universe. The typical size of this kind of micro black hole is about the same as an atom, and its mass is about 100 million tons (10-11, 10 to the power of 11 kilograms). There is no evidence that such objects exist, but it is also difficult to prove that they do not exist.