It is easy to imagine a "black hole" as a "big black hole", but it is not. The so-called "black hole" is such a celestial body: its gravitational field is so strong that even light cannot escape.
According to the general theory of relativity, the gravitational field will bend space-time. When the star is large, its gravitational field has little influence on time and space, and the light emitted from a certain point on the surface of the star can be emitted in any direction in a straight line. The smaller the radius of the star, the greater the bending effect on the surrounding space-time, and the light emitted at some angles will return to the surface of the star along the curved space.
When the radius of a star is less than a certain value (called "Schwartz radius" in astronomy), even the light emitted from the vertical plane will be captured. At this time, the star becomes a black hole. To say it is "black" means that once anything falls in, it can't escape, including light. In fact, black holes are really invisible, which we will talk about later.
So, how are black holes formed? In fact, like white dwarfs and neutron stars, black holes probably evolved from stars.
When a star ages, its thermonuclear reaction has exhausted the fuel (hydrogen) in the center, and the energy generated by the center is running out. In this way, it no longer has enough strength to bear the huge weight of the shell. Therefore, under the heavy pressure of the shell, the core began to collapse, until finally a small and dense star was formed, which could balance the pressure again.
Stars with smaller mass mainly evolve into white dwarfs, while stars with larger mass may form neutron stars. According to scientists' calculations, the total mass of neutron stars cannot be more than three times that of the sun. If it exceeds this value, there will be no force to compete with its own gravity, which will lead to another big collapse.
This time, according to scientists' guesses, matter will move relentlessly towards the center point until it becomes a small volume and tends to be very dense. When its radius shrinks to a certain extent (it must be smaller than that of schwarzschild radius), as we mentioned above, the huge gravity makes it impossible to shoot out even light, thus cutting off all the connection between the star and the outside world-a "black hole" is born.
Compared with other celestial bodies, black holes are too special. For example, a black hole is invisible, so people can't directly observe it, and even scientists can only make various guesses about its internal structure. So how does a black hole hide itself? The answer is-bending space. As we all know, light travels in a straight line. This is a basic common sense. But according to the general theory of relativity, space will bend under the action of gravitational field. At this time, although the light still propagates along the shortest distance between any two points, it is not a straight line, but a curve. Figuratively speaking, it seems that light should go straight ahead, but strong gravity pulls it away from its original direction.
On earth, because the gravitational field is very small, this bending is very small. Around the black hole, this space deformation is very large. In this way, even if the light emitted by the star is blocked by the black hole, although part of it will fall into the black hole and disappear, the other part will bypass the black hole in the curved space and reach the earth. So we can easily observe the starry sky on the back of the black hole, just as the black hole does not exist. This is the invisibility of black holes.
More interestingly, some stars not only send light energy directly to the earth, but also send light in other directions, which may be refracted by the strong gravity of nearby black holes and reach the earth. In this way, we can see not only the "face" of this star, but also its side and even its back!
"Black hole" is undoubtedly one of the most challenging and exciting astronomical theories in this century. Many scientists are trying to uncover its mystery, and new theories are constantly put forward. However, these latest achievements in contemporary astrophysics cannot be explained clearly here in a few words. Interested friends can refer to special works.
Black holes can be divided into two categories according to their composition. One is a dark energy black hole and the other is a physical black hole. Dark energy black holes are mainly composed of huge dark energy rotating at high speed, and there is no huge mass inside. Huge dark energy rotates at a speed close to the speed of light, and a huge negative pressure is generated inside to devour objects, thus forming a black hole. See "Cosmic Black Hole Theory" for details. Dark energy black holes are the basis of galaxy formation, as well as galaxy clusters and galaxy clusters. Physical black holes are formed by the collapse of one or more celestial bodies, and their mass is huge. When the mass of a physical black hole is equal to or greater than that of a galaxy, we call it a strange black hole. Dark energy black holes are very big, and can be as big as the solar system. But physical black holes are small and can be turned into singularities.
adhesion
Translation by Ramesh narayan and Eliot Quartal
Black holes are usually found because they gather around gas to produce radiation. This process is called accretion. The efficiency of high temperature gas radiating heat energy will seriously affect the geometric and dynamic characteristics of accretion flow. At present, thin disks with high radiation efficiency and thick disks with low radiation efficiency have been observed When accretion gases approach the central black hole, their radiation is extremely sensitive to the rotation of the black hole and the existence of the horizon. The photometric and spectral analysis of accretion black holes provides strong evidence for the existence of rotating black holes and horizons. The numerical simulation also shows that relativistic jets often appear in accretion black holes, some of which are driven by the rotation of black holes.
Astrophysicists use the word "accretion" to describe the flow of matter to a central gravitational body or a central expanding material system. Accretion is one of the most common processes in astrophysics, and it is precisely because of accretion that many common structures around us are formed. In the early universe, galaxies were formed when gas flowed to the center of gravitational potential well caused by dark matter. Even today, stars are still formed by the collapse and fragmentation of gas clouds under their own gravity, and then accreted by the surrounding gas. Planets, including the earth, are also formed by the accumulation of gas and rocks around newly formed stars. But when the central celestial body is a black hole, accretion will show its most spectacular side.
However, black holes do not absorb everything. They also emit protons outward.
Explosive black hole
Black holes will glow, shrink in size and even explode. When British physicist Stephen Hawking made this language in 1974, the whole scientific community was shocked. Black holes were once thought to be the ultimate destination of the universe: nothing can escape from them. They devour gas and stars, and their mass increases, so the volume of holes will only increase. Hawking's theory is an inspiration-led thinking leap. He combined general relativity with quantum theory. He found that the gravitational field around the black hole releases energy and consumes the energy and mass of the black hole. This "Hawking radiation" is negligible for most black holes, while small black holes radiate energy at a very high speed until the black hole explodes.
Wonderful shrinking black hole
When a particle escapes from a black hole without repaying the borrowed energy, the black hole will lose the same amount of energy from its gravitational field. Einstein's formula E = MC 2 shows that the loss of energy will lead to the loss of mass. So black holes will become lighter and smaller.
Boil until destruction.
All black holes will evaporate, but big black holes boil very slowly, and their radiation is very weak, so it is difficult to be detected. But as the black hole becomes smaller, this process will accelerate and eventually get out of control. When the black hole becomes insignificant, the gravity will become steeper, producing more escaping particles, and the more energy and mass will be plundered from the black hole. Black holes are becoming more and more trivial, which makes the evaporation speed faster and faster, and the surrounding gas field becomes brighter and hotter. When the temperature reaches 10 15℃, the black hole will be destroyed in the explosion.