Please briefly say, what is a black hole?

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 as small as a certain value (called "schwarzschild radius" in astronomy), even the light emitted from the vertical plane is captured. At this time, the star becomes a black hole. To call it "black" means that it is like a bottomless pit in the universe. Once anything falls in, it seems that it can't escape. 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.

We have introduced the formation process of white dwarfs and neutron stars in detail. 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 to the center point until it becomes a "point" with zero volume and infinite density. And once its radius shrinks to a certain extent (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.