I think the landlord knows that stars emit light and heat through nuclear fusion, and these reactions mainly occur inside the stars, which makes the whole volume of the stars expand. It's like blowing a balloon, but instead of inflating it with your mouth, the air inside the balloon expands when heated.
Then when the star is too old to be old, nuclear fusion can't go on because of lack of raw materials, so the star will cool down after some weightlessness and extrusion, and retain most of its mass.
Because of the original size, the surface gravity of the star is very large (for example, our sun is only a small star, but its surface gravity is nearly 30 times that of the surface gravity), so it is conceivable how big its surface gravity should be when its size is greatly reduced without reducing its mass too much.
After the theory of relativity came out, researchers speculated that even light could not fly away under such a strong gravity. So even if the remaining part can shine, we can't see the outside, so there is a dark area without optical base, which is a black hole. Of course, the surface of this area (which seems to be defined as "line of sight") is not necessarily on the surface of the remnant star. At this time, the black hole is regarded as a gluttony that only eats and does not pull.
Further research puts forward new properties: the star is rotating at first, so the black hole will also rotate; At first, stars may be charged (for example, the total amount of positive and negative charges on the earth itself is not equal), so black holes may also be charged; Wait a minute.
Then Hawking made a new theoretical discovery from the perspective of information theory, but I don't quite understand it, so I won't talk about it.
Later, observation technology and data increased greatly. In the past few years, it seems that there really are black holes. Although a black hole doesn't make people look at themselves, it seems that there is always a circle of matter around it, ready to be put into a dark area at any time. This is that there is no money here.
Then the actual observation also found some phenomena that could not be explained by the original theory, such as "jet flow". This phenomenon seems to exist on neutron stars.
Finally, it seems that Hawking has recently discovered a new theory that the swallowed substance will be spit out in some way, which seems to explain the mechanism of jet flow. So the black hole changed from a bottomless pit to a garbage reprocessing center.