The name "black hole" is always imaginative. So, what is a "black hole"?
The first word of the name "black" means that it will not emit or reflect any light to the outside world, nor will it emit or reflect other forms of electromagnetic waves - whether the radio waves with the longest wavelength or those with the shortest wavelength γ Ray. So people can't see it. It's definitely "black". The second word "hole" means that as soon as anything enters its boundary, it can't slip out again. It's like a real "bottomless hole".
Some people may think: if I aim a super huge searchlight at the black hole and shine it like a monster mirror, wouldn't the black hole "show its original shape"? Wrong! No matter how strong the light directed at the black hole is, it will be "swallowed" by the black hole without any reflection. This "bottomless hole" is still so "black". It's wonderful to call this strange celestial body a "black hole". Black holes are not suddenly thought of by scientists overnight. As early as 1798, the French scientist Laplace speculated according to the mechanical theory established by Newton: "a luminous star with a density like the earth and a diameter of 250 times that of the sun will not allow any of its light to reach us under its gravity."
What does that mean? Let's start with spaceships. To get rid of the earth's gravity and enter interplanetary space, the speed of the spacecraft must reach at least 11.2 km / s, otherwise it will never escape the control of the earth's gravity. The speed of 11.2 km / S is the minimum speed required for any object to "escape" from the earth's gravitational field, which is called the "escape speed" of the earth. The gravity of the sun is much stronger than that of the earth, so the escape speed of the sun's surface is much higher than that of the earth, which is 618 km / s. Further, if the escape speed of a celestial body reaches the speed of light, it is impossible for even light to escape from it. Such a celestial body is a black hole, and the star mentioned by Laplace is a vivid example. Light is the fastest thing in the universe. Since even light can't escape from the black hole, everything else can't escape.
With the development of science, people have a deeper understanding of black holes. Today, a more accurate statement about black holes is: "black holes are a special celestial body predicted by general relativity. Its basic feature is that they have a closed boundary, known as the 'horizon' of black holes; external matter and radiation can enter the horizon, but things in the horizon cannot escape to the outside." It is precisely because black holes are so "insatiable and insatiable", that they have an indecent nickname: "the most selfish monster in space".
However, things are not so simple. Unexpectedly, the "monster" of black hole is sometimes very "generous". What's going on? It turned out that in the 1970s, British scientists Hawking and others made a more careful investigation of black holes based on quantum mechanics, and found that black holes would stably emit particles like "evaporation". Considering this "evaporation", black holes are no longer absolutely "black". Hawking also proved that every black hole has a certain temperature, and the lower the mass of the black hole, the higher the temperature, and the higher the mass of the black hole, the lower the temperature. The temperature of the black hole is very low and the evaporation is very weak; The temperature of the small black hole is very high, and the evaporation is also very violent, similar to a violent explosion. A black hole as big as the sun takes about 1066 years (i.e. "1" followed by 66 "0") to evaporate; However, a small black hole with the same mass as an asteroid will evaporate completely within 10-22 seconds (21 "0" after the decimal point followed by a "1")!