Usually, a star initially contains only hydrogen, and the hydrogen atoms in the star collide with each other at all times, resulting in fission and fusion. Because of the huge mass of stars, the energy generated by fission and fusion competes with the gravity of stars to maintain the stability of star structure. As a result of fission and fusion, the internal structure of the hydrogen atom finally changed, broke and formed a new element-helium. Then, helium atoms also participate in fission and fusion, changing the structure and generating lithium. By analogy, beryllium, boron, carbon and nitrogen will be generated in turn according to the order of the periodic table of elements. Stars will collapse until iron is produced. This is because iron is quite stable and can't participate in fission or fusion, and iron exists in stars, which causes the energy in stars to be insufficient to compete with the gravity of massive stars, which leads to the collapse of stars and eventually the formation of black holes. Like white dwarfs and neutron stars, black holes may have evolved from stars with a mass more than 20 times that of the sun. 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. 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 schwarzschild radius), 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" was born.