In the universe, there are many interstellar diffuse matter. Dense places are like clouds, so they are called interstellar clouds. The sun is formed by interstellar clouds. In the interstellar cloud, due to the action of gravity, it will shrink, and at the same time, the thermal movement of molecules and atoms will produce expansion pressure. When the mass is large and the temperature is not too high, the gravity is greater than the expansion pressure, so the interstellar cloud shrinks under the self-absorption effect. At first, the interstellar cloud contracted rapidly. As the potential energy of gravity is transformed into kinetic energy of thermal motion, the temperature rises. When the density reaches 10-9 grams per cubic meter, eddy current appears in the cloud, thus rotating. At the same time, the surrounding substances are still gathering towards the center. With the increase of the sun, the central temperature and density increase, and energy is transferred by convection. When the central temperature reaches 10,000 degrees and the surface temperature is 2,000 to 3,000 degrees, it emits red light and forms the primitive sun. The sun has just become a star, its volume is much larger than it is now, and the total energy radiated is several times larger. When the sun becomes a star, the contraction process slows down. When the central temperature reaches10 million degrees, a strong fusion reaction begins inside the sun, releasing huge energy. Because the temperature is extremely high, the expansion pressure and gravity reach a balance, and then the sun reaches a stable stage. Now the sun is in the middle of a stable period.