What is the essence of the nebula hypothesis?

Kant-Laplace said: The solar system is formed by the contraction of a nebula. First, the sun was formed, and then the remaining nebula material further contracted and evolved to form planets.

Modern xingyun says that the solar system is formed by a dense nebula in the Milky Way. This nebula revolves around the center of the Milky Way. When it passes through the spiral arm, it is compressed and its density increases. After reaching a certain density, it gradually shrinks under its own gravity. In the process of contraction, on the one hand, the inner part of the nebula is heated, and finally the primitive sun is formed. When the temperature of the central part of the primitive sun reached 7 million degrees Celsius, the fusion of hydrogen ignited the thermonuclear reaction of helium, and the modern sun was really born. On the other hand, due to the shrinking volume of the nebula, the acceleration of rotation and the increase of centrifugal force, a nebula disk is gradually formed near the equatorial plane. In the process of condensation and annexation, the matter on the nebula disk eventually evolved into planets and other small celestial bodies.