Why are there stars?

Stars originate from nebulae, and you can compare them with gases. For example, under normal circumstances, the force between gas molecules is molecular attraction, but why doesn't gas condense and contract? Because gas molecules are in irregular motion, thermal motion and molecular gravity are two factors that restrict the formation of uniform and stable gas. If the gas is cooled to a critical value, it will liquefy.

In addition to nebulae, the Big Bang produced a large number of electrons and protons, which gradually cooled with the expansion and combined them into hydrogen atoms. This is the primary nebula. The main nebula relies on the balance between gravity and thermal motion to achieve uniformity and stability. With the expansion of the universe, the temperature of the nebula gradually decreases, thus the internal balance is destroyed, resulting in micro-clusters, attracting more substances. Gravity potential energy is converted into internal heat energy. When the mass reaches a certain critical value, the temperature inside the star will be very high, which will ignite the thermonuclear reaction and eventually form a star. The nuclear fuel of a star is hydrogen. When hydrogen is completely fused, the converted helium will converge into carbon, and finally through a series of fusion, nuclear reactions will take place in turn, and finally it will be fused into iron. Then, if the mass is too small, it will evolve into a white dwarf. If the mass is large, a supernova explosion will occur, and the exploded material will form a secondary nebula. The secondary nebula contains C, H, O, N and other elements, and is the material that constitutes the earth and terrestrial planets. The secondary nebula will produce new stars and planets, and the exploded protostellar core will form neutron stars. If the protostar is massive enough, it will form a black hole.