Life cycle of Stars
Our Sun is a perfect example of a star, and there is an incredible amount of stars in the Universe. Stars live for a very long time; millions, billions, or tens of billions of years so we can never really observe the life of a star; its birth, life, and death. In determining the life cycle of a star, astronomers observe many of the billions of stars around us and see them at different stages of life, therefore putting together a star's evolution. It is a star among hundreds of billions of stars within our Milky Way Galaxy, and our galaxy is one of billions of galaxies in the universe. The death of the star is possibly the most interesting aspect of its entire life cycle. The death of a star occurs in phases. Not all stars will visit each phase depending on the size of the star. There are quite a bit of cycles that stars go through. One scenario is that the star will continue to make energy by using hydrogen and helium outside of the core; its surface will rise and fall and the star will become a variable star. After it gets out of control, the layers of gas will pull away, forming a shell of gas known as a planetary nebula. The other scenario is that the star will continue to shine through the fusion of helium nuclei, in the triple alpha process. The star is now a white dwarf, and further contraction is prevented by the repulsion of electrons in the core. Very heavy stars will continue to fuse heavy elements in order to produce more energy. However, once iron is formed, it cannot be fused to make more energy since it has such a high binding energy and is therefore very stable. The core will collapse under gravity and huge amounts of gas on the surface of the star will explode out. This star is now called a supernova. There are also neutron stars that form but that’s another process. These were some of the steps that take place when stars go through their life cycle. I hope you understand this topic more clearly now.