Explain the full life cycle of a star that is similar in size to the Sun, starting from a nebula.

A nebula is a cloud of dust and gas that collapses under its own gravity to form a protostar.
1. In the protostar the temperature and pressure rise until the core reaches ~10^7 K, where hydrogen nuclei fuse to form helium, releasing energy. The star then enters the main sequence.
2. On the main sequence the outward pressure from fusion balances the inward gravitational force, giving the star a stable structure.
3. When core hydrogen is exhausted the core contracts, heating the outer layers and the star expands into a red giant.
4. In the red giant core heavier elements are produced by fusion (e.g. helium to carbon and oxygen).
5. Once fusion ceases the star can no longer support itself, it contracts, cools and becomes a white dwarf.