What is the suns life cycle?

Answer 1

Very simply (glossing over some of the details):-

The sun formed from a cloud of space gas made up from material formed in the big bang (hydrogen, helium mainly) and debris from older stars which have exploded (all the rest of the elements). This cloud of gas began to clump together under the influence of gravity and formed a swirling dusty disk with a dense core. As more gas collapsed into the gore it got hotter and hotter and was crushed more and more by gravity until the middle of the core formed a plasma and started nuclear fusion changing hydrogen into helium. The sun lit up as a star with the planets forming out the the remaining gas disk.

When a star the size of our sun is fusing hydrogen there is enough hydrogen there to keep this process up for 90% of its 10 billion year life (the sun is now 4.6 billion years old) and it called a main sequence star. At the end of this hydrogen fusing period when the hydrogen runs out the sun converts to fusing helium in its core (there are details I have missed out here) to make oxygen and carbon. This fusion releases more energy and the core will get hotter and this will cause the sun to expand its outer layers. The sun will turn into a red giant star and its surface will be near where the earth now orbits. It will also throw of clouds of gas containing oxygen and carbon as what is called a planetary nebular (the debris may eventually be incorporated in new stars).

As was the case with hydrogen burning, the fuel for helium burning will eventually run out in the core. Once again the core, now carbon and oxygen, will collapse in an attempt to increase its temperature to the point of carbon burning. However, in a star like the sun that core temperature will never be attained and the naked core of the sun will turn into a white dwarf [See related question] star and slowly cool to a dark ember, a black dwarf [See related question]

NOTE: Other stars which are bigger (more massive) or smaller (less massive) than our sun follow markedly different life cycles. The same is true of some binary stars.