Read Chapters 17-19 from Discovering Astronomy.
WebAssign -- Chapter 17-19. Answer homework questions covering Chapters 17-19.
Chapters 17-19 discuss stellar evolution--how stars begin, how they live (on the main sequence), and how they die.
When we look at stars, we see them at various stages in their life cycle and from this "instantaneous picture" we have to deduce how stars are born, how they grow, and how they die.
Imagine that an alien looks at the following picture and from this picture alone has to figure out how humans are born, how they grow, and how they die. Well, it's not a perfect analogy, but "you get the picture!"
Our best tool for understanding stellar evolution is to look at star clusters. In a cluster, all stars are born at approximately the same time (at least within a time interval that is small compared to the ages of the clusters).
But clusters are of various ages, so we see young clusters where many stars are still on the main sequence and old clusters where fewer stars are left on the main sequence (the rest have become red giants and white dwarfs). This can be seen in the following H-R diagram that shows open clusters of various ages.