|MadSci Network: Astronomy|
We have to use a lot of different information to know what happens to stars in their lifetimes.
Some of this information is observational---how bright stars are, how hot they are, what they are made out of, etc. [We can determine what stars are made out of or their composition by analyzing the light we get from them, a process called spectroscopy. Moderator]
Some of this information is theoretical. We know how the stuff stars have (mostly hydrogen and helium) behaves when it is hot or dense, and so we can use computers to calculate what stars ought to do.
Stellar astronomy uses both methods to figure out the life cycles of stars.
For example, gravity ought to pull parts of clouds of gas together to form young stars. Some of these young stars should be hot, and emit a lot of blue or ultraviolet light. Other stars should be cool but brighter than our Sun. Sure enough, in many areas where there is a lot of gas and dust in space, we see hot stars and cool, bright stars. We can even see blobs of dense gas coming together to form stars.
We can also do things like calculate what will happen to stars like the Sun in the future---they should get quite bright and then shed parts of their outer layers. We can look at the brightest stars in certain star clusters, and can see this happening.
So in many cases the observations and the calculations agree quite well.
Try the links in the MadSci Library for more information on Astronomy.