In about five billion years, the sun's core will contract, heat up and force its outer layers to expand as it becomes a red giant star. Although this area of astronomy is usually called stellar evolution, that term is misleading since we usually think of evolution as a change from one generation to the next. This study of stars is really an examination of the various stages through which stars go. Stellar life cycles last so long, millions and billions of years, that it almost seems like it must be evolution rather than just stages that each star endures.
Stars are simple things, giant spheres of mainly hydrogen gas.
The one characteristic that separates one star from another is its original mass. Although the sun often is called an average star, it is far above average. The sun probably is more massive, larger and brighter than about 90 percent of the stars in the Milky Way galaxy.