26.3 Life Cycles of Stars

Do stars change over time? If so, how do they change? To help answer these questions, think about predicting how a baby will look when it grows up. Pictures of the baby's relatives, like Figure 15, would probably help. Astronomers use a similar approach to understand how stars evolve. When astronomers look at an image of a single star, they see a snapshot from a life story that often lasts for billions of years. To learn how a star might change over time, astronomers observe many stars of different ages. These observations provide clues about how stars form, how long they last, and what happens when their fuel runs out.

Figure 15 You can often predict how a baby will look as an adult by looking at other family members. In a similar way, astronomers observe stars of different ages to infer how stars evolve.

Most stars fall on the main sequence of the H-R diagram. Among nearby stars, about 10 percent are white dwarfs and fewer than 1 percent are giants or supergiants. Astronomers have inferred that these different types of stars represent different stages of a star's evolution. For most of their lives, stars are very stable. But a star must change when nuclear fusion has used up the supply of hydrogen in its core.

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