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CHAPTER [strong]12[/strong]

RIVUXG The young superstar cluster R136 is located in the Tarantula Nebula in the nearby Large Magellanic Cloud galaxy. These stars, surrounded by glowing interstellar clouds, are only a few million years old.

The Lives of Stars from Birth Through Middle Age

WHAT DO YOU THINK?

  • How do stars form?
  • Are stars still forming today? If so, where?
  • Do more massive stars shine longer than less massive ones? What is your reasoning?
  • When stars like the Sun stop fusing hydrogen into helium in their cores, do the stars get smaller or larger?

Answers to these questions appear in the text beside the corresponding numbers in the margins and at the end of the chapter.

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Time changes everything. We grow up, we grow older, and we die. We are most familiar with changes brought about by aging in our own lives and the lives of other people, but everything else in the universe also alters with time. These changes are due to the forces in nature that cause interactions between matter and energy. For example, over time, exposure to the effects of weather and pollution causes even solid stone to crumble (Figure 12-1).

Figure 12-1: Everything Ages These photos show the effects of water and corrosive gases in the atmosphere. Even without the significant pollution that humans add to the air, everything in contact with it decays.

Gravity provides the energy that enables stars to shine. As we saw in studying the Sun (Chapter 10), gravity compresses stars, thereby heating them and causing fusion to occur. As a result, stars emit huge amounts of radiation. Over time, their chemical compositions, masses, and brightnesses all vary—stars age. Major stages in the life of each star can last from days to hundreds of billions of years. Cumulative lifetimes of stars range from millions to hundreds of billions of years. We will explore their evolution in this and the next chapter.

In this chapter you will discover

  • how stars form
  • what a stellar “nursery” looks like
  • how astronomers use the physical properties of stars to learn about stellar evolution
  • the remarkable transformations of older stars into giants
  • how the Hertzsprung-Russell (H-R) diagram is your guide to the stellar life cycle
  • how pairs of orbiting stars can change each other

Insight Into Science: Beware Poetic License

All too often, the names and descriptions we use for nonliving objects seem to give them human qualities. Anthropomorphism—assigning human attributes to nonhuman creatures or even to nonliving objects, like stars—is descriptive but not scientific. In fact, scientists avoid this practice where possible, because it creates unjustified expectations. Nevertheless, anthropomorphism does creep into astronomy. For lack of better terms, astronomers describe stellar change with words related to living things, such as life, life cycle, evolution, birth, aging, maturing, growing old, and dying. This is poetic license of a sort: It piques our interest but muddles our science. As you study stellar evolution, always be mindful of that.