(Background) These frames are the conceptions of noted space artist Dana Berry. The ten frames depict a sequence of the birth, evolution, and death of a binary-star system. The sequence starts with the large rendering of a 1-solar-mass star and a 4-solar-mass star in the process of formation and then proceeds clockwise from top left. Some highlights:
(Inset B) Nearing the end of its life, the 4-solar-mass star swells, spilling gas onto its companion and forming an accretion disk around it.
(Inset E) The red giant has reached the point where the gravity of the original two stars cannot contain the gas. The result is a gentle expulsion—forming a planetary nebula rich in oxygen gas (hence the green color).
(Inset G) Much later, an accretion bridge again joins the two stars as the lower mass star becomes a red giant.
(Inset I) The end point of the system is two white dwarfs of roughly equal mass circling each other forevermore.
Studying this chapter will enable you to:
Explain why stars evolve off the main sequence.
Outline the events that occur after a Sun-like star exhausts the supply of hydrogen in its core.
Summarize the stages in the death of a typical low-mass star and describe the resulting remnant.
Contrast the evolutionary histories of high-mass and low-mass stars.
Discuss the observations that help verify the theory of stellar evolution.
Explain how the evolution of stars in binary systems may differ from that of isolated stars.
After reaching the main sequence, a newborn star changes little in outward appearance for more than 90 percent of its lifetime. However, at the end of this period, as the star begins to run out of fuel and die, its properties once again change greatly. Aging stars travel along evolutionary tracks that take them far from the main sequence as they end their lives. In this and the next two chapters, we will study the evolution of stars during and after their main-sequence burning stages. We will find that the ultimate fate of a star depends primarily on its mass, although interactions with other stars can also play a decisive role, and that the final states of stars can be strange indeed. By continually comparing theoretical calculations with detailed observations of stars of all types, astronomers have refined the theory of stellar evolution into a precise and powerful tool for understanding the universe.
