563

CHAPTER

20

R I V U X G Left: The planetary nebula NGC 7293 (the Helix Nebula). Right: The supernova remnant LMC N49.
(NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner/STScI, and T. A. Rector/NRAO; NASA and the Hubble Heritage Team, STScI/AURA)

Stellar Evolution: The Deaths of Stars

LEARNING GOALS

By reading the sections of this chapter, you will learn

20–1 What kinds of nuclear reactions occur inside a star of moderately low mass as it ages
20–2 How evolving stars disperse carbon into the interstellar medium
20–3 How stars of moderately low mass eventually die
20–4 The nature of white dwarfs and how they are formed
20–5 What kinds of reactions occur inside a high-mass star as it ages
20–6 How high-mass stars end with a supernova explosion
20–7 Why supernova SN 1987A was both important and unusual
20–8 What role neutrinos play in the death of a massive star
20–9 How white dwarfs in close binary systems can explode
20–10 What remains after a supernova explosion
20–11 How neutron stars and pulsars are related
20–12 How novae and X-ray bursts come from binary systems

When a star of 0.4 solar mass or more reaches the end of its main-sequence lifetime and becomes a red giant, it has a compressed core and a bloated atmosphere. Finally, the star devours its remaining nuclear fuel and begins to die. As we will learn in this chapter, the character of the star’s death depends crucially on the value of its mass.

A star of relatively low mass—such as our own Sun—ends its evolution by gently expelling its outer layers into space. These ejected gases form a glowing cloud called a planetary nebula such as the one shown here in the left-hand image. The burned-out core that remains is called a white dwarf.

In contrast, a high-mass star ends its life in almost inconceivable violence. At the end of its short life, the core of such a star collapses suddenly, which triggers a powerful supernova explosion that can be as luminous as an entire galaxy of stars. A white dwarf, too, can become a supernova if it accretes gas from a companion star in a close binary system.

In supernovae, nuclear reactions produce a wide variety of heavy elements, which are ejected into the interstellar medium. (The supernova remnant shown above in the right-hand image is rich in these elements.) Such heavy elements are essential building blocks for terrestrial worlds like our Earth. Thus, the deaths of massive stars can provide the seeds for planets orbiting succeeding generations of stars.

564