Universe Tenth Edition Chapter 20 Stellar Evolution: The Deaths of Stars Roger Freedman Robert Geller William Kaufmann III

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 By reading this chapter, you will learn

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 What remains after a supernova explosion How neutron stars and pulsars are related How novae and X-ray bursts come from binary systems By reading this chapter, you will learn

The Post-Main-Sequence Evolution of a 1-M  Star 20-1: Stars of between 0.4 and 4 solar masses go through two distinct red giant stages

The Post-Main-Sequence Evolution of a 1-M  Star

The Structure of an Old, Moderately Low-Mass AGB Star 20-2: Dredge-ups bring the products of nuclear fusion up to a giant star’s surface

Stellar Evolution in a Globular Cluster

A Carbon Star

The Cat’s Eye Nebula 20-3: Stars of moderately low mass die by gently ejecting their outer layers, creating planetary nebulae

Further Stages in the Evolution of the Sun

Some Shapes of Planetary Nebulae

Making an Elongated Planetary Nebula

Sirius A and its White Dwarf Companion 20-4: The burned-out core of a moderately low-mass star cools and contracts until it becomes a white dwarf

The Mass-Radius Relationship for White Dwarfs

Evolution from Giants to White Dwarfs

Our Sun: The Next Eight Billion Years

Mass Loss from a Supergiant Star

The Structure of an Old High-Mass Star 20-5: High mass stars create heavy elements in their cores

A Core-Collapse Supernova

Turbulence in a Core-Collapse Supernova 20-6: High mass stars violently blow apart in core-collapse supernova explosions

SN 1987A – Before and After 20-7: In 1987 a nearby supernova gave us a close-up look at the death of a massive star

SN 1987A and its “Three Ring Circus” 20-8 Neutrinos emanate from supernovae like SN 1987A

SN 1987A and its “Three Ring Circus”

Supernova Types

20-9 White dwarfs in close binary systems can also become supernovae

A Type IA Supernova

Supernova Light Curves

The Veil Nebula – A Supernova Remnant 20-10: A supernova remnant can be detected at many wavelengths for centuries after the explosion

The Vela Nebula – A Supernova Remnant

Cassiopeia A – A Supernova Remnant

A Summary of Stellar Evolution

A Supernova Pictograph

The Crab Pulsar 20-11: Neutron Stars

The Crab Pulsar

A Recording of a Pulsar

Analogy for How Magnetic Field Strengths Increase

A Rotating, Magnetized Neutron Star

The Light Curve of a Nova 20-12: Explosive nuclear processes on white dwarfs and neutron stars produce novae and bursters

The Light Curve of a Nova

The Light Curve of an X-Ray Burster

Gas Shells in Planetary Nebula IC 418

Key Ideas Late Evolution of Low-Mass Stars: A star of moderately low mass (about 0.4 M  to about 4 M  ) becomes a red giant when shell hydrogen fusion begins, a horizontal-branch star when core helium fusion begins, and an asymptotic giant branch (AGB) star when the helium in the core is exhausted and shell helium fusion begins. As a moderately low-mass star ages, convection occurs over a larger portion of its volume. This takes heavy elements formed in the star ’ s interior and distributes them throughout the star.

Key Ideas Planetary Nebulae and White Dwarfs: Helium shell flashes in an old, moderately low-mass star produce thermal pulses during which more than half the star ’ s mass may be ejected into space. This exposes the hot carbon-oxygen core of the star. Ultraviolet radiation from the exposed core ionizes and excites the ejected gases, producing a planetary nebula. No further nuclear reactions take place within the exposed core. Instead, it becomes a degenerate, dense sphere about the size of the Earth and is called a white dwarf. It glows from thermal radiation; as a white dwarf cools, it becomes dimmer.

Key Ideas Late Evolution of High-Mass Stars: Unlike a moderately low-mass star, a high-mass star (initial mass more than about 4 M  ) undergoes an extended sequence of thermonuclear reactions in its core and shells. These include carbon fusion, neon fusion, oxygen fusion, and silicon fusion. In the last stages of its life, a high-mass star has an iron-rich core surrounded by concentric shells hosting the various thermonuclear reactions. The sequence of thermonuclear reactions stops here, because the formation of elements heavier than iron requires an input of energy rather than causing energy to be released.

Key Ideas Core-Collapse Supernovae: A star with an initial mass greater than 8 M  dies in a violent cataclysm in which its core collapses and most of its matter is ejected into space at high speeds. The luminosity of the star increases suddenly by a factor of around 10 8 during this explosion, producing a supernova. More than 99% of the energy from a core-collapse supernova is emitted in the form of neutrinos from the collapsing core. The matter ejected from the supernova, moving at supersonic speeds through interstellar gases and dust, glows as a nebula called a supernova remnant.

Key Ideas A Type II supernova is the result of the collapse of the core of a massive star, as are supernovae of Type Ib and Type Ic White Dwarf Supernovae: An accreting white dwarf in a close binary system can also become a supernova when carbon fusion ignites explosively throughout such a degenerate star. This is called a thermonuclear supernova. One scenario for Type Ia supernovae is an accreting white dwarf in a close binary with an star; another scenario involves the merger of two white dwarfs.

Key Ideas Neutron Stars: A neutron star is a dense stellar corpse consisting primarily of closely packed degenerate neutrons. Neutron stars form in a Type II core-collapse supernova. They are held up from further collapse by degenerate neutron pressure A neutron star typically has a diameter of about 20 km, a mass less than 3 M , a magnetic field times stronger than that of the Sun, and a rotation period of roughly 1 second. Pulsars: A pulsar is a source of periodic pulses of radio radiation. These pulses are produced as beams of radio waves from a neutron star ’ s magnetic poles sweep past the Earth.

Key Ideas Novae and Bursters: Material from an ordinary star in a close binary can fall onto the surface of the companion white dwarf or neutron star to produce a surface layer in which thermonuclear reactions can explosively ignite. Explosive hydrogen fusion may occur in the surface layer of a companion white dwarf, producing the sudden increase in luminosity that we call a nova. Explosive helium fusion may occur in the surface layer of a companion neutron star. This produces a sudden increase in X-ray radiation, which we call a burster.