1
Chapter 11: The Deaths of Stars
Chapter 11
The Deaths of Stars
Chapter Outline
11-1 Giant Stars
11-1a Expansion into a Giant
11-1b Helium Fusion
11-1c Star Clusters: Evidence of Evolution
11-2 Deaths of Low-Mass Stars
11-2a Red Dwarfs
11-2b Medium-Mass (Sunlike) Stars
11-2c Planetary Nebulae
11-2d White Dwarfs
Article: Star Custer H-R Diagrams
11-2eThe Fate of the Sun and the End of the Earth
11-3 The Evolution of Binary Systems
11-3a Mass Transfer and Accretion Disks
11-3b Novae
11-4 The Deaths of Massive Stars
11-4a Nuclear Fusion in Massive Stars
11-4b Supernova Explosions
11-4c Types of Supernovae
11-4d Observations of Supernovae
11-5 Neutron Stars
11-5a Theoretical Prediction of Neutron Stars
11-5b The Discovery of Pulsars
11-5c The Evolution of Pulsars
11-5d Binary Pulsars
Article: The Lighthouse Model of a Pulsar
11-5e The Fastest Pulsars
11-5f Pulsar Planets
How Do We Know? 11-1 Hypotheses, Theories, and Proofs
11-6 Black Holes
11-6a Escape Velocity
11-6b Schwarzschild Black Holes
11-6c Leaping into a Black Hole
11-6d The Search for Black Holes
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Chapter 11: The Deaths of Stars
11-6e Energy from Compact Objects—Jets
11-6f Energy from Compact Objects—Gamma-Ray Bursts
What Are We? Stardust
Summary
This chapter answers the question, “What happens to a star after it runs out of hydrogen in its
core?” The most important aspect of this chapter is that the evolution of a star depends critically
on its mass. The mass of a star determines to what extent it can squeeze the core and generate
certain core temperatures that determine which nuclear reactions occur.
Also discussed in this chapter is how binary stars evolve. This should be treated as more than an
auxiliary chapter to be covered if time permits. It is from binary-star evolution that novae and
type I supernovae are produced. Additionally, over half of the stars in the universe are believed
to be in multiple star systems, so binary-star evolution could be more common than single-star
evolution.
The life and death stories of stars are important because Earth depends on one star: the Sun.
Perhaps even more important is the fact that the lives and deaths of previous generations of stars
created the atomic elements of which Earth and you are made. If those stars hadn’t lived and
died, you would not exist. This chapter explores details of the life and death stories of stars and
how the types of remaining stars produced depends on the initial mass of the individual star.
Medium-mass stars become giants, but on their way they develop degenerate cores. The
degeneracy of the electrons in the cores of these stars leads to the helium flash. The helium flash
can be understood only if students clearly understand the pressure–temperature thermostat and
the fact that the rate of nuclear fusion increases as the temperature increases. In degenerate
material, the pressure of the gas does not depend on temperature and consequently the pressure–
temperature thermostat cannot regulate the nuclear reaction until the degenerate state of the
matter is reversed. It must be emphasized that the helium flash is not an observable phenomenon
because all of the energy that is produced in the core is absorbed in the massive envelope of the
star. Ultimately, these stars will produce white dwarfs, which are again controlled by the
presence of degenerate electrons.
Massive stars avoid the consequences of degenerate matter by heating their cores more rapidly as
they collapse so that helium begins to fuse before the electrons become degenerate. In these stars
it is important to understand how supernovae occur. As a massive star begins to fuse iron in its
core, a shock wave develops near the core and moves outward through the envelope and rips the
outer layers of the star apart. This supernova is a violent explosion, and the matter that escapes
into the interstellar medium is incorporated into new stars and the planets that form with them.
, 3
Chapter 11: The Deaths of Stars
The stellar remnant from the explosion is either a neutron star or a black hole, and these
remnants are the strangest beasts in the cosmic zoo. Neutron stars are very low luminosity
objects (due to their very small size) and are difficult to observe unless they also happen to be
pulsars. Pulsars have become very important in astronomy and physics because binary pulsars
have allowed us to test the predictions of the general theory of relativity and to locate extrasolar
planets. Black holes are one type of object that most students want to know more about, and
many have misconceived ideas about these objects. The key concept of the section is how black
holes can be detected, and this means understanding how material accretes onto a black hole.
Black holes will be encountered again in chapters 12 and 14, so understanding the ways in which
material accreting onto them produces detectable energy is important.
The deaths of stars are part of a great cycle of stellar birth and death that includes the Sun, Earth,
and you, and by understanding the deaths of stars, students can get a better understanding of their
role in the evolution of the universe.
Demonstration Ideas
Students may have difficulty relating to the lighthouse effect, since many, if not most, have never
seen a lighthouse at work. A simple pulsar model can be built with a large Styrofoam ball, two
small flashlights, a thin rod, and perhaps a spinning motor. Make sure to implant the flashlights
off of the rotation axis. A model can be made with less effort if you can darken your classroom
and spin around in a chair with a flashlight in your hands. Remark to students afterwards that
while they can see the flashlight beam when it doesn’t point at them, this is only because there’s
dust in the air. This won’t happen to a pulsar beam in space.
The rapid spin of pulsars and the formation of accretion disks as a result of conservation of
angular momentum can be demonstrated with water going down a drain. The whole tub before
draining shows virtually no evidence of rotation but swirls rapidly when it reaches the drain.
Supply companies, such as Edmunds, offer a kit of two large interconnected soft drink bottles for
this purpose, but you may wish to make your own. Also, an online video of a twirling figure
skater will show this.
Show your class two strikingly different supernova remnants, and have them write or call out
questions about each. See if other members of the class can answer the questions. If not, write
the unanswerable questions and assign students to try and find answers for the next class. A good
source for images is the astronomy picture of the day website:
https://apod.nasa.gov/apod/astropix.html.
Key Terms
, 4
Chapter 11: The Deaths of Stars
Accretion disk (p. 252)
Angular momentum (p. 252)
Black hole (p. 266)
Chandrasekhar limit (p. 251)
Compact object (p. 250)
Degenerate matter (p. 250)
Event horizon (p. 267)
Gamma-ray burst (p. 270)
General theory of relativity (p. 264)
Giant stars (p. 244)
Gravitational radiation (p. 264)
Gravitational redshift (p. 268)
Horizontal branch (p. 244)
Hypernova (p. 270)
Lighthouse model (p. 260)
Millisecond pulsar (p. 250)
Neutron star (p. 258)
Nova (p. 242)
Planetary nebula (p. 247)
Pulsar (p. 259)
Roche lobe (p. 251)
Schwarzschild radius, RS (p. 267)
Singularity (p. 266)
Supergiant stars (p. 244)
Supernova (p. 242)
Supernova remnant (p. 257)
Synchrotron radiation (p. 256)
Time dilation (p. 268)
Type I supernova (p. 255)
Type II supernova (p. 255)
Answers to Review Questions
1. Why does helium fusion require a higher temperature than hydrogen fusion?
Helium fusion requires a greater temperature than hydrogen fusion because the Coulomb
barrier is greater in helium fusion. In helium fusion, a helium nucleus must be combined
with a beryllium nucleus. The helium nucleus contains two positively charged protons, and
the beryllium nucleus contains four protons. In the proton–proton chain, the largest
Coulomb force exists between the two helium-3 (3He) nuclei, each having only two
Chapter 11: The Deaths of Stars
Chapter 11
The Deaths of Stars
Chapter Outline
11-1 Giant Stars
11-1a Expansion into a Giant
11-1b Helium Fusion
11-1c Star Clusters: Evidence of Evolution
11-2 Deaths of Low-Mass Stars
11-2a Red Dwarfs
11-2b Medium-Mass (Sunlike) Stars
11-2c Planetary Nebulae
11-2d White Dwarfs
Article: Star Custer H-R Diagrams
11-2eThe Fate of the Sun and the End of the Earth
11-3 The Evolution of Binary Systems
11-3a Mass Transfer and Accretion Disks
11-3b Novae
11-4 The Deaths of Massive Stars
11-4a Nuclear Fusion in Massive Stars
11-4b Supernova Explosions
11-4c Types of Supernovae
11-4d Observations of Supernovae
11-5 Neutron Stars
11-5a Theoretical Prediction of Neutron Stars
11-5b The Discovery of Pulsars
11-5c The Evolution of Pulsars
11-5d Binary Pulsars
Article: The Lighthouse Model of a Pulsar
11-5e The Fastest Pulsars
11-5f Pulsar Planets
How Do We Know? 11-1 Hypotheses, Theories, and Proofs
11-6 Black Holes
11-6a Escape Velocity
11-6b Schwarzschild Black Holes
11-6c Leaping into a Black Hole
11-6d The Search for Black Holes
, 2
Chapter 11: The Deaths of Stars
11-6e Energy from Compact Objects—Jets
11-6f Energy from Compact Objects—Gamma-Ray Bursts
What Are We? Stardust
Summary
This chapter answers the question, “What happens to a star after it runs out of hydrogen in its
core?” The most important aspect of this chapter is that the evolution of a star depends critically
on its mass. The mass of a star determines to what extent it can squeeze the core and generate
certain core temperatures that determine which nuclear reactions occur.
Also discussed in this chapter is how binary stars evolve. This should be treated as more than an
auxiliary chapter to be covered if time permits. It is from binary-star evolution that novae and
type I supernovae are produced. Additionally, over half of the stars in the universe are believed
to be in multiple star systems, so binary-star evolution could be more common than single-star
evolution.
The life and death stories of stars are important because Earth depends on one star: the Sun.
Perhaps even more important is the fact that the lives and deaths of previous generations of stars
created the atomic elements of which Earth and you are made. If those stars hadn’t lived and
died, you would not exist. This chapter explores details of the life and death stories of stars and
how the types of remaining stars produced depends on the initial mass of the individual star.
Medium-mass stars become giants, but on their way they develop degenerate cores. The
degeneracy of the electrons in the cores of these stars leads to the helium flash. The helium flash
can be understood only if students clearly understand the pressure–temperature thermostat and
the fact that the rate of nuclear fusion increases as the temperature increases. In degenerate
material, the pressure of the gas does not depend on temperature and consequently the pressure–
temperature thermostat cannot regulate the nuclear reaction until the degenerate state of the
matter is reversed. It must be emphasized that the helium flash is not an observable phenomenon
because all of the energy that is produced in the core is absorbed in the massive envelope of the
star. Ultimately, these stars will produce white dwarfs, which are again controlled by the
presence of degenerate electrons.
Massive stars avoid the consequences of degenerate matter by heating their cores more rapidly as
they collapse so that helium begins to fuse before the electrons become degenerate. In these stars
it is important to understand how supernovae occur. As a massive star begins to fuse iron in its
core, a shock wave develops near the core and moves outward through the envelope and rips the
outer layers of the star apart. This supernova is a violent explosion, and the matter that escapes
into the interstellar medium is incorporated into new stars and the planets that form with them.
, 3
Chapter 11: The Deaths of Stars
The stellar remnant from the explosion is either a neutron star or a black hole, and these
remnants are the strangest beasts in the cosmic zoo. Neutron stars are very low luminosity
objects (due to their very small size) and are difficult to observe unless they also happen to be
pulsars. Pulsars have become very important in astronomy and physics because binary pulsars
have allowed us to test the predictions of the general theory of relativity and to locate extrasolar
planets. Black holes are one type of object that most students want to know more about, and
many have misconceived ideas about these objects. The key concept of the section is how black
holes can be detected, and this means understanding how material accretes onto a black hole.
Black holes will be encountered again in chapters 12 and 14, so understanding the ways in which
material accreting onto them produces detectable energy is important.
The deaths of stars are part of a great cycle of stellar birth and death that includes the Sun, Earth,
and you, and by understanding the deaths of stars, students can get a better understanding of their
role in the evolution of the universe.
Demonstration Ideas
Students may have difficulty relating to the lighthouse effect, since many, if not most, have never
seen a lighthouse at work. A simple pulsar model can be built with a large Styrofoam ball, two
small flashlights, a thin rod, and perhaps a spinning motor. Make sure to implant the flashlights
off of the rotation axis. A model can be made with less effort if you can darken your classroom
and spin around in a chair with a flashlight in your hands. Remark to students afterwards that
while they can see the flashlight beam when it doesn’t point at them, this is only because there’s
dust in the air. This won’t happen to a pulsar beam in space.
The rapid spin of pulsars and the formation of accretion disks as a result of conservation of
angular momentum can be demonstrated with water going down a drain. The whole tub before
draining shows virtually no evidence of rotation but swirls rapidly when it reaches the drain.
Supply companies, such as Edmunds, offer a kit of two large interconnected soft drink bottles for
this purpose, but you may wish to make your own. Also, an online video of a twirling figure
skater will show this.
Show your class two strikingly different supernova remnants, and have them write or call out
questions about each. See if other members of the class can answer the questions. If not, write
the unanswerable questions and assign students to try and find answers for the next class. A good
source for images is the astronomy picture of the day website:
https://apod.nasa.gov/apod/astropix.html.
Key Terms
, 4
Chapter 11: The Deaths of Stars
Accretion disk (p. 252)
Angular momentum (p. 252)
Black hole (p. 266)
Chandrasekhar limit (p. 251)
Compact object (p. 250)
Degenerate matter (p. 250)
Event horizon (p. 267)
Gamma-ray burst (p. 270)
General theory of relativity (p. 264)
Giant stars (p. 244)
Gravitational radiation (p. 264)
Gravitational redshift (p. 268)
Horizontal branch (p. 244)
Hypernova (p. 270)
Lighthouse model (p. 260)
Millisecond pulsar (p. 250)
Neutron star (p. 258)
Nova (p. 242)
Planetary nebula (p. 247)
Pulsar (p. 259)
Roche lobe (p. 251)
Schwarzschild radius, RS (p. 267)
Singularity (p. 266)
Supergiant stars (p. 244)
Supernova (p. 242)
Supernova remnant (p. 257)
Synchrotron radiation (p. 256)
Time dilation (p. 268)
Type I supernova (p. 255)
Type II supernova (p. 255)
Answers to Review Questions
1. Why does helium fusion require a higher temperature than hydrogen fusion?
Helium fusion requires a greater temperature than hydrogen fusion because the Coulomb
barrier is greater in helium fusion. In helium fusion, a helium nucleus must be combined
with a beryllium nucleus. The helium nucleus contains two positively charged protons, and
the beryllium nucleus contains four protons. In the proton–proton chain, the largest
Coulomb force exists between the two helium-3 (3He) nuclei, each having only two
