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Note that the following lectures include animations and PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode). The Deaths of Stars. Chapter 13. Guidepost.

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Note that the following lectures include animations and PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode).


As you read this chapter, take a moment to be astonished and proud that the human race knows how stars die. Finding the masses of stars is difficult, and understanding the invisible matter between the stars takes ingenuity, but we humans have used what we know about stars and what we know about the physics of energy and matter to solve one of nature’s deepest mysteries. We know how stars die.

The previous chapter made heavy use of theory in describing the internal structure of stars. We tested that theory against observations of clusters of stars and variable stars. In this chapter, we have more direct observational evidence to help us understand how stars die. We still need theory, but images and spectra of

guidepost continued
Guidepost (continued)

dying stars give us landmarks to guide our theoretical explorations.

This chapter leads us onward, not only to discuss the remains of dead stars—neutron stars and black holes—but it also leads us into discussions of the formation of our galaxy, the origin of the universe, the formation of planets, and the origin of life. Our world is made of atoms that were born in the deaths of stars. This chapter of endings is also a chapter of beginnings.


I. Lower-Main-Sequence Stars

A. Red Dwarfs

B. Sunlike Stars

C. Mass Loss from Sunlike Stars

D. Planetary Nebulae

E. White Dwarfs

II. The Evolution of Binary Stars

A. Mass Transfer

B. Recycled Stellar Evolution

C. Accretion Disks

D. Nova Explosions

E. The End of Earth

outline continued
Outline (continued)

III. The Deaths of Massive Stars

A. Nuclear Fusion in Massive Stars

B. The Iron Core

C. The Supernova Deaths of Massive Stars

D. Types of Supernovae

E. Observations of Supernovae

F. The Great Supernova of 1987

G. Local Supernovae and Life on Earth

the end of a star s life
The End of a Star’s Life

When all the nuclear fuel in a star is used up, gravity will win over pressure and the star will die.

High-mass stars will die first, in a gigantic explosion, called a supernova.

Less massive stars will die in a less dramatic event, called a nova

red dwarfs
Red Dwarfs

Stars with less than ~ 0.4 solar masses are completely convective.


Hydrogen and helium remain well mixed throughout the entire star.

No phase of shell “burning” with expansion to giant.

Star not hot enough to ignite He burning.

sunlike stars
Sunlike Stars

Sunlike stars (~ 0.4 – 4 solar masses) develop a helium core.


Expansion to red giant during H burning shell phase

Ignition of He burning in the He core

 Formation of a degenerate C,O core

inside stars
Inside Stars


mass loss from stars
Mass Loss From Stars

Stars like our sun are constantly losing mass in a stellar wind (solar wind).

The more massive the star, the stronger its stellar wind.


WR 124

the final breaths of sun like stars planetary nebulae
The Final Breaths of Sun-Like Stars: Planetary Nebulae

Remnants of stars with ~ 1 – a few Msun

Radii: R ~ 0.2 - 3 light years

Expanding at ~10 – 20 km/s ( Doppler shifts)

Less than 10,000 years old

Have nothing to do with planets!

The Helix Nebula

the formation of planetary nebulae
The Formation of Planetary Nebulae

Two-stage process:

Slow wind from a red giant blows away cool, outer layers of the star

The Ring Nebula in Lyra

Fast wind from hot, inner layers of the star overtakes the slow wind and excites it => Planetary Nebula

planetary nebulae
Planetary Nebulae

Often asymmetric, possibly due to

  • Stellar rotation
  • Magnetic fields
  • Dust disks around the stars

The Butterfly Nebula

the remnants of sun like stars white dwarfs
The Remnants of Sun-Like Stars: White Dwarfs

Sunlike stars build up a Carbon-Oxygen (C,O) core, which does not ignite Carbon fusion.

He-burning shell keeps dumping C and O onto the core. C,O core collapses and the matter becomes degenerate.

Formation of a White Dwarf

white dwarfs
White Dwarfs

Degenerate stellar remnant (C,O core)

Extremely dense:1 teaspoon of WD material: mass ≈ 16 tons!!!

Chunk of WD material the size of a beach ball would outweigh an ocean liner!

White Dwarfs:

Mass ~ Msun

Temp. ~ 25,000 K

Luminosity ~ 0.01 Lsun

white dwarfs 2
White Dwarfs (2)

Low luminosity; high temperature => White dwarfs are found in the lower left corner of the Hertzsprung-Russell diagram.

the chandrasekhar limit
The Chandrasekhar Limit

The more massive a white dwarf, the smaller it is.

Pressure becomes larger, until electron degeneracy pressure can no longer hold up against gravity.

WDs with more than ~ 1.4 solar masses can not exist!

mass transfer in binary stars
Mass Transfer in Binary Stars

In a binary system, each star controls a finite region of space, bounded by the Roche Lobes (or Roche surfaces).

Lagrange points = points of stability, where matter can remain without being pulled towards one of the stars.

Matter can flow over from one star to another through the Inner Lagrange Point L1.

recycled stellar evolution
Recycled Stellar Evolution

Mass transfer in a binary system can significantly alter the stars’ masses and affect their stellar evolution.

white dwarfs in binary systems
White Dwarfs in Binary Systems

X-ray emission

T ~ 106 K

Binary consisting of WD + MS or Red Giant star => WD accretes matter from the companion

Angular momentum conservation => accreted matter forms a disk, called accretion disk.

Matter in the accretion disk heats up to ~ 1 million K => X-ray emission => “X-ray binary”.

nova explosions
Nova Explosions

Hydrogen accreted through the accretion disk accumulates on the surface of the WD

  • Very hot, dense layer of non-fusing hydrogen on the WD surface

Nova Cygni 1975

  • Explosive onset of H fusion
  • Nova explosion
recurrent novae
Recurrent Novae

T Pyxidis

In many cases, the mass transfer cycle resumes after a nova explosion.

R Aquarii

Cycle of repeating explosions every few years – decades.

future of the sun
Future of the Sun


the fate of our sun and the end of earth
The Fate of Our Sun and the End of Earth
  • Sun will expand to a Red giant in ~ 5 billion years
  • Expands to ~ Earth’s radius
  • Earth will then be incinerated!
  • Sun may form a planetary nebula (but uncertain)
  • Sun’s C,O core will become a white dwarf
the deaths of massive stars supernovae
The Deaths of Massive Stars: Supernovae

Final stages of fusion in high-mass stars (> 8 Msun), leading to the formation of an iron core, happen extremely rapidly: Si burning lasts only for ~ 1 day.

Iron core ultimately collapses, triggering an explosion that destroys the star:

A Supernova

observations of supernovae
Observations of Supernovae

Supernovae can easily be seen in distant galaxies.

type i and ii supernovae
Type I and II Supernovae

Core collapse of a massive star: Type II Supernova

If an accreting White Dwarf exceeds the Chandrasekhar mass limit, it collapses, triggering a Type Ia Supernova.

Type I: No hydrogen lines in the spectrum

Type II: Hydrogen lines in the spectrum

supernova remnants
Supernova Remnants


The Crab Nebula:

Remnant of a supernova observed in a.d. 1054

Cassiopeia A

The Veil Nebula


The Cygnus Loop

synchrotron emission and cosmic ray acceleration
Synchrotron Emission and Cosmic-Ray Acceleration

The shocks of supernova remnants accelerate protons and electrons to extremely high, relativistic energies.

“Cosmic Rays”

In magnetic fields, these relativistic electrons emit

Synchrotron Radiation.

the famous supernova of 1987 sn 1987a
The Famous Supernova of 1987: SN 1987A


At maximum

Unusual type II Supernova in the Large Magellanic Cloud in Feb. 1987

the remnant of sn 1987a
The Remnant of SN 1987A

Ring due to SN ejecta catching up with pre-SN stellar wind; also observable in X-rays.

local supernovae and life on earth
Local Supernovae and Life on Earth

Nearby supernovae (< 50 light years) could kill many life forms on Earth through gamma radiation and high-energy particles.

At this time, no star capable of producing a supernova is < 50 ly away.

Most massive star known (~ 100 solar masses) is ~ 25,000 ly from Earth.

new terms
New Terms



thermal pulse

planetary nebula

compact object

black dwarf

Chandrasekhar limit

Roche lobe

Roche surface

Lagrangian point

accretion disk

supernova (type I)

supernova (type II)

carbon deflagration

supernova remnant

synchrotron radiation

discussion questions
Discussion Questions

1. How certain can we be that the sun will never explode as a supernova? What does it mean when a scientist uses the word “certainty”?

2. If we lived for a billion years, we might notice a star that exploded occasionally as a nova and finally destroyed itself in a supernova explosion. What could account for this evolution?

quiz questions
Quiz Questions

1. What event marks the end of every star's main sequence life?

a. The end of hydrogen fusion in the core.

b. The beginning of the CNO cycle.

c. The beginning of the triple-alpha process.

d. The formation of a planetary nebula.

e. Both a and c above.

quiz questions1
Quiz Questions

2. Why can't the lowest-mass stars become giants?

a. They never get hot enough for the triple-alpha process.

b. Their gravity is too weak to stop them from expanding beyond the giant phase.

c. They live so long that none has ever left the main sequence.

d. The rate of hydrogen-shell fusion is too slow to cause the star to expand.

e. They are fully connective, and never develop a hydrogen shell fusion zone.

quiz questions2
Quiz Questions

3. Why do we suspect that all white dwarfs observed in our galaxy were produced by the death of medium-mass stars?

a. The range of white dwarf masses is narrow.

b. High-mass stars do not produce white dwarfs.

c. Our galaxy's age is less than the life expectancy of low-mass stars.

d. Both a and b above.

e. Both b and c above.

quiz questions3
Quiz Questions

4. What observational evidence do we have that stars are losing mass?

a. The solar wind.

b. Stellar emission lines at ultraviolet and X-ray wavelengths.

c. Some absorption lines in the spectra of giant stars are blue shifted.

d. Both a and b above.

e. All of the above.

quiz questions4
Quiz Questions

5. What type of spectrum does the gas in a planetary nebula produce?

a. A continuous spectrum.

b. An emission line spectrum.

c. An absorption line spectrum.

d. An emission line spectrum superimposed on a continuous spectrum.

e. All of the above.

quiz questions5
Quiz Questions

6. Why are the stars found inside planetary nebulae only at temperatures above 25,000 K?

a. These stars are fusing hydrogen at their surface.

b. These stars have at least two active layers of fusion.

c. These stars have multiple concentric layers of active fusion.

d. We cannot see the interior stars that are below this temperature, as they are too dim.

e. Planetary nebulae glow due to the ionization of low-density gas by a hot interior star.

quiz questions6
Quiz Questions

7. What happens to white dwarfs as they age?

a. Their surface temperature decreases.

b. Their luminosity decreases.

c. Their size decreases.

d. Both a and b above.

e. All of the above.

quiz questions7
Quiz Questions

8. Why have no black dwarfs yet been observed in our galaxy?

a. They can only be detected by their gravitational influence on a binary companion.

b. They are too dim for our present-day telescopes to detect.

c. Astronomers are not motivated to search for such objects.

d. They are all too distant (in theory) to be detected.

e. Our galaxy is too young for any to have formed.

quiz questions8
Quiz Questions

9. What unusual property do all higher-mass white dwarfs have?

a. They are cooler than lower-mass white dwarfs.

b. They are smaller than lower-mass white dwarfs.

c. They are less dense than lower-mass white dwarfs.

d. They are less luminous than lower-mass white dwarfs.

e. All of the above.

quiz questions9
Quiz Questions

10. What prevents gravity from shrinking a white dwarf to a smaller size?

a. Helium core fusion.

b. Helium shell fusion.

c. Hydrogen core fusion.

d. Hydrogen shell fusion.

e. Degenerate electrons.

quiz questions10
Quiz Questions

11. Which stars have high rates of mass loss due to intense stellar winds?

a. High-mass stars.

b. Newly forming stars.

c. Stars approaching death.

d. Both a and b above.

e. All of the above.

quiz questions11
Quiz Questions

12. What happens to a star when it becomes a giant if it has a close binary companion?

a. Radiation from the giant's surface can ionize the companion's gases.

b. Radiation from the companion's surface can vaporize the giant.

c. Matter can be transferred from the companion to the giant

d. Matter can be transferred from the giant to the companion.

e. The giant can explode as a nova or supernova.

quiz questions12
Quiz Questions

13. What can happen to the white dwarf in a close binary system when it accretes matter from the companion giant star?

a. The white dwarf can become a main sequence star once again.

b. The white dwarf can ignite the new matter and flare up as a nova.

c. The white dwarf can accrete too much matter and detonate as a supernova type Ia.

d. Both a and b above.

e. Both b and c above.

quiz questions13
Quiz Questions

14. What evidence do we have that some close binary pairs have merged to become a single giant star?

a. Two sets of spectral lines, one from each star, have been observed for some giants.

b. Alternating radial motion of a giant is revealed by an alternating Doppler shift.

c. Some giants are between luminosity classes.

d. Some giants are pulsating variable stars.

e. Some giant stars have rapid rotation.

quiz questions14
Quiz Questions

15. Which type of star eventually develops several concentric zones of active shell fusion?

a. Low-mass stars.

b. Medium-mass stars.

c. High-mass stars.

d. White dwarfs.

e. Neutron stars.

quiz questions15
Quiz Questions

16. Which of the following trends accurately represents the characteristics of the several different fusion zones inside a late-stage high-mass star going from the outer to inner-most zone?

b. Mass of individual nuclei increases.

c. Fusion lifetime decreases.

a. Temperature decreases.

d. Both a and b above.

e. All of the above.

quiz questions16
Quiz Questions

17. Why can't massive stars generate energy from iron fusion?

a. The temperature at their centers never gets high enough.

b. The density at their centers is too low.

c. Iron fusion consumes energy.

d. Not enough iron is present.

e. Both a and b above.

quiz questions17
Quiz Questions

18. Which of the following statements accurately describe some observed properties of type Ia and type II supernovae?

a. Type Ia supernovae have hydrogen lines in their spectra.

b. Type II supernovae have hydrogen lines in their spectra.

c. Type Ia supernovae are more luminous.

d. Both a and c above.

e. Both b and c above.

quiz questions18
Quiz Questions

19. Which type of supernova leaves NO core remnant?

a. Type Ia supernovae.

b. Type Ib supernovae.

c. Type II supernovae.

d. Both a and b above.

e. All of the above.

quiz questions19
Quiz Questions

20. Why do old supernova remnants emit X-rays?

a. Electrons accelerated by magnetic fields produce synchrotron radiation.

b. The expanding hot gas collides with the interstellar medium.

c. Short-lived unstable isotopes of nickel and cobalt emit X-rays.

d. The remnant gas is excited by the neutrino burst.

e. Radiation from the central black hole excites the gas.


1. a

2. e

3. e

4. e

5. b

6. e

7. d

8. e

9. b

10. e

11. e

12. d

13. e

14. e

15. c

16. d

17. c

18. e

19. a

20. b