Download White Dwarfs

The Deaths of Stars
Outline
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)
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
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
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
(~ 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
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.
Farinfrared
WR 124
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
Two-stage process:
The Ring Nebula
in Lyra
Slow wind from a red giant blows
away cool, outer layers of the star
Fast wind from hot, inner
layers of the star overtakes
the slow wind and excites it
=> Planetary Nebula
The Dumbbell Nebula in Hydrogen and
Oxygen Line Emission
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
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
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)
Low luminosity; high temperature => White
dwarfs are found in the lower left corner of the
Hertzsprung-Russell diagram.
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
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
Mass transfer in a binary
system can significantly
alter the stars’ masses and
affect their stellar evolution.
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
Hydrogen accreted
through the accretion
disk accumulates on the
surface of the WD
Nova Cygni 1975
 Very hot, dense layer
of non-fusing hydrogen
on the WD surface
 Explosive onset of H
fusion
 Nova explosion
Recurrent Novae
T Pyxidis
R Aquarii
In many
cases, the
mass transfer
cycle
resumes after
a nova
explosion.
 Cycle of
repeating
explosions
every few
years –
decades.
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
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
Supernovae can easily be seen in distant galaxies.
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
Xrays
The Crab Nebula:
Remnant of a
supernova
observed in a.d.
1054
Cassiopeia A
Optical
The Cygnus Loop
The Veil Nebula
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
Before
At maximum
Unusual type II Supernova in the
Large Magellanic Cloud in Feb. 1987
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
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
nova
supernova
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
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 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 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. Both a and b above.
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 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 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 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 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 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 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 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 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 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 Questions
**14. What might be evidence that some close binary pairs
have merged to become a single giant star? Remember
conservation principles!
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 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 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 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 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 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 Questions
20. Why do old supernova remnants emit X-rays?
a. Electrons accelerated by magnetic fields produce radiation.
b. The expanding hot gas collides with the interstellar medium.
c. Short-lived unstable isotopes of nickel and cobalt emit Xrays.
d. The remnant gas is excited by the neutrino burst.
e. Radiation from the central black hole excites the gas.
Answers
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a
e
b
e
b
e
d
e
b
e
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e
d
e
e
c
d
c
b
a
b