The Deaths of Stars

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The Deaths of Stars
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• Chapter 10

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The End of a Stars Life
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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
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Evolution off the Main Sequence Expansion into a
Red Giant
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Hydrogen in the core completely converted into He
? Hydrogen burning (i.e. fusion of H into He)
ceases in the core.
H burning continues in a shell around the core.
He core H-burning shell produce more energy
than needed for pressure support
Expansion and cooling of the outer layers of the
star ? red giant
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Expansion onto the Giant Branch
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Expansion and surface cooling during the phase of
an inactive He core and a H-burning shell
Sun will expand beyond Earths orbit!
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Degenerate Matter
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Matter in the He core has no energy source left.
? Not enough thermal pressure to resist and
balance gravity
? Matter assumes a new state, called degenerate
matter
Electron energy
Pressure in degenerate core is due to the fact
that electrons can not be packed arbitrarily
close together and have small energies.
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Red Giant Evolution
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H-burning shell keeps dumping He onto the core.
He core gets denser and hotter until the next
stage of nuclear burning can begin in the core
He fusion through the triple-alpha process
4He 4He ? 8Be g 8Be 4He ? 12C g
The onset of this process is termed the helium
flash
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Evidence for Stellar Evolution Star Clusters
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Stars in a star cluster all have approximately
the same age!
More massive stars evolve more quickly than less
massive ones.
If you put all the stars of a star cluster on a
HR diagram, the most massive stars (upper left)
will be missing!
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HR Diagram of a Star Cluster
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High-mass stars evolved onto the giant branch
Turn-off point
Low-mass stars still on the main sequence
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Estimating the Age of a Cluster
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The lower on the MS the turn-off point, the older
the cluster.
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Main Sequence Dwarfs
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Recall
Stars with less than 0.4 solar masses are
completely convective.
Mass
? 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.
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Sunlike Stars
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Sunlike stars ( 0.4 4 solar masses) develop a
helium core.
Mass
? Expansion to red giant during H burning shell
phase
? Ignition of He burning in the He core
? Formation of a degenerate C,O core
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White Dwarfs
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Degenerate stellar remnant (C,O core)
Extremely dense 1 teaspoon of white dwarf
material mass 16 tons!!!
Chunk of white dwarf material the size of a beach
ball would outweigh an ocean liner!
white dwarfs Mass Msun Temp. 25,000
K Luminosity 0.01 Lsun
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Low luminosity high temperature gt White dwarfs
are found in the lower center/left of the H-R
diagram.
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The Chandrasekhar Limit
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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!
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Mass Loss from Stars
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Stars like our sun are constantly losing mass in
a stellar wind (? solar wind).
The more massive the star, the stronger its
stellar wind.
Far-infrared
WR 124
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The Final Breaths of Sun-Like Stars Planetary
Nebulae
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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
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The Formation of Planetary Nebulae
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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 gt
planetary nebula
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Planetary Nebulae
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Often asymmetric, possibly due to

• Stellar rotation

• Magnetic fields

• Dust disks around the stars

The Butterfly Nebula
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Mass Transfer in Binary Stars
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In a binary system, each star controls a finite
region of space, bounded by the Roche lobes (or
Roche surfaces).
Lagrangian points points of stability, where
matter can remain without being pulled toward one
of the stars.
Matter can flow over from one star to another
through the inner lagrange point L1.
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Recycled Stellar Evolution
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Mass transfer in a binary system can
significantly alter the stars masses and affect
their stellar evolution.
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White Dwarfs in Binary Systems
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Binary consisting of white dwarf main-sequence
or red giant star gt WD accretes matter from the
companion
X ray emission
Angular momentum conservation gt accreted matter
forms a disk, called accretion disk.
T 106 K
Matter in the accretion disk heats up to 1
million K gt X ray emission gt X ray binary.
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Nova Explosions
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Hydrogen accreted through the accretion disk
accumulates on the surface of the white dwarf

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

Nova Cygni 1975

• Explosive onset of H fusion

• Nova explosion

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Recurrent Novae
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T Pyxidis
In many cases, the mass transfer cycle resumes
after a nova explosion.
? Cycle of repeating explosions every few years
decades.
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The Fate of our Sunand the End of Earth
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• Sun will expand to a red giant in 5 billion
  years
• Expands to Earths orbit
• Earth will then be incinerated!
• Sun may form a planetary nebula (but uncertain)
• Suns C,O core will become a white dwarf

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The Deaths of Massive Stars Supernovae
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Final stages of fusion in high-mass stars (gt 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 Supernova
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Numerical Simulations of Supernova Explosions
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The details of supernova explosions are highly
complex and not quite understood yet.
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Supernova Remnants
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X rays
The Crab Nebula Remnant of a supernova observed
in a.d. 1054
The Veil Nebula
Optical
Cassiopeia A
The Cygnus Loop
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Synchrotron Emission and Cosmic-Ray Acceleration
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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.
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The Famous Supernova of 1987 Supernova 1987A
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Before
At maximum
Unusual type II supernova in the Large Magellanic
Cloud in Feb. 1987
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The Remnant of Supernova 1987A
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Ring due to supernova ejecta catching up with
pre-supernova stellar wind also observable in X
rays.
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Observations of Supernovae
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Supernovae can easily be seen in distant galaxies.
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Type I and II Supernovae
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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