The Deaths of Stars - Kruger Physics & Astronomy
Transcript of The Deaths of Stars - Kruger Physics & Astronomy
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The Deaths of Stars
Chapter 10
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Evidence that Stars Die 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.
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Evolution off the Main Sequence:
Expansion into a Red Giant
Hydrogen in the core
completely converted into He:
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
→ “Hydrogen burning”
(i.e. fusion of H into He)
ceases in the core.
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Expansion onto the Giant Branch
Expansion and
surface cooling during
the phase of an
inactive He core and
a H- burning shell
The Sun will expand
beyond Earth’s orbit!
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Degenerate Matter
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:
Pressure in degenerate
core is due to the fact that
electrons can not be
packed arbitrarily close
together and have small
energies.
Ele
ctr
on
en
erg
y
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Red Giant Evolution
He-core gets denser
and hotter until the
next stage of nuclear
burning can begin in
the core:
He fusion
begins!
The onset of this
process is called
the Helium Flash
H-burning shell keeps
dumping He onto the core.
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Evidence for Stellar
Evolution: Star Clusters
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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High-mass stars
evolved onto the
giant branch
Low-mass stars
still on the main
sequence
Turn-off point
HR Diagram of a Star Cluster
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Estimating
the Age of
a Cluster
The lower on
the MS the
turn-off
point, the
older the
cluster.
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Red Dwarfs Recall:
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
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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
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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.
Far-infrared
WR 124
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The Final Breaths of Sun-Like Stars:
Planetary Nebulae
The Helix Nebula
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!
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The Ring Nebula in Lyra
The Formation of
Planetary Nebulae Two-stage process:
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
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Planetary Nebulae
The Butterfly Nebula
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White Dwarfs Degenerate stellar remnant (C,O core)
Extremely dense:
1 teaspoon of WD material: mass ≈ 16 tons!!!
White Dwarfs:
Mass ~ Msun
Temp. ~ 25,000 K
Luminosity ~ 0.01 Lsun
Chunk of WD material the size of a beach ball
would outweigh an ocean liner!
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Low luminosity; high temperature => White dwarfs are found in
the lower center/left of the Herzsprung-Russell diagram
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Size of a White Dwarf
• White dwarfs with the same mass as the Sun are about the same size as Earth.
• Higher-mass white dwarfs are smaller.
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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!
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What can happen to a white dwarf in a close binary system?
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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).
Matter can flow over from one star to another through the
Inner Lagrange Point L1.
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Recycled Stellar
Evolution
Mass transfer in a binary
system can significantly
alter the stars’ masses and
affect their stellar evolution.
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Accretion Disks
• Mass falling
toward a white
dwarf from its
close binary
companion has
some angular
momentum.
• The matter
therefore orbits
the white dwarf
in an accretion
disk.
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Matter in the
accretion disk heats
up to ~ 1 million K
=> X-ray emission
=> “X-ray binary”
T ~ 106 K
X-ray
emission
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Nova • The
temperature of accreted matter eventually becomes hot enough for hydrogen fusion.
• Fusion begins suddenly and explosively, causing a nova.
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Nova
• The nova star
system
temporarily
appears much
brighter.
• The explosion
drives accreted
matter out into
space.
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Nova Explosions
Nova Cygni 1975
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Recurrent Novae The nova does not destroy the white dwarf or binary star.
In many cases, the mass transfer cycle resumes after a
nova explosion.
→ Cycle of repeating explosions every
few years – decades
T Pyxidis
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Type I Supernova
If enough material accretes onto the white dwarf, it can
cause the white dwarf to begin runaway nuclear fusion.
When this happens, it explodes as a Type I Supernova.
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Tycho’s Supernova Remnant
Type I Supernova that
burst in 1572, getting
as bright as Venus in
our sky.
Its position was
mapped by Tycho
Brahe. The remnant
was not found until
1952.
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The Fate of our Sun
and the End of Earth
• The 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
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The Deaths of Massive Stars:
(Type II) 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
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Iron builds up
in the core until
degeneracy
pressure can no
longer resist
gravity.
The core then
suddenly
collapses,
creating a Type
II Supernova
explosion.
The Death Sequence of a High-Mass Star
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Numerical
Simulations of
Supernova
Explosions
The details of
supernova
explosions are
highly complex
and not quite
understood yet.
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Supernova Remnants
The Cygnus Loop
The Veil Nebula
The Crab Nebula:
Remnant of a
supernova observed
in a.d. 1054
Cassiopeia A Optical
X-rays
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The Famous Supernova
of 1987: SN 1987A
Before At maximum
Unusual type II Supernova in the Large
Magellanic Cloud in Feb. 1987
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The Remnant of SN 1987A
Ring due to SN ejecta catching up with pre-SN
stellar wind; also observable in X-rays
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Observations of Supernovae
Supernovae can easily be seen in distant galaxies.
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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 I Supernova.
Type I: No hydrogen lines in the spectrum
Type II: Hydrogen lines in the spectrum