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Stellar Evolution
Where do gold earrings come from?
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Goals
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Explain why stars evolve off the main sequence.
What happens when they leave the main sequence?
How does mass affect what happens?
How do stars die?
Where does gold come from?
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From main sequence
to the red giant stage:
100 million years.
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Clusters
• Groups of stars all at the same
distance and age.
• Watch evolution along the H-R
diagram.
• See differences between low-mass
and high-mass.
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The Pleiades
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Less than 20 million years old.
New stars.
Lots of different spectral types.
All on the main sequence.
Or are they?
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Open Clusters
• 600 million years old.
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Globular Cluster
• 12 billion years old.
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The Main Sequence
• A star is a delicate
balance between the
force of gravity
pulling in, and
pressure from the
heat of fusion
pushing out.
• Stars on the main
sequence burn
hydrogen in their
core to produce heat.
• Longest phase of a
star’s life.
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What then?
• When the hydrogen in the core is almost
consumed the balance between gravity pulling in
and pressure pushing out is disturbed.
• The structure and appearance of the star changes
dramatically.
• What happens then, depends on the star’s mass.
• Two cases:
– Low-mass (< 8 x mass of Sun)
– High-mass (> 8 x mass of Sun)
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Low-Mass Stars
• Recall: lowmass stars are
the lower right
side of the H-R
diagram.
• They are small
so they take
longer to
exhaust their
core hydrogen
fuel.
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Helium Ash
• After 10 billion years the stellar
core is “choked” with helium
“ash”.
• Hydrogen fusion continues in a
shell around non-burning core.
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The Red Giant Branch
• Without fusion energy pressure in
core:
– Helium core collapses (no counter to
gravity)
– Gravitational energy heats up core
• Also heats up hydrogen-burning
shell.
• Fuses hydrogen faster in shell.
• Result: More energy is generated
inside the star than before! 
More outward pressure
• The star gets bigger while its
outside gets cooler.
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Helium Fusion
• Outside radius = Mercury’s orbit
• Core radius = Diameter of Earth!
• Pressure and temperature great
enough for the Helium Flash!
3He  C + Energy
• Red Giant Branch:
– hydrogen fusing in a shell
– and helium fusing in core.
•Time: Lasts for 10-20 million years
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The Onion Sun
• Red Giant Stars
• Layers of:
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Non-fusing H
Fusing H
Fusing He
Non-fusing C “ash”
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…And the Solar System?
• A few million years from now:
– Sun becomes slightly brighter
– Ocean’s begin to evaporate
– “Hot House” Earth
• A few billion years from now:
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Sun swells up
Engulfs the inner Solar System
Certain death for terrestrial planets
Possible “spring” on the Jovian ocean-moons!
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Red Supergiant
• What happens when the
Sun runs out of helium in
its core?
• Same as before.
• Core shrinks, surface
expands.
• Radius ~ 3 AU!
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Supergiants, Giants and Dwarfs
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Death
• Core is contracting and heating.
– Surface is cooling and expanding.
• Will it finally become hot enough in core for
Carbon to fuse?
• For the Sun: No.
• Gravity keeps contracting the core: 1000 kg/cm3!
• What stops it?
• Electron degeneracy pressure!
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Electron Degeneracy
Pressure from motion of atoms
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Electron Degeneracy
Pressure from electron shells
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Where are we now?
• Core dead – nothing happening.
• Shells – burning H and He, but soon stop too.
• Outside atmosphere of star still cooling and
expanding.
• …and expanding
• …and expanding
• Force of radiation from burning shells blows the
atmosphere away.
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NGC3242 – HST – Bruce Balick
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M57 – Ring Nebula
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M27 – Dumbbell Nebula – copyright NOAO
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Cat’s Eye
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Eskimo Nebula
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Hourglass Nebula
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White Dwarfs
• Q: What’s left once the
atmosphere blows away?
• A: The exposed electron
degenerate carbon core.
• Size of Earth.
• No more fusion.
• Glow by their heat alone.
• Eventually cool and fade
away  black dwarf.
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NGC2440 – HST – Bruce Balick
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High-Mass Stars
• Think back to the first
carbon core.
• How they get from main
sequence to the carbon
core stage is a little
different.
• Now however, there is
enough mass that it
becomes hot enough to
fuse carbon?
• Hot enough to eventually fuse lots of elements.
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The Iron Core
4H  He + Energy
3He  C + Energy
2C  O + Energy
Fusion takes place in the core when the end result
also yields energy.
• This energy causes pressure which counters
gravity.
• But Iron doesn’t fuse.
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Core-Collapse
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Iron core – no outward pressure.
Gravity wins!
Star collapses rapidly!
Electron degeneracy can’t stop it.
Atomic structure can’t stop it.
Electrons and protons crushed together to produce
neutrons.
• Neutrons pushed together by force of gravity.
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Supernova
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Supernova
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Supernova
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Supernova
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Supernovae
• The result of the catastrophic collapse is the
rebound and explosion of the core.
• From start of collapse to now: 1 second!
• Matter thrown back into the interstellar medium.
• Matter rushing outwards, fuses with matter
rushing inwards.
• Every element after Fe is made in the instant of a
supernova!
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M1 – Crab Nebula – copyright VLT
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NGC 4526 – 6 Million parsecs away
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