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Stellar EvolutionStellar Evolution

Where do gold earrings come from?

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GoalsGoals

• 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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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

• New stars.• Lots of different spectral

types.• All on the main sequence.• Or are they?Less than 20 million years old.

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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: low-mass 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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From main sequence to the red giant stage: 100 million years.

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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:

– Non-fusing H– Fusing H– Fusing He– Non-fusing C

“ash”

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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.

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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 know?

• 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.

8B8BM57 – Ring Nebula

8B8BM27 – Dumbell Nebula – copyright NOAO

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Cat’s Eye

8B8BEskimo Nebula

8B8BHourglass 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.

8B8BNGC2440 – HST – copyright 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

• 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!

8B8BM1 – Crab Nebula – copyright VLT

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NGC 4526 – 6 Million parsecs away