astrophysics i: the stellar lifecycle kathy cooksey
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Astrophysics I:Astrophysics I:The Stellar LifecycleThe Stellar Lifecycle
Kathy Cooksey
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How to Make a Star• Collapse interstellar clouds
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•Visible and IR image of the hot protostars in Orion Nebula.
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How to Make Systems
• Cloud around protostar spins faster
• Flattens to a disk– Think pizza dough
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Protostars and Disks• Dust and gas
condense onto dust grains
• Small clumps grow bigger
• Bigger clumps have more mass and attract more matter
• Planetesimals become building blocks of planets Orion Nebula – Copyright O’Dell and Wong
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Now what?• Mass of the star
determines rest of its life!
• More massive star more pressure in core
• More pressure more fusion
• More fusion:– More energy produced
– Hotter
– Shorter life span
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Stellar Stellar EvolutionEvolution
The Life of a Star(like our Sun)
(which does not move in a circle as it evolves
Artist’s rendition)
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The Main Sequence• Balance between:
– Force of gravity pulling in
– Pressure from the heat of fusion pushing out
• Stars on main sequence burn hydrogen in their core to produce heat
• Longest phase of a star’s life
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What then?
• Gravity-pressure balance disturbed when hydrogen in core depleted
• Big change in structure and appearance of the star• “THE END” depends on star’s mass• Two cases:
– Low-mass (< 8 mass of Sun)
– High-mass (> 8 mass of Sun)
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Red Giants
After hydrogen exhausted in core: • Core collapses, releasing energy to
outer layers• Outer layers expand• Increasing temperature and pressure
in core helium fuses
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The End for Low Mass Stars
• 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
atmosphere away.
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M57 – Ring Nebula
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White Dwarfs
• Leftover once atmosphere blows away
• 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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High-Mass Stars• H and He burned in core• Core collapses hot enough
to fuse heavier elements (C, N, O …)
• Iron is most stable element and cannot be fused further– Instead of releasing energy,
it uses energy
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End for High Mass Stars• Fuel runs out• Core collapses and rebounds• Supernova!• Matter thrown back into the
interstellar medium• Matter rushing outwards,
fuses with matterrushing inwards
• Every element heavier than iron is made in instant of supernova!
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We are stardust!
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About the core…
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First Stop: Electron Degeneracy
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Last Stop: Neutron Degeneracy
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Otherwise…
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M1 – Crab Nebula – copyright VLT
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NGC 4526 – 6 Million parsecs away
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Stellar Lifecycle Summary
Low-mass Stars
• Like Sun
• Long lived (measure in billion years)
• Fuse to mostly helium
• Planetary nebula and white dwarf end state
• Most common
High-mass Stars
• 8 more massive
• Short lived (measure in million years)
• More fusion (C, N, O …)
• Supernova and neutron star or black hole end
• Makes most important elements