QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Astrophysics I:Astrophysics I:The Stellar LifecycleThe Stellar Lifecycle
Kathy Cooksey
How to Make a Star• Collapse interstellar clouds
•Visible and IR image of the hot protostars in Orion Nebula.
How to Make Systems
• Cloud around protostar spins faster
• Flattens to a disk– Think pizza dough
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
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
Stellar Stellar EvolutionEvolution
The Life of a Star(like our Sun)
(which does not move in a circle as it evolves
Artist’s rendition)
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
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)
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
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!
Electron Degeneracy
Pressure from motion of atoms
Electron Degeneracy
Pressure from electron shells
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.
M57 – Ring Nebula
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
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
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!
We are stardust!
About the core…
First Stop: Electron Degeneracy
Last Stop: Neutron Degeneracy
Otherwise…
M1 – Crab Nebula – copyright VLT
NGC 4526 – 6 Million parsecs away
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