stellar evolution. birth main sequence post-main sequence death
DESCRIPTION
Star BirthTRANSCRIPT
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Stellar Evolution
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• Birth
• Main Sequence
• Post-Main Sequence
• Death
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Star Birth
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Giant Molecular Cloud
• 103-106 Msun
• 10-30 K– H2
• Very Dense• 1-100 lyr across• Only GMCs can form
stars– Gravity must be stronger
than pressure
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Collapse Triggered
• Cloud collapses • Cloud becomes lumpy• Lumps collapse to
become protostars
• Collapsing gas efficiently radiates away heat, so it does not get very hot– Bright in IR
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• Collapsing gas becomes rotating cloud
• Formation of disk can force ejection of material via jet
• Jet likely due to strong magnetic fields
• Ejected material carries away some angular momentum, allowing the star to slow down
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• Once core reaches 107 K fusion can begin
• Low mass stars are protostars longer
• Low mass stars spend 10-100 Myrs as protostar
• High mass stars spend a couple million years
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Stellar Mass Limits
• Too much mass will create luminosity so high that internal pressure is stronger than gravity, blowing the star apart– M < 150 Msun
• Need enough mass to have enough gravity to collapse core enough to initiate nuclear fusion– M > 0.08 MSun
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Brown Dwarf
• Too low mass to maintain fusion
• Not supported by normal gas pressure
• Supported by electron degeneracy pressure– No temperature
dependence– Quantum mechanics
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The Life of a Low Mass Star
M < 2-4MSun
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The Main Sequence
• Stars burn H in their cores via the p-p chain
• About 90% of a star’s lifetime is spent on the Main Sequence
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Red Giant• Core depleted of H
– H burned up– Now core contains He
• Inert He core and surrounding H contract
• H shell become hot enough for fusion– Rate of fusion higher in shell
expansion– ↑L, ↓T
• Core mass keeps rising
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Horizontal Branch• He core supported by
degeneracy pressure• H burning adding more
He to core• Temp ↑ due to core
contraction, P constant• He fusion begins
– 100 Million K– Fusion rate spikes with
high temperature• He flash
– Thermal pressure takes over from degeneracy
– H burning weakens
• Triple alpha process
• 3 4He 12C + γ
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Horizontal Branch
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Asymptotic Giant Branch
• He in core runs out and fusion stops again
• He fusion begins in shell around C core– Double shell burning
• Star expands to larger size than RGB– ↑L, ↓T
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Asymptotic Giant Branch
• He in core runs out and fusion stops again
• He fusion begins in shell around C core– Double shell burning
• Star expands to larger size than RGB– ↑L, ↓T
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Planetary Nebula
• Low mass stars are too small to ignite C fusion
• Large L and size mean that outer layers are easily blown off
• Hot, inert C (and some He) core are left
• Leftover core is supported by electron degeneracy pressure
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White Dwarfs
• The leftover core of a low mass star
• Made of He and C• Supported by electron
degeneracy pressure• Extremely hot and
dense• Max mass: 1.4 MSun
The heaviest WDs are the smallest
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Binaries with a WD
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Nova• Accretion disk – when
material from the companion becomes gravitationally bound to WD in a swirling disk
• Material eventually falls onto WD
• H compressed by strong gravity of WD
• Compression increases temperature
• Fusion ignites• Deflagration
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Supernova: Type Ia
• Increase in mass causes increase in temp• WD is supported by degeneracy pressure, so
increased temp does not affect pressure• When mass nears 1.4 Msun the temperature is
high enough for C fusion• Carbon fusion ignites and completely
deflagrates the star• VIDEO