Download - 1) Proto Star
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1) Proto Star• How is a proto-star heated?
– Gravitational compression
• Why do stars below 0.08 Msun not form?– Core temp does not reach required for fusion.
• How can we observe proto-stars obscured by dust?– Infrared observations
LOW MASS TRACK
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From Protostar to Star
• Low-mass protostars become stars very slowly– Weaker gravity causes them to contract slowly, so
they heat up gradually
– Weaker gravity requires low-mass stars to compress their cores more to get hot enough for fusion
– Low-mass stars have higher density!
• High-mass protostars become stars relatively quickly– They contract quickly due to stronger gravity
– Core becomes hot enough for fusion at a lower density
– High-mass stars are less dense!
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2) Main Sequence• When do stars enter the main-sequence phase?
– Fusion of H to He begins
• About what percent of the mass is in the core?– 10%
• List in order from lowest to highest temp requirement: Triple alpha, CNO, proton-proton fusion– P-P ~ 5 million Kelvin (H->He)– CNO ~ 20 million Kelvin (H->He)– Triple alpha ~ 100 million Kelvin (He->C)
• Why are main sequence stars so stable? – Compression -> T -> fusion -> P -> expansion– Expansion -> T -> fusion -> P -> compression
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3) H Shell burning
• H in core has become depleted
• H shell burns
• Radius of the star expands
• (draw core)
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4) He core burning / H shell burning
• Temp in core is now ~ 100 million K
• Triple alpha process fusion
• He -> C
• Radius of star contracts
• (draw core)
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5) He & H shell burning
• Carbon core
• Radius of star expands
• (draw core)
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Massive fuel tankBut burns fuel quickly
Short lifetime before fuel depletionCNO engine
Small fuel tankBut runs efficiently
Long lifetime beforefuel depletion Proton-proton engine
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1) Proto Star
• While on the main sequence what do high mass stars burn in their cores?– Hydrogen
• What fusion process?– CNO
HIGH MASS TRACK
2) Main sequence
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The CNO cycle
• Low-mass stars rely on the proton-proton cycle for their internal energy
• Higher mass stars have much higher internal temperatures (20 million K!), so another fusion process dominates– An interaction involving Carbon,
Nitrogen and Oxygen absorbs protons and releases helium nuclei
– Roughly the same energy released per interaction as in the proton-proton cycle.
– The C-N-O cycle!
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3) H shell to He core/H shell burning
• Why are massive stars able to fuse He on this leg?– Started off with hotter cores; requires relatively
less contraction to heat to necessary temp. (100 million)
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4) Up to iron burning• Onion structure of the core
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concepts
• Convection: (think of boiling water) buoyant hot bubbles rise while cooler bubbles sink. No net mass transfer, but heat transfer!
• Opacity: measure of light’s ability to penetrate.