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

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!

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

3) H Shell burning

• H in core has become depleted

• H shell burns

• Radius of the star expands

• (draw core)

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)

5) He & H shell burning

• Carbon core

• Radius of star expands

• (draw core)

Massive fuel tankBut burns fuel quickly

Short lifetime before fuel depletionCNO engine

Small fuel tankBut runs efficiently

Long lifetime beforefuel depletion Proton-proton engine

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

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!

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)

4) Up to iron burning• Onion structure of the core

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.