1) Proto Star LOW MASS TRACK 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

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 tank But burns fuel quickly Short lifetime before fuel depletion CNO engine Small fuel tank But runs efficiently Long lifetime before fuel depletion Proton-proton engine

1) Proto Star 2) Main sequence HIGH MASS TRACK While on the main sequence what do high mass stars burn in their cores? Hydrogen What fusion process? CNO

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.