1) Proto Star

2. LOW MASS TRACK 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

4. 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!

5. 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

6. 3) H Shell burning • H in core has become depleted • H shell burns • Radius of the star expands • (draw core)

7. 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)

8. 5) He & H shell burning • Carbon core • Radius of star expands • (draw core)

11. 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

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

14. 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!

15. 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)

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

18. 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.