2. 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

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. 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

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.