Stellar Evolution

1. Stellar Evolution

2. Birth • Main Sequence • Post-Main Sequence • Death

3. Star Birth

4. Giant Molecular Cloud • 103-106 Msun • 10-30 K • H2 • Very Dense • 1-100 lyr across • Only GMCs can form stars • Gravity must be stronger than pressure

5. Collapse Triggered • Cloud collapses • Cloud becomes lumpy • Lumps collapse to become protostars • Collapsing gas efficiently radiates away heat, so it does not get very hot • Bright in IR

8. Collapsing gas becomes rotating cloud • Formation of disk can force ejection of material via jet • Jet likely due to strong magnetic fields • Ejected material carries away some angular momentum, allowing the star to slow down

10. Once core reaches 107 K fusion can begin • Low mass stars are protostars longer • Low mass stars spend 10-100 Myrs as protostar • High mass stars spend a couple million years

11. Stellar Mass Limits • Too much mass will create luminosity so high that internal pressure is stronger than gravity, blowing the star apart • M < 150 Msun • Need enough mass to have enough gravity to collapse core enough to initiate nuclear fusion • M > 0.08 MSun

12. Brown Dwarf • Too low mass to maintain fusion • Not supported by normal gas pressure • Supported by electron degeneracy pressure • No temperature dependence • Quantum mechanics

14. The Life of a Low Mass Star M < 2-4MSun

15. The Main Sequence • Stars burn H in their cores via the p-p chain • About 90% of a star’s lifetime is spent on the Main Sequence

16. Red Giant • Core depleted of H • H burned up • Now core contains He • Inert He core and surrounding H contract • H shell become hot enough for fusion • Rate of fusion higher in shell  expansion • ↑L, ↓T • Core mass keeps rising

17. Horizontal Branch • He core supported by degeneracy pressure • H burning adding more He to core • Temp ↑ due to core contraction, P constant • He fusion begins • 100 Million K • Fusion rate spikes with high temperature • He flash • Thermal pressure takes over from degeneracy • H burning weakens • Triple alpha process • 3 4He  12C + γ

18. Horizontal Branch

19. Asymptotic Giant Branch • He in core runs out and fusion stops again • He fusion begins in shell around C core • Double shell burning • Star expands to larger size than RGB • ↑L, ↓T

20. Asymptotic Giant Branch • He in core runs out and fusion stops again • He fusion begins in shell around C core • Double shell burning • Star expands to larger size than RGB • ↑L, ↓T

21. Planetary Nebula • Low mass stars are too small to ignite C fusion • Large L and size mean that outer layers are easily blown off • Hot, inert C (and some He) core are left • Leftover core is supported by electron degeneracy pressure

22. White Dwarfs • The leftover core of a low mass star • Made of He and C • Supported by electron degeneracy pressure • Extremely hot and dense • Max mass: 1.4 MSun The heaviest WDs are the smallest

24. Binaries with a WD

25. Nova • Accretion disk – when material from the companion becomes gravitationally bound to WD in a swirling disk • Material eventually falls onto WD • H compressed by strong gravity of WD • Compression increases temperature • Fusion ignites • Deflagration

26. Supernova: Type Ia • Increase in mass causes increase in temp • WD is supported by degeneracy pressure, so increased temp does not affect pressure • When mass nears 1.4 Msun the temperature is high enough for C fusion • Carbon fusion ignites and completely deflagrates the star • VIDEO