Stellar Evolution

Birth Main Sequence Post-Main Sequence Death

Star Birth

Giant Molecular Cloud M sun K – H 2 Very Dense lyr across Only GMCs can form stars – Gravity must be stronger than pressure

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

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

Once core reaches 10 7 K fusion can begin Low mass stars are protostars longer Low mass stars spend Myrs as protostar High mass stars spend a couple million years

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

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

The Life of a Low Mass Star M < 2-4M Sun

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

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

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 4 He  12 C + γ

Horizontal Branch

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

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

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

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 M Sun The heaviest WDs are the smallest

Binaries with a WD

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

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 M sun the temperature is high enough for C fusion Carbon fusion ignites and completely deflagrates the star VIDEO