Stellar Evolution. Birth Main Sequence Post-Main Sequence Death

Stellar Evolution

• Birth

• Main Sequence

• Post-Main Sequence

• Death

Star Birth

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

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 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

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

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 < 2-4MSun

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 4He 12C + 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 MSun

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 Msun the temperature is

high enough for C fusion• Carbon fusion ignites and completely

deflagrates the star• VIDEO