Chapter 11: Life Cycles of Stars
Gas cloud contracts, forms protostar
Protostar contracts, accretes matter
Center heats up
At Tcenter ~ 10 x 106 K, fusion starts
Contraction stops, star on main sequence
Evolutionary track: path in H-R diagram
Pre-Main Sequence
Protostar initially above and right of main sequence
At first, temperature and luminosity increase
Star hidden in cloud of gas, dust
As star contracts, convection becomes important
Star contracts at ~ constant Tsurface
Luminosity decreases, evolutionary track moves vertically downward
Gas, dust cloud disperses
At Tcenter ~ 10 x 106 K, fusion starts
Contraction stops, star on main sequence
Slight increase in Tsurface as star joins main sequence
The Main Sequence
Zero-age main sequence - location of newly formed stars on main sequence
MASS IS DESTINY
Higher mass => higher luminosity
Higher mass => higher Tsurface
Main sequence is a mass sequence
Mass-luminosity relationship (main sequence only)
L = M 3.5
Massive stars collapse faster than less massive stars
Brown dwarfs
Mass less than 0.08 solar masses
Tcenter never high enough to fuse H
'Failed stars'
Have been observed
Hydrostatic equilibrium - pressure balances weight
Stars are stable on main sequence
Energy generated in center equals energy radiated away at surface
Energy Generation in Main-Sequence Stars
Proton-proton reaction
Stars with masses ~ 1 solar mass or less
Tcenter < 16 x 106 K
CNO cycle
4 H => 1 He
Intermediate steps involve C, N, O
More massive stars
Tcenter ~ 16 x 106 K and higher
Lifetime on main sequence
Eventually H in core converted to He
No energy generation to provide pressure that supports outer layers of star
Structural changes in star
More massive stars use up H faster than low mass stars - much more luminous
Lifetime on main sequence:
ex. (1): star = 20 Msun
ex. (2): star = 0.5 Msun
Post-Main-Sequence Evolution
H in core depleted
Star contracts
Temperature increases
H burning shell surrounds core
Temperature increases, shell burning increases
Outer envelope expands
Surface temperature decreases
Luminosity increases
Star moves to red giant region H-R diagram
He flash
Core continues to contract, heat up
At Tcenter ~ 100 x 106 K , He fusion stars
Triple-alpha reaction
3 He => 1 C + energy
Sometimes goes further
C + He => O + energy
Sudden and rapid in solar-mass stars
He-burning core + H burning shell
Star uses up He in core
Core contracts
He burning shell
H burning outer shell
Triple alpha process very temperature dependent
Star unstable
Ejects shell - planetary nebula
Multiple episodes of mass ejection
Enhanced stellar winds
Ultraviolet radiation from hot inner layers of star ionize planetary nebula
Enhanced stellar wind collides with ejected material
Planetary nebula lasts about 50,000 years
Remaining stellar core contracts
Cannot burn carbon
Becomes white dwarf
About size of earth
Very faint
Mostly carbon, oxygen
Contraction halted by degeneracy pressure of electrons
Cools and fades
Becomes black dwarf
Medium mass stars
Masses ~ 5 solar masses
He burning core - outer layers unstable
Become pulsating variables
Most produce planetary nebulae and become white dwarfs
Maximum mass of white dwarf is 1.4 solar masses (Chandrasekhar limit)
High mass stars
C, O core contracts.
Stars > 3-4 Msun T ~ 6x108 oK
C fusion -> Ne, Na, Mg, O
Cycle of contraction and fusion can repeat, ever more rapidly, as more massive stars (> 8 Msun) generate even heavier elements.
Star with shells like an onion.
T ~ 3x109 K, silicon => iron
Eventually fusion energy sources exhausted.
Gravity overcomes internal pressure.
Prof. Donna Weistrop
University of Nevada, Las Vegas