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


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