Core

He

H burning shell

Main Sequence EvolutionHydrogen is depleted in the last fusion

zoneReaction rate fallsCore can no longer support its weight

core shrinksTemperature in the core increasesExtra energy produced to -

Begin fusion reaction in next core zoneBegin fusion reaction in next core zoneLift the envelopeStar brightens significantly (Red

Giant)

The Sun as a Red Giant

diameter = 1 AU

The Sun as a Main Sequence Star

diameter = 1/100 AU

H-R Diagram

Temperature

Lu

min

osi

ty

0.1R

1 R

10 R

100 R

ZAMS

Core is collapsing and heating up◦ He Fusion (Triple Alpha Process)

Threshold Temperature◦ 108 K = 100,000,000 K◦ Massive stars do this easily

Core is still ideal gas◦ Low mass stars (like the Sun) struggle

Core becomes degenerate

The ideal gas safety valve◦ Heat an ideal gas and it expands and cools◦ Cool an ideal gas and it contracts and heats up◦ Pressure Temperature

Degenerate electron gas◦ Electrons are forced to be very close together

and fill every energy state◦ Pressure and Temperature not related

Extra energy from He fusion causes the core to expand very little

Fusion energy raises the temperature of the core and takes place all over the core◦ He Flash◦ End of ascent up the Red Giant Branch

Eventually core does expand enough to become ideal again◦ Fusion now at much higher temperature

Star is quasi-stable◦ Envelope contracts (luminosity should )◦ Temperature increases (lum. should )◦ These two effects nearly offset

Horizontal Branch◦ RR Lyrae Variables

Periods of 0.05 to 1.2 days Amplitudes of 1 to 2 magnitude

All have M = -0.5 Measure m and calculate distance

Mag

nit

ud

e

Time

H-R Diagram

Temperature

Lu

min

osi

ty

ZAMS

Periods between 1 and 70 days Amplitudes of 0.1 to 2 mag

H burning shell

He burning shell

Carbon

Core

He is depleted in the last fusion zone Reaction rate falls Core can no longer support its weight

◦ core shrinks Temperature in the core increases

◦ Carbon fusion threshold 600 million K◦ Low Mass stars cannot reach this temperature

Envelope expands to supergiant

H-R Diagram

Temperature

Lu

min

osi

ty

ZAMS

Periods several hundred days Amplitudes several magnitudes

Mag

nit

ud

e

Time

Core is once again degenerate◦ He fusion in the shell becomes explosive

Send shocks of energy into the expanded envelope Recombination energy also drives envelope

out

The Ring Nebula

The Helix Nebula

Wings of a Butterfly Nebula

H-R Diagram

Temperature

Lu

min

osi

ty

ZAMS

Enhanced stellar wind in giant phases Planetary Nebula

Chandresakhar Limit◦ Maximum amount of mass that can be supported

by electron degeneracy◦ 1.4 M

Mass < 2.5 M

◦ He ignites in degenerate core (He flash)◦ Nuclear processing stops at He fusion

Mass between 2.5 and 8 M

◦ He ignites in a non-degenerate core No He flash required

◦ Nuclear processing stops at He fusion Mass between 8 and 10 M

◦ C fusion possible

Core was collapsing◦ Can never reach 600 million K for C fusion◦ Much of the original envelope has left in Planetary

Nebula◦ Material becomes degenerate◦ Electron Degeneracy Pressure halts collapse

Core must be < 1.4 M

Carbon

Thin atmosphere of H and He

Mass 1 M

Radius R

Sirius A◦ A1V, m = -1.46◦ T = 9550 K

Sirius B◦ m = 8.3◦ T = 25,000 K◦ gives R = 92%R

H-R Diagram

Temperature

Lu

min

osi

ty

ZAMS Animation