core he h burning shell main sequence evolution star brightens significantly (red giant)
TRANSCRIPT
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Core
He
H burning shell
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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)
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The Sun as a Red Giant
diameter = 1 AU
The Sun as a Main Sequence Star
diameter = 1/100 AU
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H-R Diagram
Temperature
Lu
min
osi
ty
0.1R
1 R
10 R
100 R
ZAMS
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Core is collapsing and heating up◦ He Fusion (Triple Alpha Process)
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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
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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
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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
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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
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All have M = -0.5 Measure m and calculate distance
Mag
nit
ud
e
Time
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H-R Diagram
Temperature
Lu
min
osi
ty
ZAMS
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Periods between 1 and 70 days Amplitudes of 0.1 to 2 mag
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H burning shell
He burning shell
Carbon
Core
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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
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H-R Diagram
Temperature
Lu
min
osi
ty
ZAMS
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Periods several hundred days Amplitudes several magnitudes
Mag
nit
ud
e
Time
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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
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The Ring Nebula
The Helix Nebula
Wings of a Butterfly Nebula
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H-R Diagram
Temperature
Lu
min
osi
ty
ZAMS
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Enhanced stellar wind in giant phases Planetary Nebula
Chandresakhar Limit◦ Maximum amount of mass that can be supported
by electron degeneracy◦ 1.4 M
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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
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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
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Carbon
Thin atmosphere of H and He
Mass 1 M
Radius R
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Sirius A◦ A1V, m = -1.46◦ T = 9550 K
Sirius B◦ m = 8.3◦ T = 25,000 K◦ gives R = 92%R
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H-R Diagram
Temperature
Lu
min
osi
ty
ZAMS Animation
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