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Lecture 17
Post-ms evolution II
Review
Review
Review
Second dredge-up: He-shell burning
• A Helium-burning shell ignites around a C,O core.
 Similar to the H-shell burning phase
• Again, the envelope expands and cools, becoming
convective and causing a second dredge-up.
Instability strip
Review: Horizontal branch
• H-burning shell is compressed, increasing the luminosity it
produces
• He-shell burning: CO core collapses, while envelope expands
Start of HB
End of HB
H →He burning
He →C,O burning
Convection
Asymptotic giant branch
As the envelope cools it eventually reaches the Hayashi
track and bends upward. This is the asymptotic giant
branch.
He-burning dominates
the luminosity
Thermal pulses
• He ash is dumped on the slightly
degenerate He-burning shell, causing
shell flashes
Further nucleosynthesis
For stars with 4<M/MSun<8, nuclear reactions can
continue:
 O  2 He

 Ne He
12
12
6C 6C  
 Mg  n
 Na  p
16
8
20
10
23
12
23
11
4
2
4
2
24
4
 12
Mg  22 He
 28
4
Si

 14
16
16
2 He
8 O 8 O  
31
 15 P  p
 1631S  n
AGB stars
• High mass-loss rates, and cool effective temperatures
(~3000 K)
• A dust shell hides most of the stellar luminosity and so
the stars are seen only in the infrared.
Mass loss
• Mass loss driven
by the high
luminosity and
thermal pulses.
 As mass
decreases, and
luminosity
increases, the
mass loss rate
increases.
High-resolution radio image of mass-loss
from an AGB star, TX Cam
Break
Post-AGB phase
• The cloud expands and becomes optically thin
• Exposes the hotter interior
Planetary nebulae: Fate of low mass stars
• The hot core lights up the expanding envelope, for about 20,000 years.
Planetary Nebulae
• Bluish-green colour due to [OIII] forbidden lines
• Reddish colour from ionized hydrogen and nitrogen
Cat’s eye nebula
The Helix Nebula
• Looking along the rotation axis
 Gas is being ejected in “rings” preferentially along the
equator
Outflow velocities
• Typically shell expands at 10-30 km/s
• Some are much faster: the Ant nebula has an outflow
velocity of about 1000 km/s
Fate of planetary nebulae
• Release the envelope into the ISM, on a timescale of ~10000
years.
• There are probably about 15,000 in the Milky Way today
 ISM is being enriched at a rate of about 1 Msun/year.
Planetary nebulae and white dwarfs
• When the helium and hydrogen shells are extinguished, the luminosity drops
abruptly
White dwarfs
• When the envelope has dispersed, only the hot, dense,
small core is left