Post Main Sequence Evolution

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Post Main Sequence Evolution

• Physical Astronomy
• Professor Lee Carkner
• Lecture 16

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Post Main Sequence

• As a star evolves off of the main sequence, the
  surface properties change
• This in turn changes the luminosity
• Exact path depends on mass
• Tends to follow pattern of up and to the right
  followed by down and to the left

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Subgiant

• The shell adds more He to the core until it
  exceeds the Schonberg-Chandrasekhar limit
• This energy expands the atmosphere and the
  surface cools
• Move to the right along the subgiant branch (SGB)

4
Red Giant

• This increases energy production and the energy
  expands the atmosphere further
• Star moves up along the red giant branch (RGB)

5
Dredge Up

• As the star evolves along the red giant branch,
  the convective zone deepens
• This causes a lot of mixing in the star and the
  products of the fusion reactions move towards the
  surface
• We see more He and less Li in the photosphere

6
Helium Ignition

• The H burning shell gets pushed out and actually
  decreases in luminosity
• Star moves down and left on HR diagram

7
Helium Flash

• For solar mass stars the He burning phase is very
  fast
• Triple alpha very strongly temperature dependant
• In the very dense core, the energy from the start
  of triple alpha cause the next wave of reactions
  to speed up
• Much energy goes into lifting electron degeneracy
  and so we dont see it
• After this the core will expand and slow reaction
  rate

8
Asymptotic Giant

• After the He core stops burning, the star has an
  inert He core and an active He shell
• Just like the RGB the star moves up on the HR
  diagram
• Expanding atmosphere increase luminosity
• Also increases convection producing a second
  dredge up

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Thermal Pulse

• As the star goes up the AGB, the H burning shell
  drops He on the He burning shell
• The flash pushes the H shell out, slowing the
  generation of He
• These thermal pulses cause the atmosphere to
  expand and contract, altering the luminosity
  rapidly

10
AGB Mass Loss

• These thermal pulses can throw material off the
  star
• Since the outer atmosphere is very cool, dust
  grains form in the ejecta
• The cores of the stars are now filled with C and
  O
• Due to the mass loss, stars less than 8 Msun
  cant overcome degeneracy
• Carbon, oxygen, neon,
• Stars less than 4 Msun will have carbon-oxygen
  cores

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Light Pressure

• For fully absorbing dust grains, the light
  pressure is approximately
• Prad F/c
• F sT4 at the stellar surface

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Post-AGB

• Mass loss rates 10-4 Msun/year
• Called OH/IR sources
• OH molecules produce masers
• IR from the dust

13
PN Formation

• Forms planetary nebula
• Core has no more nuclear reactions and forms
  white dwarf

14
Planetary Nebula

• We can see the planetary nebula as a fuzzy ball
• White dwarf emits a lot of UV light that excites
  or ionizes gas
• Gas is very thin so PNs emit forbidden lines
• Makes PNs look green

15
PN Structure

• We would expect PNs to be spherical
• Can get high resolution images with HST
• Gas may be preferentially ejected along equator
  or focused by magnetic fields
• PNs are expanding at 10-30 km/s

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Next Time

• Stellar talks
• Be prepared to give talks and ask questions
• For Monday
• Read 15.1-15.3
• Homework 15.2, 15.6, 15.8, 15.11