1. Post Main Sequence Evolution PHYS390 (Astrophysics) Professor Lee Carkner Lecture 15

2. Questions 1)Many show evidence for interaction with another star. How might this explain blue stragglers?  Blue stragglers started out as lower mass stars, and then either through mergers or mass transfer gained enough mass to move up the main sequence past the turn-off point 2)What would cause the Hertzsprung gap to have few observed stars in it?  Stars evolve very quickly through this part of the diagram, so at any one time very few stars in the galaxy are in the gap

3. Post Main Sequence   A star makes two basic “moves”  Up and to the right   Down and to the left   Higher mass stars repeat this pattern more times

4. 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)

5. Red Giant   This increases energy production and the energy expands the atmosphere further   Star moves up along the red giant branch (RGB)

6. Dredge Up   As the star evolves along the red giant branch, the convective zone deepens   We see more He and less Li in the photosphere

7. Helium Ignition  At the tip of the red giant branch the core becomes dense and hot enough for triple alpha hydrogen burning   The atmosphere contracts and becomes hotter  Star moves down and left on HR diagram

8. Helium Flash  For solar mass stars the He burning phase is very fast   Triple alpha very strongly temperature dependant   Very hard to model  Much energy goes into lifting electron degeneracy and so we don’t see it  After this the core will expand and slow reaction rate

9. 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

10. Thermal Pulse   Causes helium shell flashes  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

11. 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 M sun can’t overcome degeneracy   Carbon, oxygen, neon, …  Stars less than 4 M sun will have carbon-oxygen cores

12. Light Pressure  P rad = F/c   F =  T 4 at the stellar surface

13. Post-AGB  After the AGB, stars can develop a superwind   Wind causes star to be enshrouded in dust  Called OH/IR sources   Millimeter lasers  IR from the dust

14. PN Formation   Forms planetary nebula   Core has no more nuclear reactions and forms white dwarf

15. 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  [O III] emits at 500.68 and 495.89 nm  Can make PNs look green

16. 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   Dissipate after about 50000 years

17. Next Time  Stellar talks  Be prepared to give talks and ask questions  Email (or give to me on flash drive) your PP file  For Wednesday:  Read 15.1-15.3  Homework: 15.2, 15.6, 15.8, 15.11