1. Stellar Evolution Please press “1” to test your transmitter.

2. What is the expected main- sequence life time of the sun? 1.10 million years 2.100 million years 3.1 billion years 4.10 billion years 5.100 billion years

3. Evolution on the Main Sequence Zero-Age Main Sequence (ZAMS) Main-Sequence stars live by fusing Hydrogen (H) to Helium (He). Finite supply of H => finite life time. Main Sequence evolution

4. Evolution on the Main Sequence

5. Evolution off the Main Sequence: Expansion into a Red Giant H in the core completely converted into He: “H burning” (i.e. fusion of H into He) continues in a shell around the core. Expansion and cooling of the outer layers of the star → Red Giant

6. Question: Which way will a star at point X move in the HR diagram when it swells up to a red giant, so that its luminosity increases slightly, and its color changes to red? X A E D C B

7. Expansion onto the Giant Branch Expansion and surface cooling during the phase of an inactive He core and a H-burning shell Sun will expand beyond Earth’s orbit!

8. Red Giant Evolution 4 H → He He He-core gets denser and hotter until the next stage of nuclear burning can begin in the core : He fusion: 3 4 He → 12 C “Triple-Alpha Process” Fusion of Helium into Carbon

9. Red Giant Evolution (5 solar-mass star) Inactive He C, O

10. Nuclear fusion can, in principle, continue (i.e., produce energy) until which element is reached? 1.Helium 2.Carbon 3.Oxygen 4.Iron 5.Plutonium H → He He → C, O C → Ne, Mg, O Ne → O, Mg …

11. The Life “Clock” of a Massive Star (> 8 M sun ) Let’s compress a massive star’s life into one day… 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 Life on the Main Sequence + Expansion to Red Giant: 22 h, 24 min. H burning H → He He → C, O He burning: (Red Giant Phase) 1 h, 35 min, 53 s

12. H → He He → C, O C → Ne, Na, Mg, O Ne → O, Mg H → He He → C, O C → Ne, Na, Mg, O 12 1 2 3 4 5 6 7 8 9 10 11 C burning: 6.99 s Ne burning: 6 ms 23:59:59.996

13. H → He He → C, O C → Ne, Na, Mg, O Ne → O, Mg O burning: 3.97 ms 23:59:59.99997 O → Si, S, P H → He He → C, O C → Ne, Na, Mg, O Ne → O, Mg Si burning: 0.03 ms The final 0.03 msec!! O → Si, S, P Si → Fe, Co, Ni

14. Summary of Post-Main-Sequence Evolution of Stars M > 8 M sun M < 4 M sun Evolution of 4 - 8 M sun stars is still uncertain. Fusion stops at formation of C,O core. Fusion proceeds to formation of Fe core. Red dwarfs: He burning never ignites M < 0.4 M sun

15. Evidence for Stellar Evolution: Star Clusters Stars in a star cluster all have approximately the same age!

16. Take all stars of a 5-billion-year-old cluster, and put them onto a Hertzsprung-Russell diagram. Do you expect to see a complete Main Sequence? 1.Yes 2.No, stars near the upper end will be missing. 3.No, stars near the lower end will be missing.

17. High-mass stars evolve off the main sequence (to become red giants) earlier than low-mass stars. => For a given age, low-mass stars are still on the MS, while high-mass stars are already red giants!

18. Example: HR diagram of the star cluster M 55 High-mass stars evolved onto the giant branch Low-mass stars still on the main sequence Turn-off point

19. What can we infer from the location of the turn-off point? 1.The average mass of stars in the cluster. 2.The distance of the cluster. 3.The age of the cluster. 4.The size of the cluster. 5.All of the above.

20. The lower on the MS the turn-off point, the older the cluster.

21. Evidence for Stellar Evolution: Variable Stars Some stars show periodic brightness variations. Most important example:  Cephei Light curve of  Cephei

22. Cepheid Variables: The Period-Luminosity Relation The variability period of a Cepheid variable is correlated with its luminosity. => Measuring a Cepheid’s period, we can determine its absolute magnitude! The more luminous it is, the more slowly it pulsates.

23. What can we infer from a Cepheid’s absolute magnitude? (Of course, we can also measure its apparent magnitude.) 1.Its distance. 2.Its age. 3.Its mass. 4.Its temperature. 5.Its radius.

24. Comparing absolute and apparent magnitudes, we can measure a star’s distance (using the 1/d 2 law)! The Cepheid distance measurements were the first distance determinations that worked out to distances beyond our Milky Way!Cepheid distance measurements Cepheids are up to ~ 40,000 times more luminous than our sun => can be identified in other galaxies.