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0. Stellar Evolution. Please press “1” to test your transmitter. 0. What is the expected main-sequence life time of the sun?. 10 million years 100 million years 1 billion years 10 billion years 100 billion years. 0. Evolution on the Main Sequence. Main Sequence evolution.
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0 Stellar Evolution Please press “1” to test your transmitter.
0 What is the expected main-sequence life time of the sun? • 10 million years • 100 million years • 1 billion years • 10 billion years • 100 billion years
0 Evolution on the Main Sequence Main Sequence evolution Main-Sequence stars live by fusing Hydrogen (H) to Helium (He). Zero-Age Main Sequence (ZAMS) Finite supply of H => finite life time.
0 Evolution on the Main Sequence
0 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
0 Question: B A 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? C X D E
0 Expansion onto the Giant Branch Expansion and surface cooling during the phaseof an inactive He core and a H-burning shell Sun will expand beyond Earth’s orbit!
0 Red Giant Evolution He-core gets denser and hotter until the next stage of nuclear burning can begin in the core: 4 H → He He He fusion: 3 4He → 12C “Triple-Alpha Process” Fusion of Helium into Carbon
0 Red Giant Evolution (5 solar-mass star) C, O Inactive He
0 Nuclear fusion can, in principle, continue (i.e., produce energy) until which element is reached? • Helium • Carbon • Oxygen • Iron • Plutonium H → He He → C, O C → Ne, Mg, O Ne → O, Mg …
0 The Life “Clock” of a Massive Star (> 8 Msun) Let’s compress a massive star’s life into one day… H → He 12 11 1 Life on the Main Sequence + Expansion to Red Giant:22 h, 24 min. H burning 2 10 9 3 4 8 7 5 6 H → He He → C, O 12 11 1 2 10 He burning: (Red Giant Phase)1 h, 35 min, 53 s 9 3 8 4 7 5 6
He → C, O 0 H → He 12 11 1 2 10 C → Ne, Na, Mg, O 9 3 4 C burning: 6.99 s 8 7 5 6 C → Ne, Na, Mg, O H → He He → C, O Ne → O, Mg Ne burning: 6 ms 23:59:59.996
0 C → Ne, Na, Mg, O H → He Ne → O, Mg He → C, O O → Si, S, P O burning: 3.97 ms 23:59:59.99997 C → Ne, Na, Mg, O H → He Ne → O, Mg He → C, O O → Si, S, P Si → Fe, Co, Ni The final 0.03 msec!! Si burning: 0.03 ms
0 Summary of Post-Main-Sequence Evolution of Stars Fusion proceeds to formation of Fe core. Evolution of 4 - 8 Msun stars is still uncertain. Fusion stops at formation of C,O core. M > 8 Msun Red dwarfs: He burning never ignites M < 4 Msun M < 0.4 Msun
0 Evidence for Stellar Evolution: Star Clusters Stars in a star cluster all have approximately the same age!
0 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? • Yes • No, stars near the upper end will be missing. • No, stars near the lower end will be missing.
0 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!
0 Example:HR diagram of the star cluster M 55 High-mass stars evolved onto the giant branch Turn-off point Low-mass stars still on the main sequence
0 What can we infer from the location of the turn-off point? • The average mass of stars in the cluster. • The distance of the cluster. • The age of the cluster. • The size of the cluster. • All of the above.
0 The lower on the MS the turn-off point, the older the cluster.
0 Evidence for Stellar Evolution: Variable Stars Some stars show periodic brightness variations. Most important example: d Cephei Light curve of d Cephei
0 Cepheid Variables:The Period-Luminosity Relation The variability period of a Cepheid variable is correlated with its luminosity. The more luminous it is, the more slowly it pulsates. => Measuring a Cepheid’s period, we can determine its absolute magnitude!
0 What can we infer from a Cepheid’s absolute magnitude? (Of course, we can also measure its apparent magnitude.) • Its distance. • Its age. • Its mass. • Its temperature. • Its radius.
0 Comparing absolute and apparent magnitudes, we can measure a star’s distance (using the 1/d2 law)! The Cepheid distance measurements were the first distance determinations that worked out to distances beyond our Milky Way! Cepheids are up to ~ 40,000 times more luminous than our sun => can be identified in other galaxies.
