1. The Sun in the Red Giant Phase (view from the Earth!)

2. Evolution Low-Mass Stars Beyond the Main Sequence M < 4M_Sun Once the star reaches the MS, it spends most of its lifetime in the H  He nuclear burning phase When the hydrogen in the center is exhausted, the star forms a He-core and the H-burning shell moves outward; the star expands and cools, and becomes a Red Giant moving up from the MS Helium in the center of core remains inert until the density, pressure, and temperature increase to 10 8 K needed to ignite it  Helium Flash

3. Helium Burning: Triple-  Reaction Intermediate step: Beryllium formation 4 He + 4 He  8 Be + energy  photons) Fusion to Carbon 8 Be + 4 He  12 C + energy  (photons) Helium core is highly dense and electrons are packed together in a degenerate state Electrons as close together as possible and therefore exerting degeneracy pressure against further gravitational contraction But temperature rises  explosive He burning

4. He-Burning: He  C Triple-Alpha (He-nuclei) Reaction At temperatures T > 10 8 K Oxygen: Notation: 4 He 2 2 protons + 2 neutrons # Protons: Atomic Number in Periodic Table

5. Solar-type star

6. Main Sequence Lifetime of Solar-type Star

11. Evolution beyond the Red Giant L does not increase at the onset of the He-flash itself since the central region of the core is quite opaque The H-burning shell is slowly extinguished and L decreases, even as the star shrinks and temperature rises; the star moves leftward along a nearly Horizontal Branch on the H-R diagram Luminosity rises again as the energy from the He- burning core of the RG rises to the surface The star then resumes its climb up the H-R diagram along a second vertical branch – the Asymptotic Giant Branch (AGB)

12. Evolution Beyond the AGB Phase He-burning via the triple-alpha fusion is highly temperature sensitive The AGB star is unstable; radiation pressure from the interior push away the envelope – hot core separates from the envelope Hot core is mainly C-O (products of triple-alpha) Hot core is very luminous initially, but rapidly cools through a Planetary Nebula (PN) phase (NO relation to planets!) The PN C-O core surrounded by the brightly lit ejected envelope appears as a ‘ring’ The PN core cools and collapses to White Dwarf

14. Central Star and Spherical Ejected Shell

15. Cat’s Eye Planetary Nebula

16. Planetary Nebulae and White Dwarfs The ring shaped PN is ionized and heated by the hot central core; takes about 10,000 years Hot PNe have C-O stellar core at about 100,000 K Moves left on the H-R diagram as it is exposed Moves BELOW the MS as it cools, shrinks, and becomes less luminous Matter in the cold core is ‘degenerate electron gas’, not an ideal gas; Pressure is independent of temperature; contraction of the core stops when the pressure equals gravity; star becomes White Dwarf R (WD) ~ 0.01 R (Sun) ~ R (Earth) WD cools away into a ‘stellar corpse’ ! BUT, may turn into a huge DIAMOND (Carbon crystal) !!

17. Pne  WD Tracks

20. Post-MS Evolution of Low-Mass Stars 1.End of H  He burning in the core of MS star 2. Red Giant phase with inert He-core and outer H-burning shell; star expands and cools, but is brighter 3. Climbs up the RG branch until He-flash in the core 4.Core expands and cools; H-burning decreases; outer layers contract; luminosity decreases but temperature increases; star moves LEFT on the H-R diagram along the Horizontal Branch 5. He-burning shell eventually moves outward and the star becomes more luminous and climbs up the AGB, with He- and H-burning outer shells but inert C-O core 6. The envelope of the AGB star is radiatively pushed away, separates from the core, and the star becomes a Planetary Nebula 7. The C-O core eventually becomes a White Dwarf

21. Stellar Lifetimes Lifetimes depend on Mass M and Luminosity L L determines the rate of energy production, and is proportional to M 3.5 A fraction of M is converted to energy E = fMc 2 If t is the lifetime of the star then L t = fMc 2 OR lifetime t is proportional to M / L e.g. If M = 2 M(Sun), then L = 12 times L (Sun), and has a lifetime about 6 times shorter

22. Ages of Stellar Clusters H-R diagram yields information on L, M, T, R, and color of stars; most characteristics except age But may determine the age of a stellar cluster, formed at the same time and composition, from the evolution of stars in the cluster with different masses  isochrones High mass stars evolve off the MS (“turn off”) before low mass stars

23. Evolution and nucleosynthesis of High Mass Stars Very different structure and evolution from low mass star Mass more than about 4 times M(Sun), but luminosity up to 10,000 times L(Sun) or more Burn brightly, evolve rapidly, die relatively quickly CNO cycle is more efficient in H  He fusion than the p-p chain; requires higher temperatures prevalent in cores of high-mass stars At over 600 million K elements heavier than CNO are fused, e.g. 12 C + 12 C  24 Mg + energy

24. H  He Nuclear Fusion Via the C-N-O Cycle in Massive Stars Ordinary Isotopes: 12 C, 14 N, 16 O act as catalysts e + positron Positive electron annihilates negative electron (matter- antimatter) e - + e + =  energy

25. Evolution of Supergiants: Constant Luminosity

27. Evolution of Supergiants Beyound He-buring

28. Evolution of High-Mass Stars Beyond the MS M > 4 M (Sun) – O and B stars Burn H  He via the more efficient CNO cycle After H-core exhaustion the He-core contracts and heats up, but the H-burning continues around the He-core and the star puffs up The star expands and cools, but the luminosity remains constant since the huge outer layers are opaque It moves right on the H-R diagram as a Red Supergiant Takes about a million years to cross the H-R diagram

29. Blue Supergiant Phase Core temperature reaches T > 100 million K; the He-flash ignites He-burning to C and O via the Triple-alpha nuclear fusion reaction With a H-burning shell, a He-burning core, the star builds up a C-O core and becomes a Blue Supergiant, moving leftward on the H-R diagram, following the He-flash After He-core exhaustion, the C-O core collapses and heats up, with H and He burning outer shells, and the star expands and becomes a Red SG again, moving right on the H-R diagram Carbon ignites when core T > 600 MK, density > 150,000 g/cc

30. Crisscrossing the HR Diagram

31. Intermediate and High Mass Stars A dichotomy emerges: 1. Intermediate mass star: 4 M(Sun) < M < 8 M(Sun) - Carbon burning reactions produce O, Ne, Mg - no further burning, inert O-Ne-Mg core WD, after about 1000 years 2. High mass stars: M > 8-10 M(sun) - evolve rapidly with strong stellar winds (radiation driven) - O-Ne-Mg core heats up to T ~ 1.5 billion K, density ~ 10 million g/cc, and ignites Neon burning to Mg and Si; lasts only a few years - Oxygen shell burns up to Si, S, P…(Si-core)

33. SUPERNOVA Fiery Explosive Death of Massive Stars In M > 8 M(Sun) stars the Si-core ignites and burns up to Fe-Ni No further fusion possible since fusion beyound iron requires energy rather than produce it Once an iron-core has been formed, the star no longer has any fuel source When M (Fe-core) > 1.4 – 2 M(Sun), the Fe core contracts, heats up, and explodes….SUPERNOVA The envelope is ejected and the iron core collapses into Neutron Star or BLACK HOLE