The Sun in the Red Giant Phase view

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

Evolution Low-Mass Stars Beyond the Main Sequence • M < 4 M_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 envelope expands and cools; the core contracts and heats; star 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 108 K needed to ignite it Helium Flash

Helium Burning: Triple-a Reaction • Intermediate step: Beryllium formation 4 He + 4 He 8 Be + energy (photons) • Fusion to Carbon 8 Be + 4 He 12 C + energy g (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

He-Burning: He C Triple-Alpha (He-nuclei) Reaction At temperatures T > Oxygen: 108 K Notation: 4 He 2 2 protons + 2 neutrons # Protons: Atomic Number in Periodic Table The most energetic electromagnetic radiation or g-rays are produced in nuclear reactions

Solar-type star

Main Sequence Lifetime of Solar-type Star

Helium Flash

Low-Mass Stellar Evolution

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 Heburning 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)

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

Central Star and Spherical Ejected Shell

Cat’s Eye Planetary Nebula

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) !!

Pne WD Tracks

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

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

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

Evolution of Supergiants: Constant Luminosity

Evolution of Supergiants Beyound He-buring

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

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

Crisscrossing the HR Diagram

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)

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 • BH if M (core) > 3 M(Sun)