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

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 108 K needed to ignite it Helium Flash

Helium Burning: Triple- Reaction• Intermediate step: Beryllium formation 4He + 4He 8Be + energy photons)

• Fusion to Carbon

8Be + 4He 12C + 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

He-Burning: He CTriple-Alpha (He-nuclei) Reaction

At temperatures T > 108 K

Oxygen:

Notation:4He2

2 protons +2 neutrons

# Protons: Atomic Number in PeriodicTable

Solar-type star

Main Sequence Lifetime of Solar-type Star

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)

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

Post-MS Evolution of Low-Mass Stars1. 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

Stellar Lifetimes• Lifetimes depend on Mass M and Luminosity L• L determines the rate of energy production, and

is proportional to M3.5

• A fraction of M is converted to energy E = fMc2

• If t is the lifetime of the star then

L t = fMc2

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

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.

12C + 12C 24Mg + energy

H He Nuclear Fusion Via the C-N-O Cycle in Massive Stars

Ordinary Isotopes: 12C, 14N, 16O act as catalysts

e+ positronPositiveelectronannihilatesnegative electron(matter-antimatter)e- + e+ = energy

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