While Stars Are on the Main Sequence

• What holds stars up? Equilibrium between thermal pressure and gravity (and radiation pressure in massive stars); lasts for 90% of their life: 10 Gyr for the Sun, more for smaller stars.
• How is the energy produced? Between the time when T > 10 million K and when they run out of H, He production in the core, by the proton-proton chain (T < 20 million K) or CNO cycle (dominant at T > 20 million K).
• How does the energy get out? Radiation and convection, like in the Sun; May take a million years to reach the surface; In very low mass stars, deep convection zone and intense flare activity; In high-mass stars, no convection near the surface.

   What Happens after the Main Sequence?

  Evolution of Low-Mass Stars (M < 10 solar masses)

• H shell burning production: H depleted in the core, He core shrinks; T rises around the core, energy production by H fusion continues at a faster rate in a shell, and the star becomes brighter.
• Star growth: Envelope expands so it cools down, while the core shrinks and heats up; > Subgiant and red giant branch, with red giant winds.
• Very-low-mass stars: They end their lives as He white dwarfs (with electron degeneracy pressure maintaining their size).
• Sunlike stars: He burning to C in the core starts with a flash when T = 10⁸ K; The core expands and the luminosity decreases, as the star moves to the horizontal branch of the HR diagram; Example: Beta Ceti.
• He shell burning: He depleted, C core shrinks, T rises, faster fusion, envelope expands > Double-shell burning and asymptotic giant branch.

What Does a Sun-Like Star Eventually Become?

• Planetary nebula: He shell flashes and ejection of the star's envelope, ionized by the star's UV radiation; May look round (Ring Nebula or NGC 3132) but most have bipolar lobes (NGC 2346 or Henize 3-401), probably because of magnetic fields, and possibly due to orbiting companions; [The closest known in the Helix nebula, 450 ly away].
• Dying star: By this time, it has shed almost half its mass, and no more matter remains around it either, including nearby planets it may have had; Distant ones may survive, but they become warmer and ice on their moons or "Kuiper belt" objects may turn to gas.
• White dwarf: From the hot remnant of the C core, cooling down; Earth-sized but with the Sun's mass, stabilized by electron degeneracy pressure.
• Examples: The first one discovered was Sirius B (small but very hot); We know other ones, but they are hard to see unless they are in binary systems...

• End of evolution: The star becomes a black dwarf (unless more matter falls on it), some of its matter is added to the surrounding interstellar medium.

  Evolution of High-Mass Stars

• Summary: Heavier, bigger stars lead a much shorter, more violent life.
• Faster changes: The more massive the star, the faster the p-p chain proceeds; [and at the higher T the presence of C, N and O accelerates H fusion (CNO cycle)].
• Phases: H shell burning; gradual onset of He burning in the core; He shell burning; ... Intermediate mass stars stop here.
• C burning in the core: It requires a temperature of 600 MK, or an initial mass of 8 suns; Produces heavier elements; Last significant process is Si burning and Fe piling up in the core; Fe cannot fuse...

  Evolution of Very Massive Stars

• Summary: Stars heavier than 20 solar masses emit such strong stellar winds that they appear surrounded by their ejected matter as in a slow explosion (Wolf-Rayet stars).

Nucleosynthesis - Formation of the Elements

• Starting points: The very first stars started out with mostly H and the little He that was formed in the early universe, and basically nothing else; Old (population II) stars have 0.1% heavy elements; Younger stars incorporate 2-3% of heavy elements, formed by previous generations of stars.
• Main process in stars: All main sequence stars produce He from H by the p-p chain.
• After the main sequence: C is produced from He [by the triple-alpha process], and elements heavier than C are then formed [by helium capture], up to Fe; Evidence from cosmic abundances.
• Beyond iron: Smaller quantities of elements beyond Fe are also formed [by neutron capture], especially during supernova explosions; Some slow-burning, metal-poor red giants between 0.8 and 8 solar masses can produce elements up to lead!
• Example: Where do the H and O in our water come from?
• Puzzle: Why don't we see any very old, population III stars? Were they all massive and exploded long ago?

 Final Stages

• Photodisintegration: With T = 10 billion K, nuclei turn back into p's and n's.
• Core collapse: e + p -> n + n; collapse from sudden release of pressure.
• Supernova: The star is blown apart by an explosion from the core bounce, or neutrino shock wave; Debris spreads the heavy elements, which are recycled in newer star generations.
• Final state: We see the SN remnant as a nebula, and the core remains as a compact object, a neutron star or a black hole...