While Stars Are on the
- 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
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
- 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
- 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.
- 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
- 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...
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
Nucleosynthesis - Formation of the
- 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
- 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
- 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
- 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...
page by luca bombelli <bombelli at olemiss.edu>,
modified 21 nov 2013