Astronomy

1
Internal structures of O3, G2 and M8 stars
15 RS
1 RS
.10 RS
2
Pre-Main Sequence Facts

• Vogt-Russell theorem initial mass and
  composition sets fate of star
• Kelvin-Helmholtz time pre-nuclear burning
• ? 106 years for massive stars
• ? 107 years for lighter stars
• gravity collapse ? heating initially size
  drops so L drops then T grows so L grows
  droopy path on H-R
• isochrone curve of constant age
• nuclear burning (fusion of H to He via
  proton-proton cycle) begins when the star arrives
  at main sequence

3
Pre-MS evolutionary tracks with isochrones
4
The whole life story for a star like the sun
5
Main Sequence Facts

• mass .1 MS to 60 MS
• temperature 2500 K to 50,000 K
• radius .1 RS to 15 RS
• luminosity .001 LS to 106 LS
• lifetime 3 trillion yrs to 1 million yrs
• tS ? 10 billion yrs
• Since M Lt, LM/t so since (L/LS) (M/MS)3.5
• (MtS/MSt) (M/MS)3.5 ? mass-lifetime relation
• (t/ tS ) (M/ Ms )-2.5

6
t M-2.5
7
Evolution off the main sequence, to red giant
8
Red Giant Evolution

• core H burning stops (ash is He), so core
  shrinks and heats (more gravitational collapse)
• H burning shell moves out while denser He stays
  in core
• size GROWS, star cools (moves to right on H-R ?
  red giant (as big as 1 AU!!)
• lasts gt 1 billion yrs to lt 1 million yrs
• in stars with enough mass (like the sun) core
  gets hot enough from collapse that electrons
  there become degenerate a weird quantum state in
  which pressure in independent of temperature (in
  an ideal gas, increase of T increases P too)
• if degenerate, pressure is maintained even if T
  drops

9
Headed to red giant stage
10
Evolution after core helium burning starts
11
Helium begins to fuse in the core

• core of star is so dense that electrons are
  degenerate a million times as dense as water
• at 100 million K, if density is sufficient,
  helium fusion to carbon begins (triple a process)
  and oxygen
• a LOT of energy is produced if not too massive
  (?MS) there is a helium flash not visible to us
• star shrinks but grows hotter ? L constant
  moves to left on H-R on the horizontal branch
• when He core burning stops
• Core collapse so grows hotter core electrons
  become degenerate again, star grows ? L grows a
  lot moves up to right on H-R on the asymptotic
  giant branch
• pulsations, blowing off material, instability

12
On the horizontal branch
13
On the horizontal branch and the asymptotic giant
branch
14
On the AGB
15
Degenerate core of oxygen and carbon blows off
cool debris and what remains is hotter moves to
left on H-R on the planetary nebula stage
16
The formation of the elements

• if star is massive enough, C O can burn (fuse)
  to form heavier elements (Silicon (Si) and some
  others)
• profusion of gamma rays breaks up nuclei and all
  sorts of recombinations occur, many yielding
  energy
• free neutrons get caught by nucleii and make
  bigger ones
• iron (Fe) nucleus is the stablest one of all
• to make heavier nuclei, it COSTS energy
• This stuff occurs as stars die