End State of Stars

1
End State of Stars

• Core remnant less than
• 1.4 solar masses

2
H shell fusion

• Core becomes clogged with He ash
• H fusion stops in core
• Core contracts and gets hotter
• Heats up a shell of H around core
• H fusion occurs in this shell
• Central temperature increases to
• 100 million K

3
He core fusion

• The increased temperature in He core
• Increased speed of particles
• Means electrostatic repulsion of He can be
  overcome
• He fusion begins in core

4
Size of star

• With an increase in core temperature
• Hydrostatic balance is upset
• Outer layers of star expand
• L increases
• Outer layers however cool
• Red cool
• Giant High L

5
The Sun as a Red giant
6
Fusion ends

• He fusion produces C and O
• Core pressure and temperature not high enough for
  fusion of heavier elements
• After about 100 million years fusion ends

7
Planetary nebula

• Alternate contractions and expansions of the core
  cause variations in luminosity
• These thermal pulses cause the whole star to
  expand and contract
• They last about a million years
• During these pulses the outer envelope of the
  star can be blown off

• This outer layer expands into space
• Driven out by radiation pressure from the exposed
  core
• The hot white core surface temp about
• 100 000 K
• Emits u-v light so the gases glow

8
Planetary Nebula

• These do not last for more than 50 000 years
• 25 to 60 of original stars can be lost in
  planetary nebula
• They are quite common
• Up to 50 000 or so in our galaxy

9
White dwarf

• The planetary nebula fades
• Leaving the white hot core
• About the size of the earth
• But the mass of a star
• Very dense
• White ? hot
• Dwarf ? small surface area
• Eventually will cool to a black dwarf

10
Typical Sizes

• Red giants can be
• Between 0.4 and 8 solar masses
• More massive stars form red supergiants
• And have an altogether different fate