Stellar Evolution

1
Stellar Evolution

• Pressure vs. Gravity

2
Pressure vs. Gravity

• Two determining forces that govern a stars
  existence
• Gravity pushes inwardly
• Pressure pushes outwardly

3
Pressure

• Stars support themselves against gravitational
  collapse by generating thermonuclear energy
• Most stars are composed mainly of Hydrogen and
  thus fuse together to form Helium.

4
Proton-Proton Chain

• p p ? d e ?e
• p d ? 3He ?
• 3He 3He ? 4He p p
• These reactions take about 5 billion years to go
  into effect.

5
Low Mass Star Evolution

• Our sun is a example of low mass star that uses
  Hydrogen as fuel
• Lower mass stars have less energy available to
  fuse heavier elements together
• Slower burning process longer lives

6
Gravity

• When star runs out of fuel gravity wins out
• Contraction produces heat
• Helium core then becomes hot enough to begin
  burning Helium for fuel to produce Carbon
• To radiate the energy produced by the Helium
  burning, the star expands into a Red Giant.

7
White Dwarf

• Outer Hydrogen envelope, i.e. planetary nebula
  eventually drifts off
• Hot remnant core is a white dwarf
• No more support from burning fuel.
• Thermal motion of the ions will become less
  important and eventually degenerate electron
  pressure opposes gravitational collapse.

8
Importance of White Dwarfs

• More accurate estimate of the age of the universe
• At these extremes of temperature and pressure we
  expect to observe deviations from existing
  theories of matter

9
High Mass Star Evolution

• Approx. 8 solar masses or greater
• More massive stars have higher core temperatures
  so, burn beyond C.
• H? He? C? Ne? O? Si? Fe
• Cannot burn beyond Fe because it is the most
  tightly bound state. Burning Fe would no longer
  release energy, but require energy absorption.
• These stars have short violent lives.

10
Nucleosynthesis
11
Massive Star Collapse

• Burn Hydrogen up through Carbon, Neon, Oxygen
  Silicon
• Iron Core Formation burning shells
• Catastrophic collapse of Iron Core.
• Happens very quickly

12
End of Road

• Two possible outcomes for massive stars.
• Neutron Star
• Black Hole

13
Summary

• Stellar evolution depends on gravity and
  pressure.
• Less Massive stars have less energy so end up as
  white dwarfs. These stars live very long
• More massive stars have more energy so end up as
  neutron stars or black holes. They have very
  short lives