1. Stellar Evolution
Pressure vs. Gravity

2. Pressure vs. Gravity
Two determining forces that govern a star’s 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+ + νe p + d  3He + γ
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