Stellar Evolution Pressure vs. Gravity

Pressure vs. Gravity Two determining forces that govern a star’s existence Gravity pushes inwardly Pressure pushes outwardly

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.

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.

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

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.

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.

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

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.

Nucleosynthesis

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

End of Road Two possible outcomes for massive stars. Neutron Star Black Hole

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