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Stellar Evolution

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

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Stellar Evolution

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  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

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