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Annoucements

Annoucements. Go observing! Soon! The next exam is on Friday, October 8. That is only 9 days from today. How Stars Evolve. Evolution of protostars onto main sequence Gravitational contraction Luminosity versus temperature and radius The fate of the Sun Red giant phase Helium flash

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Annoucements

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  1. Annoucements • Go observing! Soon! • The next exam is on Friday, October 8. • That is only 9 days from today.

  2. How Stars Evolve • Evolution of protostars onto main sequence • Gravitational contraction • Luminosity versus temperature and radius • The fate of the Sun • Red giant phase • Helium flash • Reading: sections 19.4, 18.4, 19.1

  3. Protostars evolve into main-sequence stars Hotter

  4. Protostars evolve into main-sequence stars • Protostar collapses under gravity. • The collapse releases energy. • The star is relatively transparent, so the energy is radiated away, temperature doesn’t change much. • The star gets smaller so it get dimmer. Brighter Hotter

  5. Luminosity versus radius and temperature 2 1 R = RSun T = TSun R = 2 RSun T = TSun Which star is more luminous?

  6. Luminosity versus radius and temperature 2 1 R = RSun T = TSun R = 2 RSun T = TSun • Each cm2 of each surface emits the same amount of radiation. • The larger stars emits more radiation because it has a larger surface. It emits 4 times as much radiation.

  7. Luminosity versus radius and temperature 1 2 1 R = RSun T = TSun R = RSun T = 2TSun Which star is more luminous? The hotter star is more luminous. Luminosity varies as T4 (Stefan-Boltzmann Law)

  8. Luminosity Law L R2T4 Luminosity surface area  (Radius)2 Luminosity (Temperature)4 2 1 If star A is 2 times as hot as star B, and the same radius, then it will be 24 = 16 times as luminous.

  9. Protostars evolve into main-sequence stars • The star gets smaller so it get dimmer. • Next the protostar becomes opaque, light no longer escapes easily. • Heat is trapped and the protostar gets hotter. • Finally, the core gets hot enough to start nuclear fusion, contraction stops. Brighter Hotter

  10. More massive protostars evolve faster

  11. Choose the best match: All ____stars have ___ than ___ stars. • Main-sequence : more mass : pre-main sequence • More massive : shorter lifetimes : less massive • More luminous : larger radii : less luminous • Holllywood : worse tans : Broadway

  12. The Fate of the Sun • How will the Sun evolve over time? • What will be its eventual fate?

  13. Sun’s Structure • Core • Where nuclear fusion occurs • Envelope • Supplies gravity to keep core hot and dense

  14. Main Sequence Evolution • Core starts with same fraction of hydrogen as whole star • Fusion changes H  He • Core gradually shrinks and Sun gets hotter and more luminous

  15. Gradual change in size of Sun Now 40% brighter, 6% larger, 5% hotter

  16. Main Sequence Evolution • Fusion changes H  He • Core depletes of H • Eventually there is not enough H to maintain energy generation in the core • Core starts to collapse

  17. Red Giant Phase • He core • No nuclear fusion • Gravitational contraction produces energy • H layer • Nuclear fusion • Envelope • Expands because of increased energy production • Cools because of increased surface area

  18. Sun’s Red Giant Phase

  19. HR diagram Giant phase is when core has been fully converted to Helium

  20. Helium Flash • He core • Eventually the core gets hot enough to fuse Helium into Carbon. • This causes the temperature to increase rapidly to 300 million K and there’s a sudden flash when a large part of the Helium gets burned all at once. • We don’t see this flash because it’s buried inside the Sun. • H layer • Envelope

  21. Movement on HR diagram

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