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Stellar Evolution

Stellar Evolution. Stars must evolve because they shine. The Trifid Nebula (M20). Lagoon Nebula (M8). M24. Gaseous Nebulae. Young Clusters. Pleiades. M7. Stellar Nurseries The Eagle Nebula. Orion Nebula. The Trigger Mechanism. Spiral Density Waves Supernova. Movie.

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Stellar Evolution

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  1. Stellar Evolution Stars must evolve because they shine

  2. The Trifid Nebula (M20) Lagoon Nebula (M8) M24 Gaseous Nebulae

  3. Young Clusters Pleiades M7

  4. Stellar NurseriesThe Eagle Nebula

  5. Orion Nebula

  6. The Trigger Mechanism • Spiral Density Waves • Supernova Movie

  7. Gas Cloud Encounters Spiral Arm Spiral Arm Cloud Animation

  8. d v Angular Momentum m L = (mass)(velocity)(distance from axis)

  9. Conservation of Angular Momentum • Total angular momentum is unchanged • As collapse proceeds, d is decreasing • Cloud spins faster

  10. Animation

  11. Planet Formation • Orbits should be coplanar • Planets should orbit in the same direction • Planets should rotate in the same direction

  12. Temperature in the Solar Nebula Asteroids S M V E M J

  13. The Power Struggle • Gravity tries to crush the star • Radiation Pressure tries to blow it up • During Pre-Main Sequence Evolution, gravity is winning • Star collapses

  14. Ideal Gas • As pressure increases, there are more atomic collisions • Average particle speed increases • Temperature goes up • P  T

  15. More Ideal Gases • Ex. – fill a balloon with air • Carefully heat it up • Balloon expands to try to cool down • Put it in the freezer • Balloon shrinks to try to heat up • Ideal gases work like a safety valve

  16. Evolution on the Blackbody Curve Infrared Visible

  17. Brightness Temperature Evolution on the H-R Diagram Phase I Pre-Main Sequence Evolution Main Sequence

  18. Stellar Luminosity • Bigger objects are brighter • pre-main sequence objects are above the main sequence L  R2T4

  19. Dust Shell • Region around proto-star gets too warm • Dust is pushed back • Proplyds • Bi-polar jet

  20. Inner Region Protostar

  21. Proplyds

  22. Bi-Polar Jets

  23. Observed Protostar • Brighter than indicated by temperature • Very large • Redder than indicated by temperature • Dust shell scatters short wavelengths T Tauri Variables

  24. Phase II Pre-Main Sequence Evolution • Protostar begins a very strong solar-type wind • T Tauri Wind • Dust shell becomes vaporized • Planet formation halted • Now we see protostar for the first time • Star grows fainter (much smaller) • Temperature remains almost constant • Hyashi Track

  25. Hyashi Track

  26. The Onset of Fusion • Core temperatures reach 10 million K • Fusion begins • Gravity now balanced by radiation pressure • Hydrostatic Equilibrium established • Zero Age Main Sequence (ZAMS)

  27. Pre-Main Sequence Evolutionary Tracks

  28. Core Stellar Structure • Core • Fusion reactions here Envelope • Envelope • Supplies gravity to keep core hot and dense

  29. Stellar Models • Start at center by guessing temperature, pressure, and composition • Work outward using well-established laws of physics and chemistry • Check the surface conditions against real stars.

  30. Main Sequence Evolution • Hydrogen is depleted in the current fusion zone • Reaction rate falls • Core can no longer support its weight • core shrinks • Temperature in the core increases • Gas is ideal • Extra energy produced to - • Begin fusion reaction in next core zone • Lift the envelope • Star brightens slightly

  31. Core Current hydrogen “burning” shell New hydrogen burning shell Main Sequence Evolution He He

  32. End Section

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