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Stellar Evolution II

Stellar Evolution II. The Upper End of the Main Sequence: How massive can a star get?. Larger clouds of gas (GMCs) tend to fragment into smaller ones before collapsing to form stars – very massive stars are rare Stars with masses above 50 M SUN are unstable – nuclear reactions

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Stellar Evolution II

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  1. Stellar Evolution II

  2. The Upper End of the Main Sequence:How massive can a star get? • Larger clouds of gas (GMCs) tend to fragment into smaller ones • before collapsing to form stars – very massive stars are rare • Stars with masses above 50 MSUN are unstable – nuclear reactions • in their core produce energy at such a fast rate that they blow off • their outer layers, losing mass.

  3. Eta Carinae • Mass: 100-150MSUN • Giant eruption in 1843 • Made it 2nd brightest star • in sky for a short time • Created two giant lobes • of hot gas expanding away • from the star

  4. Evolution of Stars on the Main Sequence • Core starts with same fraction of hydrogen as whole star • Fusion changes H  He • For each reaction, the star loses 4 H and gains only one He, so pressure decreases and gravity squeezes the core more tightly • Core gradually shrinks and gets hotter, increasing the pressure to compensate • Energy generation rate gradually increases, so star gets more luminous and the surface gets hotter. Increased pressure of radiation increases the radius. H > He

  5. Evolution of Stars on the Main Sequence

  6. Evolution of Stars on the Main Sequence • Lifetime of stars on the main sequence: • More massive stars burn their fuel faster, so will use it up • quicker > have smaller lifetimes • Lifetime T = 1/M2.5 • O5 - 1,000,000 yr • A0 - 440,000,000 yr • G0 - 8,000,000,000 yr • K0 - 17,000,000,000 yr • M0 - 56,000,000,000 yr • (age of universe: 13,600,000,000 yr)

  7. Main Sequence Stars: • Energy generated by fusing H to He in their core • Luminosity and surface temperature increase as mass increases

  8. Post-MS Evolution of Low Mass Stars(M < 8MSUN) • What happens when the core runs out of Hydrogen? H H > He

  9. Post-MS Evolution of Low Mass Stars(M < 8MSUN) • Core stops producing energy – gravity causes it to contract and heat up • Layer surrounding the core also contracts and heats up enough to start fusing H to He He • Outer parts of star expand • because star is H – burning layer • is producing more energy than is • required to balance gravity H > He • Surface gets cooler because of • increased area > star becomes red • giant

  10. Post-MS Evolution of Low Mass Stars(M < 8MSUN)

  11. Post-MS Evolution of Low Mass Stars(M < 8MSUN)

  12. Degenerate Gases • Normal Gas: • Compress normal gases > particles move faster > • increased pressure and temperature • Heat a normal gas > pressure increases

  13. Degenerate Gases • Degenerate Gases: • If gas is dense enough, particles have no where to move – if you compress the gas, the particles cannot move faster; they simply ‘wiggle’ more energetically • Compress degenerate gas > • temperature increases but • pressure remains the same • Heat a degenerate gas > • pressure stays the same

  14. Helium Flash • Matter in core is fully ionized – all electrons are free of their atoms • Most pressure in the core is from the electrons • As the core of Helium ash shrinks, it becomes degenerate – its • temperature will increase but its pressure will remain the same • Density now around 1000,000 grams/cubic cm (about 1000 tonnes/cc) • As the temperature of the core passes 100,000,000 K, it can start He • fusion into Carbon

  15. Helium Flash • He ignites > produces energy > temperature increases, but pressure • stays the same – the core cannot respond to the increased temperature • by expanding • Increases temperature > increased He fusion rate > increased energy • production > increased temperature > increased He fusion rate > • increased energy production > increased temperature > increased He • fusion rate > increased energy production > increased temperature > .... • Explosion! – the Helium • Flash

  16. Helium Flash • For a few minutes the core generates 100,000,000,000,000 times • more energy per second than the sun – 100 times more energy per • second than all the stars in the Milky Way combined • Does not destroy the star – energy is absorbed in outer layers. No • outward sign of explosion • Helium flash only occurs in stars between 0.5 and 3 MSUN • After a few minutes, the core becomes so hot that the gas becomes • normal again, and pressure increases

  17. Helium Burning • He fused to C, O in core • H still fusing to He in • shell around core He > C, O H > He

  18. Helium Burning • Extra energy from He • Fusion causes core to • Expand • This forces H burning • shell to expand. He > C, O H > He

  19. Helium Burning • Expansion cools H • burning shell, which then • absorbs heat from the • envelope, causing it to • shrink a little and get • hotter He > C, O H > He

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