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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 MSUN are unstable – nuclear reactions in their core produce energy at such a fast rate that they blow off their outer layers, losing mass. • 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 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 Evolution of Stars on the Main Sequence 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) Main Sequence Stars: • Energy generated by fusing H to He in their core • Luminosity and surface temperature increase as mass increases Post-MS Evolution of Low Mass Stars (M < 8MSUN) • What happens when the core runs out of Hydrogen? H H > He Post-MS Evolution of Low Mass Stars (M < 8MSUN) • Core stops producing energy – gravity causes it to contract and heat up He H > He • Layer surrounding the core also contracts and heats up enough to start fusing H to He • Outer parts of star expand because star is H – burning layer is producing more energy than is required to balance gravity • Surface gets cooler because of increased area > star becomes red giant Post-MS Evolution of Low Mass Stars (M < 8MSUN) Post-MS Evolution of Low Mass Stars (M < 8MSUN) Degenerate Gases Normal Gas: • Compress normal gases > particles move faster > increased pressure and temperature • Heat a normal gas > pressure increases 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 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 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 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 Helium Burning • He fused to C, O in core • H still fusing to He in shell around core He > C, O H > He Helium Burning • Extra energy from He Fusion causes core to Expand • This forces H burning shell to expand. He > C, O H > He 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