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 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

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 H still fusing to He in shell around core 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