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.

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 reactionsin their core produce energy at such a fast rate that they blow offtheir outer layers, losing mass.

Eta Carinae

• Mass: 100-150MSUN

• Giant eruption in 1843Made it 2nd brightest starin sky for a short time•Created two giant lobesof hot gas expanding awayfrom 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 upquicker > 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)

H > He

• What happens when the core runs out of Hydrogen?

H

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

• Core stops producing energy – gravity causes it to contract and heat up

He

• Layer surrounding the core also contracts and heats up enough to start fusing H to He

H > He• Outer parts of star expandbecause star is H – burning layeris producing more energy than is required to balance gravity

• Surface gets cooler because ofincreased area > star becomes redgiant

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:• 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

Degenerate Gases

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 Hefusion into Carbon

Helium Flash• He ignites > produces energy > temperature increases, but pressurestays the same – the core cannot respond to the increased temperatureby expanding• Increases temperature > increased He fusion rate > increased energyproduction > increased temperature > increased He fusion rate > increased energy production > increased temperature > increased He fusion rate > increased energy production > increased temperature > ....

• Explosion! – the HeliumFlash

Helium Flash• For a few minutes the core generates 100,000,000,000,000 timesmore energy per second than the sun – 100 times more energy persecond than all the stars in the Milky Way combined• Does not destroy the star – energy is absorbed in outer layers. Nooutward 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 becomesnormal again, and pressure increases

Helium Burning

He > C, O

H > He

• He fused to C, O in core• H still fusing to He in shell around core

Helium Burning

He > C, O

H > He

• Extra energy from He Fusion causes core to Expand• This forces H burning shell to expand.

Helium Burning

He > C, O

H > He

• Expansion cools H burning shell, which then absorbs heat from the envelope, causing it to shrink a little and get hotter