Lecture 17: The Evolution of High-Mass Stars

Readings: 22-5

 

Key Ideas:

Intermediate Mass Stars (4Msun < M < 8Msun)

High Mass = O & B stars (M > 8Msun)

Main Sequence Phase

Red Supergiant Phase

         He burning

         Carbon burning M>4MSun

         Neon, Oxygen & Silicon Burning M>MSun

         Ends with Iron Core Formation

 

Intermediate-Mass Stars

 

Stars with M = 4-8 MSun

                  He fusion starts under non-degenerate conditions, so no He flash

         After asymptotic giant branch phase, core gets hot enough to fuse C. Results 

         in an O-Ne-Mg core

         Inert O-Ne-Mg core contracts & heats up

         C, He, & H burning shells

Thermal pulses destabilize envelope:

         Eject envelope in a massive stellar wind

         Leave O-Ne-Mg white dwarf core behind

 

High-Mass Stars

O&B Stars (M>8MSun)

         Burn Hot

         Live Fast

         Die Young

 

Main Sequence Phase:

         Burn H to He in core via CNO cycle

         Build up a He core, like low-mass stars

         Lasts only for ~10 Myr

 

Red Supergiant Phase

After H core exhaustion

         He core contracts & heats up

         H burning in a shell around the He core

         Huge, puffy envelope ~ size of the orbit of Jupiter!

 

Moves horizontally across the H-R diagram:

Takes ~ 1 Myr to cross H-R diagram. This is not very long, so we see very few stars in the crossing phase

 

AppleMark

 

Helium Ignition

 

Core Temperature reaches 170 Million K

 

Ignites Helium burning to Carbon & Oxygen

         Not Degenerate=No Flash

         Rapid Phase: ~ 1 Myr

         He burning in the core

         H burning in a shell

         Start building a C-O core

Star becomes a Blue Supergiant

 

He Core Exhaustion

When He runs out in the core:

         Inert C-O core collapses & heats up

         H & He burning moves into shells

         Becomes a Red Supergiant again

C-O core collapses until

         TCore> 600 million K

         Density > 150,000 g/cc

Ignites Carbon Burning in the Core

Needs to be this hot to overcome 6proton-6proton electric repulsion

 

Carbon Burning

12C+12C fuses to

         24Mg

         20Ne +4He

         16O+4He+4He

         + many side processes

Very inefficient

         Makes many neutrinos

Lasts ~1000 years before C runs out.

 

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High-Mass Stars: M>8Msun

 

At the onset of Carbon Burning:

         Evolution is so fast the envelope can no longer respond

         See little outward sign of the inward turmoil to come

 

Exception

Strong stellar winds can erode envelope, changing the starŐs outward appearance.

 

Neon Burning

 

O-Ne-Mg core contracts & heats until

         Tcore~1.5 billion K

         Density ~107 g/cc

Ignite Neon Burning

         Reaction network makes O, Mg & others

Huge neutrino losses. More energy comes out in neutrinos than in electromagnetic radiation.

Builds a heavy O-Mg core

Lasts for a few years before Ne runs out

 

Oxygen Burning

Ne runs out, core contracts and heats until

         Tcore~2.1 billion K

         Density ~ few x 107 g/cc

Ignite Oxygen Burning

         Reaction network makes silicon, sulfur, phosphorus and other elements

         Huge neutrino losses, neutrino energy > 100,000 photon energy

         Builds a heavy silicon (+other stuff) core

Lasts for ~1 year before O runs out!

 

Silicon Burning

O runs out, core contracts & heats until:

         Tcore ~ 3.5 Billion K

         Density ~103 g/cc

Ignite Si burning

         Si melts into a sea of 4He, p & n, shredded by photons

These smaller particles fuses with other particles into Nickel

(Ni) and Iron (Fe)

Builds a heavy Ni/Fe core

 

Lasts for only ~1 day before Si runs out.

 

The Nuclear Impasse

Fusion of light elements releases nuclear binding energy, as the mass of the more tightly bound nucleus is lighter than the sum of the reactants.

 

Iron (Fe) is the most bound nucleus. In general,

         Fusion lighter than Fe releases energy

         Fusion heavier than Fe absorbs energy

 

So 12C+12C -> releases energy

But 56Fe+56Fe -> requires energy

 

Once a Fe core forms, there are no fusion fuels left for the star to tap.

 

Fe has the lowest mass/nucleon of any nucleus. You can also see that the mass difference/nucleon is greatest between H and He, and much smaller as heavier nuclei are fused. He, C, Ne, O, etc. fusion release a lot less energy. And in many cases, a lot of it comes out in the form of neutrinos, instead of heating the gas and helping to support the star.

 

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The Approach to the Iron Catastrophe

 

At the end of the Silicon Burning Day

         Star builds up an inert Iron core

         Series of nest nuclear burning shells continue to add Fe to the core

Finally, the Fe core exceeds 1.2-2 MSun

         Fe core begins to contract & heat up

         This collapse is final & catastrophic