Lecture 18: Evolution of High Mass Stars

Key Ideas:

High Mass = O & B Stars (M > 4 M_sun)

Stars with 4 < M < 8 M_sun

• Burn Hydrogen up through Carbon
• Blow off their envelope
• Core becomes an O-Ne-Mg White Dwarf

Stars with M > 8 M_sun

• Burn Hydrogen up through Carbon, Neon, Oxygen & Silicon
• Iron Core Formation & burning shells
• Catastrophic collapse of Iron Core.

High Mass Stars

O & B Stars (M > 4 M_sun):

• 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
• Last for only ~ 10 Myr

Red Supergiant Phase

After H core exhaustion:

• Inert He core contracts & heats up
• H burning in a shell around the contracting He core
• Huge, puffy envelope ~ size of orbit of Jupiter

Moves horizontally across the H-R diagram, becoming a Red Supergiant star

Takes ~ 1 Myr to cross H-R diagram

Helium Flash

Core Temperature reaches 170 Million K

Ignites Helium burning to C & O:

• Rapid Phase: ~ 1 Myr
• He burning in the core
• H burning in a shell
• Starts 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 into shells
• Becomes a Red Supergiant again

C-O Core collapses until:

• T_core > 600 Million K
• density > 150,000 g/cc
• Ignites Carbon Burning in the Core.

Carbon Burning:

Complex reaction network: ¹²C+¹²C fuses to:

• ²⁴Mg
• ²⁰Ne + ⁴He
• ¹⁶O + 2 ⁴He

Build up an inert O-Ne-Mg core

Very inefficient:

• Makes many neutrinos
• Lasts only ~1000 years before C runs out.

Intermediate Mass Stars

Stars with 4 < M < 8 M_sun

After 1000 years:

• Inert O-Ne-Mg core contracts & heats up
• C, He, & H burning shells

Thermal pulses destabilize the envelope:

• Eject the envelope in a massive stellar wind.
• Leave O-Ne-Mg white dwarf core behind.

High Mass Stars: M > 8 M_sun

At the onset of Carbon Burning:

• Evolution is so fast that the envelope can no longer respond.
• Should see little outward sign of the inward turmoil to come.

Exception: Strong stellar winds can erode the envelope, changing the outward appearance of the star.

[For example, the massive star Eta Carina has immense stellar winds. Hubble reveals a furiously expanding pair of dust and gas clouds billowing off this star. See this page at STScI for more details & pictures.]

Neon Burning

O-Ne-Mg core contracts & heats up until:

• T_core ~ 1.5 Billion K
• density ~ 10⁷ g/cc

Ignite Neon burning:

• reaction network makes O, Mg, & others
• Huge neutrino losses: > L_* !
• Builds a heavy O-Mg core

Lasts for a few years before Ne runs out.

Oxygen Burning

Ne runs out, core contracts & heats up until:

• T_core ~ 2.1 Billion K
• density ~ few x 10⁷ g/cc

Ignite Oxygen burning:

• reaction network making Si, S, P, & others
• Huge neutrino losses: > 100,000 L_* !
• Builds a heavy Si core.

Lasts for ~ 1 year before O runs out.

Silicon Burning

O runs out, Si core contracts & heats up until:

• T_core ~ 3.5 Billion K
• density ~ 10⁸ g/cc

Ignite Silicon burning:

• Si melts into a sea of He, p, & n
• Fuses with rest into Nickel (Ni) & Iron (Fe)
• Builds a heavy Ni/Fe core.

Lasts for ~ 1 day... Star core at the end of Silicon Burning

The Nuclear Impasse

Fusion of light elements is the release of nuclear binding energy.

Iron (Fe) is the most tightly bound nucleus:

• Fusion of nuclei lighter than Fe release energy.
• Fusion of nuclei heavier than Fe absorb energy.

Once an Fe core forms, there are no new fusion reactions left for the star to tap.

End of the Road

At the end of the Silicon Burning Day:

• Build up an inert Fe core
• Onion Skin of nested nuclear burning shells

Finally, the Fe core exceeds 1.2-2 M_sun:

• Fe core begins to contract & heat up.
• Collapse is final & catastrophic (as we will see...).

Summary:

Stars with 4 < Mass < 8 M_sun

• Burn through Carbon
• Blow off their envelope
• Core becomes an O-Ne-Mg White Dwarf

Stars with Mass > 8 M_sun

• Burn Carbon, Neon, Oxygen & Silicon
• Build up a heavy Iron Core & burning shells.
• Final stage occurs when the Iron core begins to catastrophically collapse.