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
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.
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.
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