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Astrophysics of Life 2:
Stellar Evolution
The Lives and Deaths of Stars
Protostars. These objects radiate energy away in the form of light.
That energy comes from gravity – released by contraction.
On the Main Sequence, the
contraction stops because
gravity and internal pressure
exactly balance each other.
Nuclear reactions occur at
exactly the right rate to
balance gravity.
A stars’ mass determines its luminosity
Nuclear reactions slowly convert
H to He in the core.
Newly formed stars are typically:
~91% Hydrogen (H)
~9% Helium (He)
In the Sun’s core, the conversion of
H to He will take ~10 billion years
(its Main Sequence lifetime).
Composition of a Sun-like star
What happens when
the core Hydrogen is
used up?
Nuclear reactions stop.
Core pressure decreases.
Core contracts and gets hotter
- heating overlaying layers.
4H  1He burning moves to a
hot “shell.”
Post-Main Sequence evolution
•4H  1He reactions occur
faster than before.
•The star gets brighter (more
luminous)!
•The hot shell causes the outer
layers to expand and cool!
•The star moves off the Main
Sequence, …up the “Red
Giant” branch.
Ascension up the red giant
branch takes ~100 million years
What happens in the core as
it continues to contract and
get hotter?
Remember why Hydrogen
burning requires 107 K?
(Protons repel each other.)
Helium nuclei (2 protons) repel
each other even more…
Helium begins to fuse into Carbon at >108 K.
This reaction is called “triple alpha” = 3He  C.
The Helium Flash:
After the core reaches 108 K, Helium
“ignites” to make Carbon.

The onset of this burning causes the
temperature to rise sharply in a runaway
explosion - Helium Flash (stage 9)

Eventually the core expands, density
drops and equilibrium is re-established

Core structure is now readjusted during
Helium core burning and total luminosity
is actually decreased.

During core Helium burning, the star is
on the Horizontal Branch.

Post-Main Sequence evolution
Relative sizes of main-sequence, red giant, and
horizontal branch stars.
Stage 7
Stage 9
Stage 10
Stage 10 - Helium-to-Carbon burning
occurs stably in core, with Hydrogen-toHelium burning in shell…
Carbon
Helium
…until the core Helium runs out….
in just 20-50 million years…
Carbon
Helium
Carbon
Helium
The increased shell burning
causes the outer layers to
expand and cool (again).
The star moves up the
asymptotic giant branch
(in only ~10,000 years!).
becoming a red supergiant
(stage 11)
•During this phase the
Carbon core contracts and
heats up.
•Helium is burning to
Carbon in a shell around the
Carbon core while H-to-He
burning occurs in an outer
shell.
What do you suppose happens next in the core?
For a Sun-like star:
nothing…. Why?
Solar mass stars cannot squeeze and
heat the core enough to ignite Carbon.
So what does happen?
The Carbon core continues to
contract and heat.
Shell He burning grows more intense.
He flashes occur in the shell.
Surface layers pulsate and are finally
ejected (slowly, at ~10s of km/s).
The hot, tiny core (White Dwarf) is
revealed.
White Dwarf
And a Planetary Nebula appears!
(expanding emission line nebula
heated by intense radiation from the
hot white dwarf)
Planetary Nebulae have
nothing to do with
planets.
They emit line radiation
(hot gas) but are much
smaller than the emission
nebulae (HII regions) we
discussed in Chapter 11.
They are important
sources of heavy
elements (C, N and O),
which will go into the
next generation of stars.
White dwarfs have about ½
the Sun’s mass (the rest was
expelled).
They are about the size of the
Earth! (~0.01 solar radii)
Density: ~6600 lbs/cm3 !!
Very low luminosities,
(L = 4R2 T4)
What eventually happens
to a white dwarf?
It gets cooler and fainter
(at the same radius).
This is the End:
•White Dwarf fades away.
•Planetary Nebula dissipates
into interstellar space.
•End of story for stars like
the Sun.
Summary:
What happens to higher mass stars?
Gravity squeezes and heats the
core enough to ignite Carbon.
Then Oxygen, then Neon…as
each fuel gets exhausted in the
core, its burning moves to a
shell.
Concentric fusion shells form
an “onion skin” structure.
Formation of an Iron core is
the last stage…
Why is Iron formation the end of the line?
Creating elements
heavier than Iron
requires energy!
With no more sources
of energy, and Fe
fusion taking energy
from the gas, pressure
support in the star’s
core is lost…
The core quickly
collapses under its own
weight….
Protons and electrons are crushed
together in the collapsing core,
making neutrons.
Eventually, the neutrons are so close
together they “touch” neutron
degeneracy pressure (which is very
stiff).
The densities reach 100 billion kg/cm3
(at those densities the whole Earth would fit in our football stadium!!)
The collapsing neutron
core then bounces!
Supernova
An explosive shock wave
propagates outward,
expelling all outer layers.
This supernova was
recorded by Chinese
astronomers in 1054 AD.
Supernova ejecta
like this spread
heavy elements
throughout the
Galaxy.
Crab Nebula