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Chapter 16: Evolution of
Low-Mass Stars
While on the Main Sequence
stars of all mass burn
hydrogen into helium
How long a
star lives
on the
Main
Sequence
depends
on its
mass
As the star
burns its
hydrogen, it
accumulates
a helium ash
Because energy flow in
the central regions of
the star is by radiation,
the helium ash isn’t
being stirred out.
As time goes by, the helium
ash gets in the way
To continue to burn hydrogen with all that
helium in the way, the star gets a little hotter,
a little bigger and a little brighter .
At the center
of the star, a
dead helium
core starts
to form
The central helium core is
not fusing. It’s just being
squeezed by gravity and
added to by the hydrogen
fusing above it
Once the hydrogen runs out, the
helium ash gets compressed until
it becomes degenerate
When something is
degenerate all the low
energy states are filled.
Only the highest energy states are left for new electrons
What does it mean to be
Degenerate?
•Electron energy levels crowded together
almost continuous
•All low energy levels are full according to
the Pauli Exclusion Principle
•Only place for additional electrons to go is
in high energy levels which means
they must move very fast
•Adding more mass decreases the volume
•Temperature is same everywhere
If you add mass to a
degenerate object it shrinks
When the helium core shrinks, it heats
up. This causes hydrogen to start
fusing in a shell around the core
Because the
core is
shrinking as
more mass is
added to it, it
heats up. As it
heats up that
causes the shell
fusion around it
to speed up and
the star stars to
expand
The
expansion
to a red
giant is all
due to the
battle
between
gravity and
pressure
Evolution off the Main
Sequence is the reverse of
forming a protostar
The most massive stars
become red supergiants
Once fusion
in the core
stops the
core shrinks
and heats up
while the
outer surface
expands and
cools
The core doesn’t completely
collapse due to degeneracy
Helium Fusion starts when the
core reaches 100,000,000°
3 He C  
4
12
In stars with low mass the
helium ignition is explosive
In the degenerate core the
temperature is the same everywhere
Once helium
fusion settles
down the star
resides on the
“horizontal
branch” for a
while
The energy production will
stabilize so the star will
shrink in size (some). It will
then start a second life
burning helium into carbon
For low mass stars: a second
red giant stage when the
helium in the core runs out
Internal Structure of AGB
star
Thermal Pulses cause whole
layers of star to lift off
Near the end, shell fusion becomes unstable resulting in
thermal pulses which push layers of the star into space
Planetary nebulae recycle most of a
stars’ matter back out into space
By this time,
convection is
starting to reach
farther down into
the interior of the
star and dredge
up the products
of fusion
The Death of a Low Mass Star
<8 solar masses
Planetary nebulae
can have very
complex forms. The
details of how they
create those forms is
not well understood
but probably has
something to do with
magnetic fields or if
the star is in a binary
system.
The death of a low mass star
on the H-R Diagram
After planetary nebula dissipates
only a white dwarf is left
X-ray light
Visible light
White Dwarf Stars are
degenerate matter
Chandrasekhar
Limit
1.4 Msun
They are composed mostly of carbon with some oxygen
Many stars live and die in binary
systems
The most massive will form a
white dwarf first
Eventually, the other star evolves
off the main sequence
The white dwarf can become
very active as it gains mass
White dwarfs in binary systems are called cataclysmic
variables because they can vary cataclysmically
As hydrogen builds up on the
white dwarf it can ignite
If the white dwarf mass
exceeds the Chandrasekhar
limit it explodes in a supernova.
If not, it can undergo an
ordinary nova outburst