Download Lecture 15

Life as a Low-Mass Star
• What are the life stages of a low-mass star?
• How does a low-mass star die?
Before we begin lets have some revision
questions on how stars are born (i.e. the
previous lecture).
What is the first step in the formation of a
protostar?
A. Conservation of angular momentum causes galactic
material to make a disk.
B. Gravity causes planetesimals to begin to accumulate.
C. Gravity causes a cloud of gas and dust to begin to
contract.
D. Nuclear fusion heats material and causes it to glow.
Why do we think that clouds of gas and dust form
stars?
A.
B.
C.
D.
E.
We see young star clusters with gas and dust around them.
Infrared and microwave telescopes let us see inside dust clouds
and see protostars.
Computer models predict that if a cloud has enough mass it
will contract from the pull of gravity, heat up, and form a star.
The Hubble Telescope lets us watch stars form before our eyes.
A, B, and C
What is the energy source that heats a
contracting protostar?
A. Friction
B. Pressure, as the gas and dust are compressed
C. Gravitational potential energy released as the
material is pulled inward
D. Fusion
E. Kinetic energy
Why do protostars rotate rather fast and end up
surrounded by disks of material?
A. The galaxy is rotating, so all the stars that form are
rotating as well.
B. If a cloud spins even a little bit, the spin increases as
it contracts.
C. The conservation of angular momentum causes this
to occur.
D. All of the above.
E. B and C
Why can’t a cloud with less than 0.08 times the
mass of the Sun become a star?
A.
B.
C.
D.
It will break into pieces.
Gravity will be too weak, and it will come apart.
It will never get hot enough for fusion to start.
It will become a star, but a small and faint one.
Life as a Low-Mass Star
• What are the life stages of a low-mass star?
• How does a low-mass star die?
What are the life stages of a
low-mass star?
A star
remains on
the main
sequence as
long as it can
fuse hydrogen
into helium in
its core.
Main-Sequence Lifetimes and Stellar Masses
Thought Question
What happens when a star can no longer fuse
hydrogen to helium in its core?
A.
B.
C.
D.
Its core cools off.
Its core shrinks and heats up.
Its core expands and heats up.
Helium fusion immediately begins.
Life Track After Main Sequence
• Observations of star
clusters show that a
star becomes larger,
redder, and more
luminous after its
time on the main
sequence is over.
Broken Thermostat
• As the core contracts,
H begins fusing to He
in a shell around the
core.
• Luminosity increases
because the core
thermostat is broken—
the increasing fusion
rate in the shell does
not stop the core from
contracting.
Helium fusion does not begin right away because it
requires higher temperatures than hydrogen fusion—larger
charge leads to greater repulsion.
The fusion of two helium nuclei doesn’t work, so helium
fusion must combine three He nuclei to make carbon.
Helium Flash
• The thermostat is broken in a low-mass red giant
because degeneracy pressure supports the core.
• The core temperature rises rapidly when helium
fusion begins.
• The helium fusion rate skyrockets until thermal
pressure takes over and expands the core again.
Helium core-fusion stars neither shrink nor grow
because the core thermostat is temporarily fixed.
Life Track After Helium Flash
• Models show that
a red giant should
shrink and become
less luminous after
helium fusion
begins in the core.
Life Track After Helium Flash
• Observations of star
clusters agree with
these models.
• Helium core-fusion
stars are found in a
horizontal branch
on the H-R diagram.
How does a low-mass star die?
Thought Question
What happens when a star’s core runs out of helium?
A.
B.
C.
D.
The star explodes.
Carbon fusion begins.
The core cools off.
Helium fuses in a shell around the core.
Double Shell Fusion
• After core helium fusion stops, He fuses into
carbon in a shell around the carbon core, and H
fuses to He in a shell around the helium layer.
• This double shell–fusion stage never reaches
equilibrium—the fusion rate periodically spikes
upward in a series of thermal pulses.
• These pulse cause the star to periodically throw of
its outer layers.
Planetary Nebulae
• Double shell–fusion
ends with a pulse that
ejects the H and He
into space as a
planetary nebula.
• The core left behind
becomes a white
dwarf.
Planetary Nebulae
• Double shell–fusion
ends with a pulse that
ejects the H and He
into space as a
planetary nebula.
• The core left behind
becomes a white
dwarf.
End of Fusion
• Fusion progresses no further in a low-mass star
because the core temperature never grows hot
enough for fusion of heavier elements (some He
fuses to C to make oxygen).
• Degeneracy pressure supports the white dwarf
against gravity.
• Degeneracy pressure is due to the fact that all
electron states fill up and it gets harder to
contract\compress the star as the electrons cannot
get closer together.
Life stages of
a low-mass
star such as
the Sun
The Death Sequence of the Sun
Life Track of a Sun-Like Star
What are the life stages of a
high-mass star?
When does a star leave the main sequence?
A.
B.
C.
D.
E.
After a few million years
After a few billion years
It depends on its mass
When the hydrogen fuel in its core is used up
C and D
What happens to nuclear fusion when the
hydrogen in a star’s core runs low?
A.
B.
C.
D.
It stops.
It shifts from the core to a shell around the core.
Other elements start to fuse.
All of the above.
In a red giant star, three helium atoms (4He)
can fuse together to … ?
A.
B.
C.
D.
E.
Make more energy for the star
Give the star additional life
Create the element carbon (12C)
A and C
A, B, and C
After the Sun becomes a red giant star and
makes carbon in its core, why will it not make
heavier elements?
A. It will have run out of fuel
B. It will be near the end of its life and doesn’t have
time
C. It will not be massive enough to make it hot
enough for further reactions
D. The heavier elements will all go into a planetary
nebula
E. A and B
After the Sun becomes a red giant, it will shed
much of its atmosphere in … ?
A.
B.
C.
D.
A post-stellar nebula
A planetary nebula
A stellar nebula
A supernova explosion
• Now work on the Lecture Tutorial,
Luminosity, Temperature and Size.
Recall we can increase the Luminosity of a
blackbody by increasing its temperature or
increasing its size (radius). Stars behave in the
same way.