Life as a Low-Mass Star
What are the life stages of a low
mass star?
High-Mass Stars
(> 8 MSun )
IntermediateMass Stars
A star remains
on the main
sequence as long
as it can fuse
hydrogen into
helium in its
core.
Low-Mass Stars
(< 2 MSun)
Brown Dwarfs
Thought Question
What happens when a star can no longer fuse
hydrogen to helium in its core?
A.
B.
C.
D.
Core cools off.
Core shrinks and heats up.
Core expands and heats up.
Helium fusion immediately begins.
Thought Question
What happens when a star can no longer fuse
hydrogen to helium in its core?
A.
B.
C.
D.
Core cools off.
Core shrinks and heats up.
Core expands and heats up.
Helium fusion immediately begins.
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Thought Question
Helium fusion requires higher temperatures (around 100
million Kelvin) than hydrogen fusion, because larger charge
leads to greater electromagnetic repulsion.
Fusion of two helium nuclei does not work, so helium fusion
must combine three He nuclei to make carbon.
What happens as a star’s inert helium core starts
to shrink?
A.
B.
C.
D.
Hydrogen fuses in shell around core.
Helium fusion slowly begins.
Helium fusion rate rapidly rises.
Core pressure sharply drops.
Thought Question
What happens as a star’s inert helium core starts
to shrink?
A.
B.
C.
D.
Hydrogen fuses in shell around core.
Helium fusion slowly begins.
Helium fusion rate rapidly rises.
Core pressure sharply drops.
Thought Question
What happens in a low-mass star when core temperature
rises enough for helium fusion to begin?
A. Helium fusion slowly starts up.
B. Hydrogen fusion stops.
C. Helium fusion rises very sharply.
Broken thermostat: rising fusion rate in shell does not
expand core, so star’s luminosity continues to rise.
Thought Question
What happens in a low-mass star when core temperature
rises enough for helium fusion to begin?
A. Helium fusion slowly starts up.
B. Hydrogen fusion stops.
C. Helium fusion rises very sharply.
Hint: Degeneracy pressure is the main form of pressure in
the inert helium core.
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Helium Flash
• Thermostat is broken in low-mass red giant,
because degeneracy pressure supports core.
• Core temperature rises rapidly when helium fusion
begins.
• Helium fusion rate skyrockets until thermal
pressure takes over and expands core again.
Helium burning stars neither shrink nor grow,
because thermostat is temporarily fixed.
Thought Question
What happens when the 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.
How does a low mass star die?
Thought Question
What happens when the 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.
A star like our
Sun dies by
puffing off its
outer layers,
creating a
planetary nebula.
Only a white
dwarf is left
behind.
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A star like our
Sun dies by
puffing off its
outer layers,
creating a
planetary nebula.
A star like our
Sun dies by
puffing off its
outer layers,
creating a
planetary nebula.
Only a white
dwarf is left
behind.
Only a white
dwarf is left
behind.
A star like our
Sun dies by
puffing off its
outer layers,
creating a
planetary nebula.
Only a white
dwarf is left
behind.
Evolutionary track of a low-mass star on the H-R diagram.
Life as a High-Mass Star
What are the life stages of a
high-mass star?
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High-Mass Stars
(> 8 MSun )
High-Mass Star’s Life
IntermediateMass Stars
Early stages are similar to those of low-mass star:
Low-Mass Stars
(< 2 MSun)
Brown Dwarfs
•
Main Sequence: H fuses to He in core (CNO cycle)
•
Red Supergiant: H fuses to He in shell around inert
He core
•
Helium Core Burning: He fuses to C in core (no
helium flash)
CNO cycle is just another
way to fuse H into He,
using carbon, nitrogen, and
oxygen as catalysts.
High-mass stars
become supergiants
after core H runs
out.
CNO cycle is the main
mechanism for H fusion in
high-mass stars (>2 M Sun),
because core temperature is
higher.
Luminosity does
not change much,
but radius gets far
larger!
How do high-mass stars make the
elements necessary
for life?
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Big Bang made 75% H, 25% He – stars make everything else!
Helium fusion can make carbon in low-mass stars.
Helium-capture reactions add two protons at a time.
CNO cycle can change C into N and O.
Advanced nuclear fusion reactions require extremely high
temperatures.
Helium-capture reactions build C into O, Ne, Mg, etc.
Only high-mass stars can attain high enough core temperatures
before degeneracy pressure stops gravitational contraction.
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Advanced reactions make heavier elements
in high-mass stars.
Iron is a dead
end for fusion,
because nuclear
reactions
involving iron do
not release
energy.
Advanced nuclear burning occurs in multiple shells.
Evidence for heliumcapture reactions:
Higher abundances of
elements with even
numbers of protons.
Fe has the lowest
mass per nuclear
particle.
How does a high-mass star die?
Fe builds up in core
until degeneracy pressure
can no longer resist
gravity.
Core then suddenly
collapses, creating
supernova explosion.
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Core degeneracy
pressure goes away,
because electrons
combine with
protons, making
neutrons and neutrinos.
Neutrons collapse to
the center, forming a
neutron star.
Energy and neutrons produced in supernova explosion
enable elements heavier than Fe to form.
Elements made
during supernova
explosion
Crab Nebula: Remnant of supernova observed in 1054 A.D.
The next
nearby
supernova?
before
after
Supernova 1987A is the nearest supernova
observed in the last 400 years.
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Low-Mass Star Summary
1. Main Sequence: H fuses to He
in core.
2. Red Giant: H fuses to He in
shell around He core.
How does a star’s mass
determine its life story?
3. Helium Core Burning:
He fuses to C in core while H
fuses to He in shell.
4. Double-Shell Burning:
H and He both fuse in shells.
Not to scale!
Life Stages of High-Mass Star
Reasons for Life Stages
1. Main Sequence: H fuses to He in
core
 Core shrinks and heats until it’s
hot enough for fusion.
2. Red Supergiant: H fuses to He in
shell around He core
 Nuclei with larger charge require
higher temperature for fusion.
3. Helium Core Burning:
He fuses to C in core while H fuses
to He in shell
 Core thermostat is broken while
core is not hot enough for
fusion (shell burning).
4. Multiple-Shell Burning:
Many elements fuse in shells
 Core fusion can’t happen if
degeneracy pressure prevents
core from shrinking.
Not to scale!
5. Planetary Nebula leaves white
dwarf behind.
Not to scale!
5. Supernova leaves neutron star or
black hole behind.
How are the lives of stars with
close companions different?
Life of a 20MSun star
Life of a 1MSun star
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Thought Question
The binary star Algol consists of a 3.7 MSun main
sequence star and a 0.8 MSun subgiant star.
What’s strange about this pairing?
Stars in Algol are close
enough that matter can flow
from subgiant onto mainsequence star!
How did it come about?
> Star that is now a
subgiant was originally
more massive.
> As it reached the end of
its life and started to grow,
it began to transfer mass to
its companion.
> Now the companion star
is more massive.
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