Chapter 16: Evolution of Low-Mass Stars
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Transcript of Chapter 16: Evolution of Low-Mass Stars
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Chapter 16: Evolution of Low-Mass Stars
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While on the Main Sequence stars of all mass burn hydrogen into helium
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How long a star
lives on the Main Sequence depends
on its mass
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As the star burns its
hydrogen, it accumulates a helium
ashBecause energy flow in the central regions of the star is by radiation, the helium ash isn’t being stirred out.
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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 .
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At the center of
the star, a dead helium core starts
to formThe central helium core is not fusing. It’s just being squeezed by gravity and added to by the hydrogen fusing above it
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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
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What does it mean to be Degenerate?
• Electron energy levels crowded togetheralmost 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 meansthey must move very fast
• Adding more mass decreases the volume
• Temperature is same everywhere
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If you add mass to a degenerate object it shrinks
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When the helium core shrinks, it heats up. This causes hydrogen to start fusing in a shell around the
coreBecause 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
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The expansion to a red
giant is all due to the
battle between gravity
and pressure
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Evolution off the Main Sequence is the reverse of
forming a protostar
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The most massive stars become red supergiants
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Once fusion in the core stops the
core shrinks and heats
up while the outer
surface expands and
cools
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The core doesn’t completely collapse due to degeneracy
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Helium Fusion starts when the core reaches
100,000,000°
CHe 1243
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In stars with low mass the helium ignition is explosive
In the degenerate core the temperature is the same everywhere
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Once helium fusion settles down the star
resides on the
“horizontal branch” for a
whileThe energy production will stabilize so the star will shrink in size (some). It will then start a second life burning helium into carbon
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For low mass stars: a second red giant stage
when the helium in the core runs out
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Internal Structure of AGB star
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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
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Planetary nebulae recycle most of a stars’ matter back out into
spaceBy this time, convection is starting to reach farther down into the interior of the star and dredge up the products of fusion
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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.
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The death of a low mass star on the H-R Diagram
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After planetary nebula dissipates only a white dwarf is left
Visible light
X-ray light
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White Dwarf Stars are degenerate matter
Chandrasekhar Limit
1.4 Msun
They are composed mostly of carbon with some oxygen
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Many stars live and die in binary systems
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The most massive will form a white dwarf first
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Eventually, the other star evolves off the main sequence
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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
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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