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Guiding Questions
The Deaths of Stars
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Pathways of Stellar Evolution
GOOD TO KNOW
1. What kinds of nuclear reactions occur within a star like
the Sun as it ages?
2. Where did the carbon atoms in our bodies come from?
3. What is a planetary nebula, and what does it have to do
with planets?
4. What is a white dwarf star?
5. Why do high-mass stars go through more evolutionary
stages than low-mass stars?
6. What happens within a high-mass star to turn it into a
supernova?
7. Why was SN 1987A an unusual supernova?
8. What was learned by detecting neutrinos from SN
1987A?
9. How can a white dwarf star give rise to a type of
supernova?
10.What remains after a supernova explosion?
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Low-mass stars go through two distinct
red-giant stages
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• A low-mass star becomes
– a red giant when shell
hydrogen fusion
begins
– a horizontal-branch
star when core helium
fusion begins
– an asymptotic giant
branch (AGB) star
when the helium in the
core is exhausted and
shell helium fusion
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begins
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Bringing the products of nuclear fusion
to a giant star’s surface
• As a low-mass star ages, convection occurs over a larger
portion of its volume
• This takes heavy elements formed in the star’s interior and
distributes them throughout the star
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Low-mass stars die by gently ejecting their outer
layers, creating planetary nebulae
• Helium shell flashes in
an old, low-mass star
produce thermal
pulses during which
more than half the
star’s mass may be
ejected into space
• This exposes the hot
carbon-oxygen core of
the star
• Ultraviolet radiation
from the exposed core
ionizes and excites
the ejected gases,
producing a planetary
nebula
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Why do planetary nebulae look so different
from one another?
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The burned-out core of a low-mass star cools
and contracts until it becomes a white dwarf
• No further nuclear
reactions take place
within the exposed
core
• Instead, it becomes
a degenerate, dense
sphere about the
size of the Earth and
is called a white
dwarf
• It glows from
thermal radiation; as
the sphere cools, it
becomes dimmer
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High-mass stars create heavy elements in their cores
• Unlike a low-mass star, a high mass star
undergoes an extended sequence of
thermonuclear reactions in its core and shells
• These include carbon fusion, neon fusion,
oxygen fusion, and silicon fusion
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High-mass stars violently blow apart in
supernova explosions
• In the last stages of its life, a high-mass star has an iron-rich
core surrounded by concentric shells hosting the various
thermonuclear reactions
• The sequence of thermonuclear reactions stops here, because
the formation of elements heavier than iron requires an input
of
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energy rather than causing energy to be released
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• A high-mass star dies in a violent cataclysm in
which its core collapses and most of its matter is
ejected into space at high speeds
• The luminosity of the star increases suddenly by
a factor of around 108 during this explosion,
producing a supernova
• The matter ejected from the supernova, moving
at supersonic speeds through interstellar gases
and dust, glows as a nebula called a supernova
remnant
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In 1987 a nearby supernova gave us a
close-up look at the death of a massive star
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Neutrinos emanate from supernovae like SN 1987A
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White dwarfs in close binary systems can also
become supernovae
• An accreting white dwarf in a close binary system may
become a supernova when carbon fusion ignites
explosively throughout the degenerate star
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More than 99% of the energy from such a supernova is emitted
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in the form of neutrinos from the collapsing core
Type Ia supernovae are those produced by
accreting white dwarfs in close binaries
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Type Ib and Type Ic supernovae occur when the star
has lost a substantial part of its outer layers before
exploding
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Type II supernovae are created by the deaths of
massive stars
Most supernovae occurring in our Galaxy are hidden from our
view by interstellar dust and gases but a supernova remnant can
be detected at many wavelengths for centuries after the explosion
Jargon
• asymptotic giant branch
• asymptotic giant branch
star(AGB star)
• carbon fusion
• carbon star
• Cerenkov radiation
• Chandrasekhar limit
• core helium fusion
• dredge-up
• helium shell flash
• horizontal branch
• mass-radius relation
• neon fusion
• neutron capture
• nuclear density
• oxygen fusion
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photodisintegration
planetary nebula
progenitor star
red-giant branch
shell helium fusion
silicon fusion
supergiant
supernova (plural supernovae)
supernova remnant
thermal pulse
Type I supernova
Type Ia supernova
Type Ib supernova
Type Ic supernova
Type II supernova
white dwarf
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