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Stellar Evolution:
The Deaths of Stars
Chapter Twenty-Two
Guiding Questions
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
7. Why was SN 1987A an unusual supernova?
8. What was learned by detecting neutrinos from SN
9. How do white dwarf stars give rise to certain types of
10.What vestiges are left after a supernova explosion?
Pathways of Stellar Evolution
Low-mass stars go through two distinct
red-giant stages
• A low-mass star becomes
– a red giant when shell
hydrogen fusion
– 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
Dredge-ups bring 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
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
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
• Instead, it becomes
a degenerate, dense
sphere about the
size of the Earth and
is called a white
• It glows from
thermal radiation; as
the sphere cools, it
becomes dimmer
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
• 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
energy rather than causing energy to be released
High-mass stars violently blow apart in supernova
• 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
In 1987 a nearby supernova gave us a
close-up look at the death of a massive star
Neutrinos emanate from supernovae like SN 1987A
More than 99% of the energy from such a supernova is emitted
in the form of neutrinos from the collapsing core
White dwarfs in close binary systems can also
become supernovae
• An accreting white dwarf in a close binary system can
also become a supernova when carbon fusion ignites
explosively throughout such a degenerate star
Type Ia supernovae are those produced by
accreting white dwarfs in close binaries
Type Ib and Type Ic supernovae occur when the star has
lost a substantial part of its outer layers before exploding
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
Key Words
• 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
planetary nebula
progenitor star
red-giant branch
shell helium fusion
silicon fusion
supernova (plural supernovae)
supernova remnant
thermal pulse
Type I supernova
Type Ia supernova
Type Ib supernova
Type Ic supernova
Type II supernova
white dwarf