The Final Evolution of

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The Final Evolution of Low Mass Stars (lt 8
solar masses)
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The sun - past and future
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H
He
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H
He
The outer envelopes of
L
All surfaces become convective
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Type I Cepheids are shortlived-stages of
massive stars as the cross the HR diagram on the
way to becoming red giants.
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Betelgeuse a red supergiant. One of the very
few stars besides our own sun ever to be
directly imaged (by HST). Betelgeuse is 600 ly
distant in the constellation Orion (alpha-Orionis)
. Over 1000 times the diameter of the sun. Could
become a supernova.
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M103 an open cluster about 20 My old and 8000
ly distant in the Constellation Casseopeia. 14
ly across. About as large on the sky as ¼ of the
full moon. The bright blue stars are young
B-stars. The single red star is a red supergiant.
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Red Giant 1.3 Gy He core burning 100 My
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Helium Burning and Beyond
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The globular cluster M10. The bright yellow and
orange stars are red giants burning hydrogen in a
shell, but the bright blue stars are horizontal
branch stars, burning helium in their centers.
Both kinds of stars are much more massive than
the low mass main sequence stars in M10.
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Red Giant 1.3 Gy He core burning 100 My
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For stars of one solar mass and less, lower mass
HB stars are hotter (bluer) than higher mass ones
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The Seven Ages of the Sun

• Main sequence 10.6 Gy
• First red giant 1.3 Gy
• Helium burning 100 My
• Second red giant 20 My
• Unstable pulsation 400 Ky
• Planetary nebula 10 Ky
• White dwarf forever

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Cutaway drawing of the interior structure of an
Asymptotic Giant Branch or AGB star. Hydrogen
an helium burning shells are both active,
though not necessarily both at the same time. The
He and H burning regions are much thinner than
this diagram suggests. The outer layers are
convective. The C-O core is degenerate and
transports its radiation by conduction.
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AGB stars are known to lose mass at a prodigious
rate during their final stages, around 10-5 -
10-4 solar masses per year. This obviously cannot
persist for much over 100,000 years. The mass
loss is driven in part by the pulsational
instability of the thin helium shell. These
pulses grow more violent with time. Also, and
probably more importantly, the outer layers of
the star get so large and cool owing to the
high luminosity, that they form dust. The dust
increases the opacity and material is blown away
at speeds 10 30 km s-1
The evolution is terminated as the outer layers
of the star are blown away.
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Note the consequences for nucleosynthesis
here. The outer layers of the star contain
hydrogen and helium to be sure, but also nitrogen
from CNO processing and C and O from helium
burning. It is thought that stars in this mass
range are responsible for producing most of the
nitrogen and maybe 60 80 of the carbon in the
universe. The rest of carbon and most other
elements comes from massive stars.
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Additional Nucleosynthesis The s-Process.
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Critical Masses
Contracting protostars below this mass do not
ignite hydrogen burning on the main sequence.
They become brown dwarfs or planets.
Stars below this mass are completely convective
on the main sequence
do not ignite helium burning
Stars below this mass (and above .5) experience
the helium core flash Stars above this mass are
powered by the CNO cycle (below by the
pp-cycles) Stars above this mass have convective
cores on the main sequence (and radiative
surfaces)
Stars below this mass do not ignite carbon
burning. They end their lives as planetary
nebulae and white dwarfs. Stars above this mass
make supernovae.
Population I stars much above this mass pulse
apart on the main sequence. No heavier stars
exist.
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(cm if M is in gm)
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Equating these two one can solve for a unique
value of the mass - the Chandrasekhar mass
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