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The origin of the (lighter) elements
The Late Stages of Stellar Evolution
Supernova of 1604 (Kepler’s)
High-Mass Stars
> 8 MSun
IntermediateMass Stars
Low-Mass Stars
< 2 MSun
Brown Dwarfs
Broken thermostat
‘Helium flash’
Core of helium is supported by electron degeneracy pressure
Helium burning stars are temporarily stable.
After the Helium Flash
After He fusion stops in the core…
Variable stars:
RR Lyrae
Cepheids
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The Death of a Low-Mass Star: Planetary Nebula
Remnants of stars with ~ 1 – a few Msun
Radii: R ~ 0.2 - 3 light years

Expanding at ~10 – 20 km/s ( Doppler shifts)
Last < 10,000 years
Have nothing to do with planets!
The Helix Nebula
The Formation of Planetary Nebulae
Two-stage process:
The Ring Nebula
in Lyra
Slow wind from a red giant blows
away cool, outer layers of the star
Fast wind from hot, inner
layers of the star overtakes
the slow wind and heats it
=> Planetary Nebula
The Cat Eye Nebula:
• Approx 3000 LY away
• Central star T = 80,000 K
• Spectral class O
• Mass ~ 1 Msun
• Radius ~ 0.65 Rsun
The Cat Eye
White Dwarfs are supported by
electron degeneracy pressure
• in a low-mass star, Fusion stops after He -->C and O
• Just cools off and fizzles out
Siruis and its white
dwraf companion,
Sirius B
Summary: Evolution of a
Sun-Like Star
Earth’s Fate
•
Sun’s luminosity will rise to 1,000 times
its current level—too hot for life on Earth
Earth’s Fate
•
Sun’s radius will grow to near current
radius of Earth’s orbit
High mass stars : CNO Cycle
• H fusion is faster
because C, N and O
act as catalysts
• Same net result: 4 H
become 1 He.
• No total gain or loss
of C, N, O
Life Stages of High-Mass Stars
• high-mass stars are similar to low-mass stars:
– Hydrogen core fusion (main sequence)
– Hydrogen shell burning (supergiant)
– Helium core fusion (subgiant)
• They are also different..
– H-->He via CNO cycle not p-p chain
– Core much hotter
– fuse C, O into heavier elements
– He core is not degenerate
– no He flash!
– Lose a lot of mass
High-mass stars make the
elements necessary for life!
Big Bang made 90% H, 10% He – stars make everything else
Helium fusion can make only carbon in low-mass stars
Helium Capture occurs only in
high-mass stars
•
High core temperatures allow helium to
fuse with heavier elements
Helium capture builds C into O, Ne, Mg, …
Total # of P+N = Multiples of 4! Why?
Evidence for
helium
capture:
Higher
abundances of
elements with
even numbers
of protons
Advanced Nuclear Burning
•
Core temperatures in stars with >8MSun
allow fusion of elements up to iron
Si, S, Ca, Fe, etc. can only be made in high-mass stars
Multiple Shell Burning
• Advanced nuclear
burning proceeds in
a series of nested
shells
Fusion releases energy only when the mass of the products
< mass of the reactants
• Iron is “ash” of fusion: nuclear reactions
involving iron do not release energy
• Iron-56 has lowest mass per nuclear
particle
• Highest “binding energy” of all the
elements
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TIFF (Uncompressed) decompressor
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How does a high-mass star die?
Iron builds up in core
until degeneracy
pressure can no longer
resist gravity
Supernova Explosion
• Core degeneracy
pressure cannot
support degenerate
core of > 1.4 Msun
• electrons forced into
nucleus, combine
with protons
• making neutrons,
neutrinos and LOTS
of energy!
Collapse only takes very short amount of time
(~seconds)
Supernova!
Energy and neutrons released in supernova explosion cause elements
heavier than iron to form, including Au and U
Neutron Stars & Supernova Remnants
• Energy released by
collapse of core
drives outer layers
into space
• The Crab Nebula is
the remnant of the
supernova seen in
A.D. 1054
Supernova 1987A
•
The first visible supernova in 400 years