The origin of the (lighter) elements
The Late Stages of Stellar Evolution
Supernova of 1604 (Kepler’s)

High-Mass Stars
> 8 M
Sun
Intermediate-
Mass Stars
Low-Mass Stars
< 2 M
Sun
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 M sun
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:
Slow wind from a red giant blows away cool, outer layers of the star
The Ring Nebula in Lyra
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 M sun
•
Radius ~ 0.65 R sun

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 >8 M
Sun 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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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 M sun
• 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