Download ASTR 31: Descriptive Astronomy

Lesson 8: Star Formation, Stellar Evolution,
and Stellar Explosions
Summary of Evolutionary Endpoints
Mass
< 12 Jupiter masses
12 – 80 Jupiter masses (0.08 solar masses)
0.08 – 0.25 solar masses
0.25 – 8 solar masses
8 – ≈12 solar masses
> ≈12 solar masses
> ≈30 solar masses
Evolutionary Endpoint
Planet
Brown dwarf
Helium white dwarf
Carbon-oxygen white dwarf
Neon-oxygen white dwarf
Core-collapse supernova and neutron star
Core-collapse supernova and black hole
Summary of Stellar Nucleosynthesis
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proton: 1H
neutron: n
helium (2 protons + 2 neutrons): 4He
beryllium-8 (4 protons + 4 neutrons): 8Be
carbon-12 (6 protons + 6 neutrons): 12C
oxygen-16 (8 protons + 8 neutrons): 16O
neon-20 (10 protons + 10 neutrons): 20Ne
magnesium-24 (12 protons + 12 neutrons): 24Mg
silicon-28 (14 protons + 14 neutrons): 28Si
sulfer-32 (16 protons + 16 neutrons): 32S
argon-36 (18 protons + 18 neutrons): 36Ar
calcium-40 (20 protons + 20 neutrons): 40Ca
titanium-44 (22 protons + 22 neutrons): 44Ti
chromium-48 (24 protons + 24 neutrons): 48Cr
iron-52 (26 protons + 26 neutrons): 52Fe
iron-56 (26 protons + 30 neutrons): 56Fe
iron-57 (26 protons + 31 neutrons): 57Fe
iron-58 (26 protons + 32 neutrons): 58Fe
iron-59 (26 protons + 33 neutrons): 59Fe
cobalt-56 (27 protons + 29 neutrons): 56Co
cobalt-59 (27 protons + 32 neutrons): 59Co
cobalt-60 (27 protons + 33 neutrons): 60Co
nickel-56 (28 protons + 28 neutrons): 56Ni
nickel-60 (28 protons + 32 neutrons): 60Ni
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electron: epositron (antimatter electron): e+
electron neutrino: e
antimatter electron neutrino: <e>
photon (energy): 
Hydogen burning
 Proton-proton chain reaction:
 6(1H) → 4He + 2(1H) + 2e+ + 2e + 2
 CNO cycle:
 12C + 4(1H) → 12C + 4He + 2e+ + 2e + 3
 For a 20-solar mass star, this continues for about 10 million years.
Carbon formation
 Triple-alpha reaction:
 1. 4He + 4He → 8Be + 
 2. 8Be + 4He → 12C + 
 So ovarall:
 3(4He) → 12C + 2
 For a 20-solar mass star, this continues for about 1 million years.
 You are carbon based – this is where you come from!
Carbon burning
 Major reaction: 12C + 4He → 16O + 
 Minor reaction: 12C + 12C → 24Mg + 
 For a 20-solar mass star, this continues for about 1 thousand years.
 This is where the oxygen that you breath comes from.
Oxygen burning
 Major reaction: 16O + 4He → 20Ne + 
 Minor reaction: 16O + 16O → 32S + 
 For a 20-solar mass star, this continues for about 1 year.
 This is where the neon in neon signs come from – Viva Las Vegas!
Helium capture
 The triple-alpha reaction and the major carbon and oxygen burning reactions are
helium capture reactions. The capture of helium nuclei continues until silicon is
created, at which point the supply of helium nuclei in the star’s core is depleted:
 20Ne + 4He → 24Mg + 
 24Mg + 4He → 28Si + 
 For a 20-solar mass star, this continues for about 1 month.
 This is where the primary ingredient for silicone comes from.
Iron formation
 Alpha process:
 Fortunately, by the time that silicon is created the star’s core has grown
hot enough to make blackbody photons of high-enough energy to break up
some of the silicon. This is called photodisintegration:
 28Si +  → 7(4He)
 Consequently, helium capture can again proceed:
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 28Si + 4He → 32S + 
 32S + 4He → 36Ar + 
 36Ar + 4He → 40Ca + 
 40Ca + 4He → 44Ti + 
 44Ti + 4He → 48Cr + 
 48Cr + 4He → 52Fe + 
 52Fe + 4He → 56Ni + 
 For a 20-solar mass star, this continues for about 1 week.
 This is where the primary ingredients for gun powder, your bones,
Lieutenant Dan’s magic legs, chrome plating, etc., come from.
 Radioactive decay:
 56Ni has too many protons to be stable and radioactively decays:
 56Ni → 56Co + e+ + e + 
 56Co has too many protons to be stable and radioactively decays:
 56Co → 56Fe + e+ + e + 
 For a 20-solar mass star, this continues for about 1 day.
 This is where the primary ingredient for steel comes from.
Heavy element formation
 Slow neutron capture (s-process):
 56Fe + n → 57Fe
 57Fe + n → 58Fe
 58Fe + n → 59Fe
 59Fe has too many neutrons to be stable and radioactively decays:
 59Fe → 59Co + e- + <e> + 
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Co + n → 61Co
 61Co has too many neutrons to be stable and radioactively decays:
 61Co → 61Ni + e- + <e> + 
 This continues, making the rest of the elements through bismuth-209 (83
protons + 126 neutrons), including such famous elements as copper, silver,
lead, gold, etc.
 Rapid neutron capture (r-process):
 To make elements heavier than bismuth-209, neutron capture must
proceed more rapidly than it takes for the newly formed elements to
radioactively decay back to where they started. This is possible only for
about 15 minutes, during the supernova explosion itself. This is how such
famous radioactive elements as uranium and plutonium are produced.
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