Download Stellar Alchemy: The Origin of the Chemical Elements Rick Norman

Stellar Alchemy:
The Origin of the
Chemical Elements
Rick Norman
Dept. of Nuclear Engineering
UC Berkeley
Atomic and Nuclear Stucture
• Atom - smallest unit of a chemical
element
– Size on the order of 10-8 cm (1 Angstrom)
– Contains Z electrons (Qe = -1e)
» and
– Nucleus –
–
–
–
–
–
–
–
Size on the order of 10-13 cm (1 femtometer )
Contains more than 99.9% of the mass of the atom
Made of Z protons and N neutrons
Proton (Qp = +1e)
Neutron (Qn = 0)
A = Atomic mass = Z + N
Held together by strong nuclear force
A
ZXN
where X = chemical symbol
A look inside an atom
Stars
Visible EM Spectrum
What is the universe made of ?
• Hydrogen = 75%, by mass
• Helium = 23 %
• Everything Else = 2%
Hertzsprung-Russell Diagram
Our Sun
Mass = 2 x 1033 grams
Luminosity = 4 x 1033 ergs/second
Age = 4.5 x 109 years
Over the life of the Sun, it has
produced a total of 6 x 1050 ergs
or 3 x 1017 ergs/gram
Where does this energy come from?
Chemical Reactions ?
Maximum energy
release
= 2 x 1012 ergs/gram
Thus, chemical
reactions could power
the Sun for only 30,000
years
Gravitational Contraction ?
V = 3/5 (GM2/R) = 2 x1048 ergs
→ 1015 ergs/gram
No where near enough energy!
Nuclear Reactions Can Produce
Large Amounts of Energy
Consider combining 4 hydrogen nuclei
to make one 4He nucleus:
4 M(1H) > M(4He)
But, E = mc2
Thus, for each 4He produced,
4.2 x10-5 ergs are released
This represents 6.3 x 1018 ergs/gram
Therefore over the life of the Sun, only 5%
of the hydrogen needs to be “burned’ into
helium to account for its energy output
The Sun shines by nuclear fusion reactions!
Solar neutrino flux at Earth
Pioneers in Solar Neutrino Science
Ray Davis
Nobel Prize
2002
37Cl
+ νe →
1968 First Solar Neutrino Telescope
37Ar
+ e-
SuperKamiokande
Cherenkov
Nobel prize
1958
Cherenkov Radiation
SuperKamiokande Solar Neutrino Results
Neutrino Image
of Sun
Sudbury Neutrino
Observatory
Why did SNO use $300M worth of heavy water?
νe + d p +p +e-
Charged Current
Neutral Current
νx + d p +n +ν
νx’
Evidence
for νe νµτ
Fluxes
(106 cm-2 s-1)
ν e:
1.76(11)
νµτ:
3.41(66)
νtotal:
5.09(64)
νSSM:
5.05
Helium Burning in Red-Giant Stars
Burbidge, Burbidge, Fowler, Hoyle
and then,
12C
+ 4He 16O + γ
B2FH (1957)
Fate of Stars
For stars with masses < 10 MSun
No further nuclear reactions possible
White Dwarf (maximum mass = 1.4 MSun)
For more massive stars,
No such quiet fate possible
Neutron Star or Black Hole
Advanced stellar burning
12C
+ 12C 24Mg + γ,
16O + 16O 32S + γ,
20Ne
+ 4He
28Si + 4He
Then through successive captures of 4He,
28Si 56Ni
At this point the star is on its deathbed,
No further energy generation possible
Late stage massive star
Burning
Stage
Temperature
(keV)
Density
(g/cm3)
Timescale
Hydrogen
5
5
7 x 106 years
Helium
20
700
5 x 105 years
Carbon
80
2 x 105
600 years
Neon
150
4 x 106
1 year
Oxygen
200
1 x 107
6 months
Silicon
350
3 x 107
1 day
Collapse
600
3 x 109
seconds
Bounce
3000
1014
milliseconds
Explosion
100 - 600
varies
0.1 – 10
seconds
Supernova Explosion
Temperature goes up
Density goes up
p + e- n + ν e
e+ +e- ν + ν
99% of SN energy
comes off in neutrinos
SN 1987a
Neutrinos from SN1987a
observed by Kamiokande and IMB
underground telescopes
Compton Gamma-Ray Observatory
56Co
gamma rays observed from SN1987a
β +/EC
56Co
t1/2 = 77 days
847
keV
56Fe
stable
44Ti
gamma rays observed from CasA
Origin of Heavy elements
Neutron capture reactions
slow (s) process
produces half of nuclei from 56Fe 209Bi
occurs during He-burning in red giant stars
rapid (r) process
produces other half of nuclei heavier than
56Fe plus Th and U
occurs in supernovae ?
s and r processes
r-process path
Source of stellar energies
Nuclear reactions in stars
Origin of chemical elements
I believe a leaf of grass is no less than the
journeywork of the stars