Nucleosynthesis I Flashcards
what observations aided our understanding of how chemical elements form?
- isotopic and elemental abundances in solar system from sun and meteorites
- element abundances in different stars by spectroscopy
- presolar grains with exotic isotopic compositions that reflect formation in the outflows of different former stars
- study of nuclear reactions possible / reasonable at different inferred sites of nucleosynthesis
what are the 4 main phases/sites of nucleosynthesis?
- big bang nucleosynthesis
- stellar nucleosynthesis - by fusion and neutron capture in the interior of stars
- explosive nucleosynthesis in supernovae
- nucleosynthesis by galactic cosmic rays
which elements are formed in big bang nucleosynthesis?
primarily (>99%) 1H and 4He with minor 7Li
how many years after the big bang did H and He combine with electrons to form transparent neutral atoms (photons of which released the CMBR)
375,000years after the big bang
what is the first step of stellar nucleosynthesis? what are the reactants and products?
Hydrogen burning through the P-P 1 chain reaction. this fuses protons to produces 1H and 4He.
what is the CNO cycle?
The CNO cycle uses 12C as a catalyst to fuse protons into He. It requires carbon but allows more massive stars to fuse hydrogen much faster than through the P-P 1 chain reaction, enabling them to achieve hydrostatic equilibrium - explaining why more massive stars fuse their hydrogen much faster.
why are no elements heavier than 4 produced through hydrogen burning?
there are no stable nuclides with an atomic mass number of 5
what is helium burning? what are the reactants and products?
4He + 4He + 4He -> 12C
12C + 4He -> 16O
produces carbon and oxygen
what is carbon burning? what are the reactants and products? what solar mass is required to initiate carbon burning?
> 8 solar masses required to undergo further collapse to reach high density and temperatures.
12C + 12C -> 24Mg*
The Mg* is excited and decays to 20Ne and alpha particles (4He), but also forms some Na and Mg
what is neon burning? what are the reactants and products? what solar mass is required to initiate neon burning?
> 10 solar masses
Two sequential reactions
main producer of 24Mg
what is oxygen burning?
16O + 16O -> 32S*
produces 28Si and 32S and 31P
what is silicon burning?
last phase of nuclear fusion in star interior.
high positive charges of Si and S prevent direct interaction. high energy photons induce disintegration of Si and S into lighter species which are captured by neutrons (neutron-capture) to produce the Fe peak elements (Fe, Ni).
what is the heaviest element formed by Si burning in the core of stars?
56Fe
produced from the decay of 56Ni.
which elements have the highest nuclear binding energy?
the Fe-peak elements
why are Li, Be and B bypassed by stellar nucleosynthesis?
they have low binding energies and are not stable at the temperature of stellar cores.
