Tutorials Flashcards
Why are l=0 reactions enhanced at stellar burning temperatures?
Stellar temperatures typically correspond to low interaction energies where angular momentum penetrability is highly energy dependent, so l=0 reactions are more likely as they don’t have to overcome the angular momentum barrier.
What is the astrophysical S(E) factor?
Describes the nuclear part of the reaction cross section assuming l=0. It is used for charged particle reactions.
Why is the effective energy released in each of the pp chains different?
Some of the reactions produce neutrinos with different energies. Since neutrinos dominantly leave the star unaffected, their energy is lost and not available for heating the star, hence, the effective energy released in each of the pp chains is slightly different.
What is meant by a branching point in the s-process?
This is a nucleus for which beta decay competes with neutron capture, ie. the beta decay rate is similar to the neutron capture rate.
Direct Evidence for Nucleosynthesis in Stars
*observation of radioactive technetium in stellar spectra
* detection of neutrinos from the Sun and from SN1987A
* discovery of g-rays from radioactive 26Al in interstellar
medium
* light curve decline in SN governed by 56Co decay
* detection of g-rays from 56Co decay in SN1987A
* existence of two distinct stellar populations in our Galaxy
Star formation
- Gas cloud contracts under its gravity
- When density large enough, smaller parts contract independently –
ultimately cloud will fragment into many parts and condense to form
cluster of protostars - Minimum mass M for collapse of cloud with radius R (Jeans
criterion): Gravitational Energy > thermal kinetic energy - Contraction of protostar at first free collapse (released energy used
to dissociate H2 and ionize H, not for thermal motion) - When most H ionized, star becomes opaque, i.e. gravitational energy
is converted into thermal energy - Approach of hydrostatic equilibrium
