Question
a. Proxima Centauri is a main sequence star with a mass of 0.12 solar masses.[2]
Estimate $\frac{\text { lifetime on main sequence of Proxima Centauri }}{\text { lifetime on main sequence of Sun }}$.
b. Describe why iron is the heaviest element that can be produced by nuclear fusion processes inside stars.[2]
c. Discuss one process by which elements heavier than iron are formed in stars.[3]
▶️Answer/Explanation
Ans:
a. realization that lifetime $T \propto \frac{\text { mass }}{\text { luminosity }}$
$
\frac{\mathrm{T}}{\mathrm{T}_{\odot}}=\left(\frac{\mathrm{M}}{\mathrm{M}_{\odot}}\right)^{-2.5}=0.12^{-2.5}=200 \boldsymbol{\checkmark}
$
b. the binding energy per nucleon is a maximum for iron
formation of heavier elements than iron by fusion is not energetically possible
NOTE: For MP2 some reference to energy is needed
C. ALTERNATIVE $1-s$-process
s-process involves «slow» neutron capture
in s-process beta decay occurs before another neutron is captured
s-process occurs in giant stars «AGB stars»
s-process terminates at bismuth/lead/polonium
ALTERNATIVE 2 – r-process
r-process involves «rapid» neutron capture
in r-process further neutrons are captured before the beta decay occurs
r-process occurs in type II supernovae $\checkmark$
r-process can lead to elements heavier than bismuth/lead/polonium
NOTE: If the type of the process ( $r$ or s/rapid or slow) is not mentioned, award [2 max].
Question
a. Explain the formation of a type I a supernova which enables the star to be used as a standard candle.[3]
b. Describe the r process which occurs during type II supernovae nucleosynthesis.[2]
▶️Answer/Explanation
Ans:
a. white dwarf attracts mass from another star
explodes/becomes supernova when mass equals/exceeds the Chandrasekhar limit / 1.4M $\mathrm{M}_{\mathrm{SUN}}$
hence luminosity of all type I a supernova is the same
OWTTE
b. «successive» rapid neutron capture
faster than « $\beta$ » decay can occur
results in formation of heavier/neutron rich isotopes
OWTTE