Channel 3 and 4 Flashcards
What kind of reaction is fusion of light nuclei to make heavier ones?
Exothermic
Where does energy come from?
Mass difference
What does higher binding energy being released mean?
The nucleus is stable and formed more tightly bound
What is B.E/nucleon?
A measure of the fraction of mass converted to energy
What must the fusion of two nuclei have to overcome?
The Coulomb barrier with enough energy
What does quantum tunnelling allow?
A finite chance of nuclei fusing even when KE
When does the probability of a nucleus penetrating the Coulomb barrier increase?
It increases with increasing nuclei velocity
How does high velocity affect the number of nuclei with necessary high velocity?
It decreases with V (Maxwellian)
Why is the straight forward fusion of two protons difficult?
There isn’t a stable nucleus which contains two protons and no neutrons
What does one proton decay to?
A neutron emitting a positron and a neutrino via weak interaction whilst the two nuclei are still close enough to react (this would lead to a Deuterium nucleus)
What leads to the formation of a helium nucleus?
Further reactions with protons which lead to heavier isotopes
At which temperatures can proton-proton chain occur at?
At low temperatures around 4x10^6 K
How long does it take a proton to fuse and end up in a helium nucleus?
10^10 years
What is the CNO cycle?
A process of stellar nucleosynthesis in which stars on the Main Sequence fuse hydrogen into helium via a six-stage sequence of reactions
What kind of temperatures does the CNO need?
High temperature of around 10^7K
What are C, N,O nuclei?
They are catalysts and the reactions are fast
What stays constant which their isotope ratios change?
C+N+O=contstant
What do stars need to have to begin with?
C and N
Which element does the CNO process begin and end with?
C^12
however in the meantime the isotopes destroyed by slow reactions are enhanced compared to others e.g. 13^C and 14^N (have higher relative fractions)
What kind of stars demonstrate undergoing the CNO cycle?
Often evolved stars
What do these evolved stars show?
They have isotopes that are enhanced by exactly the values expected for the CNO cycle
What is the triple a process?
The conversion of 3 4^He nuclei (alpha particles) to 12^C
What kind of temperature is needed for triple a process?
High temperatures of around 10^8K
Why is there a small number of Be^8 nuclei in a triple a process?
The reaction is endothermic and therefore the nucleus is very unstable.
Why is the triple a process a 3 body collision?
As the reaction requires the 3 alpha particles to reach almost instantaneously due to the Be^8 nucleus decaying quickly so the third He^4 nucleus had to react quickly
What mechanism is the most efficient at low temperatures?
Proton-proton chain but it only generates small amounts of energy.
Which mechanism is the most important for the Sun?
PP chain rather than the CNO cycle
How is the Coulomb barrier overcome?
By stellar nuclear fusion using quantum tunnelling
What dictates the reaction route?
Temperature
What is intensity independent of?
Distance as once inside the beam, radiation stays within it
What is specific intensity (Iv)?
Energy crossing unit area and entering a unit solid angle per unit time per unit frequency interval (Jm^-2Hz^-1st^-1)
What is the total intensity (I)?
The specific intensity integrated over all frequencies
What is flux of radiation?
The amount of energy crossing a unit area per unit time per unit frequency interval (Jm^-2s^-1Hz^-1)
What does flux change with?
Distance
Where is the total power output (=luminosity) of an object diluted over?
A sphere with area that is 4pi , the distance from the object squared, 4pid^2
Which processes change the intensity?
Absorption, scattering, emission (which are all extinction processes)
What is opacity?
Reduction in intensity per unit path length (m^-1)
(sometimes known as the linear absorption coefficient
What is emissivity (jv)?
Amount of energy emitted per unit volume per unit time per unit frequency interval into unit solid angle (Jm^-3s^-1Hz^-1st^-1)
What kind of emissions does emissivity include?
spontaneous emission (independent of Iv) and stimulated emission (depends on Iv)
What are interactions with matter described by?
Opacity and emissivity
What is CTE?
Complete Thermodynamic Equilibrium which is the net outward energy flow
Stars are not in CTE
What is LTE?
Local Thermodynamic Equilibrium which holds in stellar interiors (not in atmospheres)
Radiation field in any small region is blackbody
What is deltaF/F?
A measure of degree of the deviation from thermal equilibrium.
What is the net outward flux and why?
It is the total luminosity of the star because most energy is generated in the inner part of the star
What kind of value indicates LTE?
A small value e.g in the inner parts of a star (deltaF/F is small)
At stellar surface why is there no LTE?
Temperatures are close to surface temperature so deltaF/F tends to 1 and no LTE
How is luminosity through a star determined?
Primarily by a temperature gradient
When is luminosity higher?
If the temperature is high and the opacity is low
