Channel 3 and 4

Question 1

What kind of reaction is fusion of light nuclei to make heavier ones?

Answer 1

Exothermic

Question 2

Where does energy come from?

Answer 2

Mass difference

Question 3

What does higher binding energy being released mean?

Answer 3

The nucleus is stable and formed more tightly bound

Question 4

What is B.E/nucleon?

Answer 4

A measure of the fraction of mass converted to energy

Question 5

What must the fusion of two nuclei have to overcome?

Answer 5

The Coulomb barrier with enough energy

Question 6

What does quantum tunnelling allow?

Answer 6

A finite chance of nuclei fusing even when KE

Question 7

When does the probability of a nucleus penetrating the Coulomb barrier increase?

Answer 7

It increases with increasing nuclei velocity

Question 8

How does high velocity affect the number of nuclei with necessary high velocity?

Answer 8

It decreases with V (Maxwellian)

Question 9

Why is the straight forward fusion of two protons difficult?

Answer 9

There isn’t a stable nucleus which contains two protons and no neutrons

Question 10

What does one proton decay to?

Answer 10

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)

Question 11

What leads to the formation of a helium nucleus?

Answer 11

Further reactions with protons which lead to heavier isotopes

Question 12

At which temperatures can proton-proton chain occur at?

Answer 12

At low temperatures around 4x10^6 K

Question 13

How long does it take a proton to fuse and end up in a helium nucleus?

Answer 13

10^10 years

Question 14

What is the CNO cycle?

Answer 14

A process of stellar nucleosynthesis in which stars on the Main Sequence fuse hydrogen into helium via a six-stage sequence of reactions

Question 15

What kind of temperatures does the CNO need?

Answer 15

High temperature of around 10^7K

Question 16

What are C, N,O nuclei?

Answer 16

They are catalysts and the reactions are fast

Question 17

What stays constant which their isotope ratios change?

Answer 17

C+N+O=contstant

Question 18

What do stars need to have to begin with?

Answer 18

C and N

Question 19

Which element does the CNO process begin and end with?

Answer 19

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)

Question 20

What kind of stars demonstrate undergoing the CNO cycle?

Answer 20

Often evolved stars

Question 21

What do these evolved stars show?

Answer 21

They have isotopes that are enhanced by exactly the values expected for the CNO cycle

Question 22

What is the triple a process?

Answer 22

The conversion of 3 4^He nuclei (alpha particles) to 12^C

Question 23

What kind of temperature is needed for triple a process?

Answer 23

High temperatures of around 10^8K

Question 24

Why is there a small number of Be^8 nuclei in a triple a process?

Answer 24

The reaction is endothermic and therefore the nucleus is very unstable.

Question 25

Why is the triple a process a 3 body collision?

Answer 25

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

Question 26

What mechanism is the most efficient at low temperatures?

Answer 26

Proton-proton chain but it only generates small amounts of energy.

Question 27

Which mechanism is the most important for the Sun?

Answer 27

PP chain rather than the CNO cycle

Question 28

How is the Coulomb barrier overcome?

Answer 28

By stellar nuclear fusion using quantum tunnelling

Question 29

What dictates the reaction route?

Answer 29

Temperature

Question 30

What is intensity independent of?

Answer 30

Distance as once inside the beam, radiation stays within it

Question 31

What is specific intensity (Iv)?

Answer 31

Energy crossing unit area and entering a unit solid angle per unit time per unit frequency interval (Jm^-2Hz^-1st^-1)

Question 32

What is the total intensity (I)?

Answer 32

The specific intensity integrated over all frequencies

Question 33

What is flux of radiation?

Answer 33

The amount of energy crossing a unit area per unit time per unit frequency interval (Jm^-2s^-1Hz^-1)

Question 34

What does flux change with?

Answer 34

Distance

Question 35

Where is the total power output (=luminosity) of an object diluted over?

Answer 35

A sphere with area that is 4pi , the distance from the object squared, 4pid^2

Question 36

Which processes change the intensity?

Answer 36

Absorption, scattering, emission (which are all extinction processes)

Question 37

What is opacity?

Answer 37

Reduction in intensity per unit path length (m^-1) (sometimes known as the linear absorption coefficient

Question 38

What is emissivity (jv)?

Answer 38

Amount of energy emitted per unit volume per unit time per unit frequency interval into unit solid angle (Jm^-3s^-1Hz^-1st^-1)

Question 39

What kind of emissions does emissivity include?

Answer 39

spontaneous emission (independent of Iv) and stimulated emission (depends on Iv)

Question 40

What are interactions with matter described by?

Answer 40

Opacity and emissivity

Question 41

What is CTE?

Answer 41

Complete Thermodynamic Equilibrium which is the net outward energy flow Stars are not in CTE

Question 42

What is LTE?

Answer 42

Local Thermodynamic Equilibrium which holds in stellar interiors (not in atmospheres) Radiation field in any small region is blackbody

Question 43

What is deltaF/F?

Answer 43

A measure of degree of the deviation from thermal equilibrium.

Question 44

What is the net outward flux and why?

Answer 44

It is the total luminosity of the star because most energy is generated in the inner part of the star

Question 45

What kind of value indicates LTE?

Answer 45

A small value e.g in the inner parts of a star (deltaF/F is small)

Question 46

At stellar surface why is there no LTE?

Answer 46

Temperatures are close to surface temperature so deltaF/F tends to 1 and no LTE

Question 47

How is luminosity through a star determined?

Answer 47

Primarily by a temperature gradient

Question 48

When is luminosity higher?

Answer 48

If the temperature is high and the opacity is low