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Stellar Structure and Evolution > Channel 3 and 4 > Flashcards

Channel 3 and 4 Flashcards

1
Q

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

A

Exothermic

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2
Q

Where does energy come from?

A

Mass difference

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3
Q

What does higher binding energy being released mean?

A

The nucleus is stable and formed more tightly bound

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4
Q

What is B.E/nucleon?

A

A measure of the fraction of mass converted to energy

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5
Q

What must the fusion of two nuclei have to overcome?

A

The Coulomb barrier with enough energy

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6
Q

What does quantum tunnelling allow?

A

A finite chance of nuclei fusing even when KE

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7
Q

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

A

It increases with increasing nuclei velocity

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8
Q

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

A

It decreases with V (Maxwellian)

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9
Q

Why is the straight forward fusion of two protons difficult?

A

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

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10
Q

What does one proton decay to?

A

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)

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11
Q

What leads to the formation of a helium nucleus?

A

Further reactions with protons which lead to heavier isotopes

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12
Q

At which temperatures can proton-proton chain occur at?

A

At low temperatures around 4x10^6 K

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13
Q

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

A

10^10 years

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14
Q

What is the CNO cycle?

A

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

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15
Q

What kind of temperatures does the CNO need?

A

High temperature of around 10^7K

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16
Q

What are C, N,O nuclei?

A

They are catalysts and the reactions are fast

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17
Q

What stays constant which their isotope ratios change?

A

C+N+O=contstant

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18
Q

What do stars need to have to begin with?

A

C and N

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19
Q

Which element does the CNO process begin and end with?

A

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)

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20
Q

What kind of stars demonstrate undergoing the CNO cycle?

A

Often evolved stars

21
Q

What do these evolved stars show?

A

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

22
Q

What is the triple a process?

A

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

23
Q

What kind of temperature is needed for triple a process?

A

High temperatures of around 10^8K

24
Q

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

A

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

25
Q

Why is the triple a process a 3 body collision?

A

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

26
Q

What mechanism is the most efficient at low temperatures?

A

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

27
Q

Which mechanism is the most important for the Sun?

A

PP chain rather than the CNO cycle

28
Q

How is the Coulomb barrier overcome?

A

By stellar nuclear fusion using quantum tunnelling

29
Q

What dictates the reaction route?

A

Temperature

30
Q

What is intensity independent of?

A

Distance as once inside the beam, radiation stays within it

31
Q

What is specific intensity (Iv)?

A

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

32
Q

What is the total intensity (I)?

A

The specific intensity integrated over all frequencies

33
Q

What is flux of radiation?

A

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

34
Q

What does flux change with?

A

Distance

35
Q

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

A

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

36
Q

Which processes change the intensity?

A

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

37
Q

What is opacity?

A

Reduction in intensity per unit path length (m^-1)

(sometimes known as the linear absorption coefficient

38
Q

What is emissivity (jv)?

A

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

39
Q

What kind of emissions does emissivity include?

A

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

40
Q

What are interactions with matter described by?

A

Opacity and emissivity

41
Q

What is CTE?

A

Complete Thermodynamic Equilibrium which is the net outward energy flow

Stars are not in CTE

42
Q

What is LTE?

A

Local Thermodynamic Equilibrium which holds in stellar interiors (not in atmospheres)

Radiation field in any small region is blackbody

43
Q

What is deltaF/F?

A

A measure of degree of the deviation from thermal equilibrium.

44
Q

What is the net outward flux and why?

A

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

45
Q

What kind of value indicates LTE?

A

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

46
Q

At stellar surface why is there no LTE?

A

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

47
Q

How is luminosity through a star determined?

A

Primarily by a temperature gradient

48
Q

When is luminosity higher?

A

If the temperature is high and the opacity is low

Stellar Structure and Evolution (8 decks)

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