Section 7

Question 1

When does fragmentation stop?

Answer 1

The collapsing core becomes so dense that it becomes opaque to its own radiation so it can no longer efficiently radiate energy released during collapse

Question 2

What happens when fragmentation stops?

Answer 2

The internal temperature and therefore pressure increases

Eventually an equilibrium is reached between internal pressure and gravity

Halts free fall collapse in centre

Question 3

What is formed as a result of the halt of free fall collapse?

Answer 3

Hydrostatic core

Question 4

What does a hydrostatic core cause to develop?

Answer 4

A shock front develops at interface between the core and the envelope

Infall velocity at shock = free fall velocity

Question 5

How is the free fall velocity derived?

Answer 5

Equating KE to GE

Question 6

What happens when the hydrostatic core heats up?

Answer 6

There is potential for further collapse

Question 7

What happens if the heat capacity ratio is greater than 4/3?

Answer 7

Jeans mass increases as T increases and core is STABLE

Question 8

What happens if the heat capacity ratio is less than 4/3?

Answer 8

Jeans mass decreases as T increases and core is UNSTABLE leading to further collapse

Question 9

What is γ?

Answer 9

The ratio of specific heat capacity at constant pressure and constant volume

f + 2/f

Question 10

What happens within the protostar as time goes on that might disrupt its stability?

Answer 10

There is a hydrostatic core that is accreting molecular material from a free-falling outer envelope

Temperature and density are both increasing

A point will be reached at which molecules and dust will dissociate

Question 11

What are degrees of freedom?

Answer 11

The threshold for a given type of particle in gas e.g atomic

Question 12

What are the degrees of freedom for monatomic gas?

Answer 12

f = 3 (translational)

Question 13

What are the degrees of freedom for diatomic molecules?

Answer 13

f = 5 (3 translational, 2 rotational)

Question 14

What happens when T = 2000K under high pressure conditions in the core?

Answer 14

Collisions lead to molecular hydrogen and dust becoming dissociated and atomised (into its constituent atoms) so f goes from 5 to 3

Question 15

What happens as a result of every dissociation of H_2 event in the core?

Answer 15

4.5 eV of energy is absorbed that goes into breaking that bond (removing energy from core and pressure, affecting the hydrostatic equilbrium)

Question 16

What does the dissociation of the gas from molecular to atomic hydrogen cause?

Answer 16

Energy from gas is removed and core becomes dynamically unstable and undergoes second collapse

Question 17

What is the size of the second core?

Answer 17

a few R_o (2R_o) and hydrostatic equilibrium is reached but accretion of mass continues

Question 18

What does the small mass of the secondary core mean?

Answer 18

It is not sufficient enough to increase the density to the point where nuclear fusion can start

Question 19

What happens to the density throughout?

Answer 19

Outer edges of core have same density as molecular cloud that collapsed (seen in graph)

There is a jump in density due to first shock front in first core

When molecular hydrogen dissociates, second phase of collapse causes density to increase again and leads to second shock front

Question 20

What are the conditions for nuclear fusion?

Answer 20

density = 100 gcm^-3
temperature = 1 million K

Question 21

What is the size of the first hydrostatic core?

Answer 21

5 AU