Jeans Mass

Question 1

State the Virial Equation

Answer 1

2U + Ω = 0

Question 2

When the molecular cloud is stable how can the virial eqn be described?

Answer 2

2U = -Ω

Question 3

When the cloud collapses how can the virial eqn be described?

Answer 3

2U > -Ω

pressure wins

Question 4

When the cloud contracts how can the virial eqn be described?

Answer 4

2U < -Ω

gravity wins

Question 5

what simplifications are giving collapse criterion?

Answer 5

• constant density
• constant pressur to Rc
• External pressure (Ps) = 0

Question 6

State short version of Jeans Mass with values

Answer 6

Mj = (10^5 * T^1.5) / (μ^2 * n^1.5) in solar mass

Question 7

typical temperature of core of molecular cloud

Answer 7

T = 10K

Question 8

typical number density of core of molecular cloud

Answer 8

n = 10^10 m^-3

Question 9

typical mean molecular weight of core of molecular cloud

Answer 9

μ = 2.5

Question 10

where does star formation occur?

Answer 10

cold dense parts in the sky, molecular clouds

Question 11

What happens in the collapse of a spherical cloud?

Answer 11

• M constant but ρ increases
• initially cloud isothermal – GPE released would heat up cloud
• increasing force from gravity/pressure leads to further collapses

Question 12

Equation of free fall time

Answer 12

tff = SQRT(3/2piG*ρ)

Question 13

tff for 5Mo star

Answer 13

3x10^5 yr