The approximate mass of dust in the Galaxy can be computed from the observed extinction of starlight. Assuming the mean radius of dust grains to be R with a uniform number density n grains/cm³,

(a) show that the mean free path d0 of a photon in interstellar dust is given by d₀ = 1/ (nπR²). 

(b) Starlight traveling toward an Earth observer a distance d from the star has intensity

In the vicinity of the Sun a measurement of I yields d₀ = 3000 c . y. If R = 10^-5 cm, calculate
n. 

(c) The average mass density of solid material in the Galaxy is 2 g/cm³ and in the disk the density of stars is about 1 M_Θ /300 (c.y)³. Compute the ratio of the mass density of dust to the mass density of stars, assuming 1 M_Θ in 300 (c . y)³.

Transcribed Image Text:

I = [e-d/d Ie