Interstellar Medium, 21 cm line

1. Oct 27, 2009

mebwtm

Somewhere in our Galaxy resides a cloud of neutral hydrogen gas with a radius of 10 pc. The gas density is 10^7 atoms/m^3.
(a)How many 21-cm photons does the cloud emit every second?
(b)If the cloud is 100 pc from the Sun, what is the energy flux of this radiation (in W/m^2) at the Sun?

Relevant Equations:
possibly Em= integral(n^2)dl=(<n>^2)L
Aligned-to-opposed ratio of hydrogen line at 21 cm 3:1
frequency=1.4204 GHz

The attempt at a solution:
So far I've done the integral for the emission measure equation by changing n into
10 atoms/cm^3
L is the diameter so it's 20 pc
From this I found Em=2000pc/cm^6

I can't figure out how to get from here to photons/second for part a

For part b i know Flux is proportional to frequency^(-alpha), but I'm not sure proportionality will help me. I know the ratio is suppose to, but again I cannot connect the two.

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