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.