If every hydrogen gas atom in a galaxy gives off a certain amount of radio light, then the total intensity of the hydrogen spectral line is proportional to the amount of hydrogen gas:

Number of Hydrogen Atoms = 3.3 x 10

^{18}x Distance^{2}x Number of boxes within the spectral line

Since the mass of a Hydrogen atom = 2 x 10^{-24}
grams:

Mass of Hydrogen gas in a galaxy (in grams) = 2 x 10

^{-24}x Number of Hydrogen Atoms

Since our Sun has a mass of 2 x 10^{33}
grams, one can use the mass of hydrogen to compare how many Sun's worth of
hydrogen gas there is in a galaxy.

Example:

From previous exercise: Distance = 3.3 x 10

^{25}cm

If you counted 150 boxes within the spectral line, then Number of Hydrogen atoms = 5.4 x 10^{66}atoms

This is a mass of 1.1 x 10^{43}gramsWhich is equivalent to 5.4 x 10

^{9}times the mass of the Sun