How to find the amount of gas in a galaxy

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 10¹⁸ x Distance² 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 10³³ 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 10²⁵ cm

If you counted 150 boxes within the spectral line, then Number of Hydrogen atoms = 5.4 x 10⁶⁶ atoms

This is a mass of 1.1 x 10⁴³ grams

Which is equivalent to 5.4 x 10⁹ times the mass of the Sun