Measuring the chemical composition of a gas cloud

This diagram illustrates how astronomers can analyse the chemical composition of distant clouds of gas using the light of a background object like a quasar as a beacon.  

When the light of the quasar passes through the gas cloud, the chemical elements in it absorb different colours or wavelengths, leaving dark lines in the spectrum of the quasar. Each element leaves a different set of lines, so by studying the spectrum astronomers can work out the chemical composition of the intervening gas cloud.