These clouds are as massive as galaxies, but unlike galaxies have no luminosity, so their spectra cannot be recorded directly.
Astronomers study these hydrogen clouds indirectly using quasars. Quasars are very remote sources of radiation. When the light emitted by a quasar passes through a massive hydrogen cloud before reaching Earth, the hydrogen cloud absorbs certain wavelengths of light, producing a spectrum that can be studied.
Astronomers believe that massive hydrogen clouds are very young progenitors of various types of galaxies, which show a picture of early star formation and chemical evolution.
Both the Kast and Hamilton spectrographs are used in this research. The Kast is used to identify quasars beyond massive hydrogen clouds. The Hamilton is used to study absorption lines in the quasars' spectra.