- Clouds of gas are a common component of galaxies including our own. They can become visible when they shine with light produced under the influence of intense radiation from nearby stars. Gas clouds can be observed by radio astronomers by the radiation they emit at a wavelength of 21cm. The doppler effects observed in 21cm radiation from gas clouds at different parts of the galaxy are a powerful means of determining the distribution of material in the galaxy and the orbital paths it pursues around the galactic centre. Gas clouds are classified into HI and HII types.