These processes are studied theoretically, experimentally and observationally. Theories of cosmology, stellar evolution and interstellar processes are involved, as are laboratory investigations of nuclear and particle physics, cosmochemical studies of elemental and isotopic abundances in the Earth and meteorites and astronomical observations of the physical nature and chemical composition of stars, galaxies and the interstellar medium. Estimates of primordial abundances from the Big Bang, based on astrophysical and cosmochemical observations and arguments.
Galaxies thus have a mixture of stars and diffuse interstellar medium.
Most observed galaxies belong to a sequence first established in the 1920s and 30s by Edwin Hubble. The disks display a spiral structure, typical of spiral galaxies such as our own Milky Way system, and the proportion of cool gas relative to stars increases along the sequence, together with the relative rate of formation of new stars from the gas. The figure illustrates very schematically some possible interactions between galaxies and the intergalactic medium. Neutral gas is detected in disk and blue compact galaxies from the hyperfine 21 cm transition of neutral atomic hydrogen H I, but very few isolated intergalactic H I clouds have ever been detected. The figure also gives a schematic illustration of the complex interactions between the ISM and stars.