The disk of our galaxy is surrounded by clouds of un-ionized or H I hydrogen. These clouds are very cold (their average temperature is around 100 degrees Kelvin), and they are often quite large (up to 100 light years in diameter). Astronomers believe H I gas is an indicator of star formation; it is thought that stars form when the gas collapses under the force of gravity. H I gas is concentrated in the Milky WayΓÇÖs galactic disk, so it seems likely that stellar nurseries are located there. (In this false color image, red and yellow represent stronger concen- trations of H I gas, blue and black weaker.)
