neutral atomic hydrogen

Neutral atomic hydrogen in astrophysics is indicated by the abbreviation HI or H I (pronounced "H one"). The abbreviation HI is used in some indications of neutral hydrogen spectral lines such as the 21-cm line, e.g., [HI]. HI consists of a gas of independent hydrogen atoms, (i.e., not joined as molecules of two hydrogen atoms) that each include a single orbiting electron. In space, with sufficient density and sufficiently low temperature, (generally less than 50 K) they can combine to form such molecules. Regions of neutral hydrogen are generally below 100 K but near early stars, may be thousands. The 21-cm line is used to detect neutral atomic hydrogen at a distance, to detect HI regions and galaxies that include them.