A region of interstellar hydrogen that is in the form of neutral, unionized atoms. The notation H I refers to the fact that the hydrogen atoms (H) are not ionized (H II is ionized hydrogen). Each neutral hydrogen atom contributes just one particle to the gas. The density of H I regions is too low for hydrogen molecules to form, and starlight would dissociate any molecules that did, so the gas remains as atoms. Neutral hydrogen constitutes about half of all interstellar matter by mass and by volume, with an average density of 1 atom/cm3. H I regions are cool (about 100 K) and emit no visible light, but they do give out the important radio spectral line at 21 cm wavelength.
Subjects: Astronomy and Astrophysics.