neutral hydrogen (H I or H0)

Un-ionized atomic hydrogen gas. It is an important component of the interstellar medium, accounting for perhaps half its mass, even though its density - typically 50 atoms per cubic centimetre - is very low. The temperature of neutral hydrogen ranges between 25 and 250 K, which is too cold for it to emit visible radiation. However, its radio emission at a wavelength of 21 centimetres has made it possible to map the distribution of neutral hydrogen in the spiral arms of our own Galaxy and other nearby galaxies.