An Etymological Dictionary of Astronomy and Astrophysics

Non-ionized → atomic hydrogen gas which constitutes an important component of the → interstellar medium, accounting for perhaps half its mass, even though its density is very low. Its radio emission
→ 21-centimeter line has made it possible to map the distribution of neutral hydrogen in the → spiral arms of our own Galaxy and other nearby galaxies.

See also: → neutral; → hydrogen.