twenty-one-centimetre line

Characteristic radio emission or absorption at a wavelength of 21 centimetres by neutral hydrogen in interstellar space. Neutral hydrogen is a major component of the interstellar medium and the 21-centimetre line provides an important means of finding its distribution, density and velocity in our own Galaxy and thousands of others. It was the first spectral line to be detected by radio astronomy (in 1951) and is now the main means for exploring the structure of galaxies through radio observations. The small energy change in the hydrogen atom responsible for the 21-centimetre emission has an intrinsically low probability of occurring. An individual hydrogen atom excited into the higher energy level typically waits 12 million years to make the energy jump spontaneously. However, the radiation is observed from interstellar hydrogen because of the vast numbers of atoms it contains and because collisions trigger the transitions.