Hydrogen Line

The hydrogen line refers to the spectral line created by changes in the energy state of neutral hydrogen and occurs at 1420.40575 MHz, or a wavelength of around 21 cm. The line is used extensively in astronomy, particularly radio astronomy, as the line falls well within the radio spectrum.

Cause

Neutral hydrogen consists of a single proton orbited by a single electron. The proton and electron are in constant motion, they have orbital angular momentum nucleus and also spin (classically it is analogous to Earth's motion). The electron and proton can spin around their axis in either direction, and are either spinning in the same direction, or the opposite direction. A Hydrogen atom that has an electron and proton spinning in the same direction (parallel) has slightly more energy than one where the electron and proton are spinning in opposite directions (anti-parallel). The lowest orbital energy state of atomic hydrogen has hyperfine splitting arising from the spins of the proton and electron changing from a parallel to antiparallel configuration. This transition is highly forbidden with an extremely small probability of 2.9×10−15 s−1. This means that the time for a single atom of neutral hydrogen to undergo this transition is around 10 million (107) years and so is unlikely to be seen in a laboratory on Earth. However, as the total number of atoms of neutral hydrogen in the interstellar medium is very large, this emission line is easily observed by radio telescopes.

The line has an extremely small natural width because of its long lifetime, so most broadening is due to doppler shifts caused by the motion of the emitting regions relative to the observer.