Hydrogen line

The HI- line (H -one - line), and hydrogen line, in astronomy the term for the characteristic radio emission of neutral hydrogen. The term also used 21- cm line originates from the corresponding vacuum wavelength ago. In radio astronomy, this radiation plays an important role because their study provides information on the density distribution, velocity and temperature of hydrogen atoms in the universe.

Formation

The emission or absorption line is produced by the hyperfine transition of neutral hydrogen atom. This is the energy difference between the parallel and antiparallel spin orientation of the electron relative to the spin of the proton. The energy difference is about 5.9 · 10-6 eV, corresponding to a radio frequency of 1420,405.751.77 MHz and a wavelength of about 21 cm.

Importance

Since the HI- line is only slightly attenuated by interstellar matter due to the very low probability of transition, it is a preferred object of observation of radio astronomy. It can be used to determine distribution of the hydrogen, which makes up a total of about 90 percent of the interstellar matter. Doppler shifts of the line provide information on the movements of astronomical objects.

1944 Hendrik Christoffel van de Hulst calculated the 21 cm line. The importance of HI- line recognized the astronomers Colin Stanley Gum, Frank John Kerr and Gart Westerhout in 1951.

The Pioneer plaque, attached to the space probe Pioneer 10 and Pioneer 11, shows the hyperfine transition of neutral hydrogen atom. The wavelength was used as a standard unit of length and the period as the standard unit time.