21-centimeter line

The 21-centimeter line is radio emission or absorption produced by a hyperfine transition in neutral hydrogen. Its frequency is about 1420.4 megahertz, making it a probe of hydrogen gas across the universe.

Hyperfine structure: Small energy-level splittings caused by interactions between nuclear and electronic magnetic moments. The line is a specific hyperfine splitting in the hydrogen ground state.
Radio astronomy: The study of celestial objects through the radio waves they emit or absorb. The 21-centimeter line is one of radio astronomy’s foundational spectral signals.
Hendrik van de Hulst: Dutch astronomer who predicted the 21-centimeter hydrogen line in 1944. His calculation identified a radio signal that could reveal interstellar hydrogen.
H-alpha: A visible hydrogen spectral line produced when an electron falls from the third to the second energy level. Unlike H-alpha, the 21-centimeter line traces neutral rather than ionized hydrogen.
Hydrogen atom: The simplest atom, consisting of one proton and one electron. Its ground-state spin states produce the transition that emits or absorbs this radio line.
Neutral hydrogen cloud: An interstellar or intergalactic region where hydrogen atoms are predominantly un-ionized. These clouds emit and absorb the line that makes their distribution measurable.
Harold Ewen: American physicist who made the first astronomical detection of the 21-centimeter line in 1951. His detection confirmed the predicted signal from neutral hydrogen.
Lyman-alpha: An ultraviolet hydrogen line produced by a transition between the first and second energy levels. Its strong absorption contrasts with the weak radio transition used to probe diffuse gas.
Spin temperature: A temperature parameter describing the relative populations of atomic hydrogen’s two hyperfine states. It determines whether hydrogen gas appears in emission or absorption against background radiation.
21-centimeter hydrogen line survey: A radio survey that maps neutral hydrogen through its 21-centimeter spectral emission. Surveys use the line to chart gas across the Milky Way and other galaxies.

Linked from 18 pages