Knowra21-centimeter lineLinked fromLinked fromThe 18 pages that link to 21-centimeter line, each with the reason it gives.All 18Broader topic 5Related 12Compared with 1Spectral lineBroader topic: Its line maps neutral hydrogen across the Milky Way and other galaxies.Hydrogen atomBroader topic: This line maps hydrogen gas across galaxies and the early universe.Hyperfine structureBroader topic: This transition traces neutral hydrogen across interstellar and intergalactic space.Neutral hydrogenBroader topic: This is the characteristic signal produced by neutral hydrogen’s spin transition.Astronomical radio sourceBroader topic: It lets radio telescopes trace neutral hydrogen across interstellar and intergalactic space.