Mondo Knows Facts
Cold hydrogen betrays itself at 21 centimetres
A flip in the hydrogen atom's spin releases a faint radio wave at 1420 megahertz Neutral hydrogen is too cold to shine visibly, but its proton and electron occasionally flip relative spin and emit a photon at 21 centimetres wavelength. Predicted in 1944, the line was detected in 1951. Because radio waves pass through dust, mapping it revealed the spiral arms of our galaxy. It sits in Mondo's space collection, one of 200 facts that each trace back to the reference below.
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Facts that can change over time are reviewed and updated. If something here looks wrong, Mondo would rather know than be confidently incorrect.
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Tags: hydrogen, radio, interstellar
Cold hydrogen betrays itself at 21 centimetres: the short answers
- Where does this come from, and can I check it?
- A flip in the hydrogen atom's spin releases a faint radio wave at 1420 megahertz. The figure is taken from NASA Science, which is linked in full on this page. Every fact on Mondo Knows names its source rather than repeating an unsourced claim.
- Is this really a space fact?
- Yes. It sits on the Space shelf, one of 2,600 sourced facts grouped into space, animals, the human body, history, science and more. Category pages collect every fact in the same field.
- What else can I read about it?
- The links under the fact point to a quiz, game, puzzle or calculator on the same subject, and the shelf below shows related facts. The wording here is original, and quoted phrases are marked as quotations.