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Astronomy Cast Ep. 775: The Hydrogen 21-cm Line
https://www.youtube.com/watch?v=2uvcumUc6Gc Hosted by: Fraser Cain ( @frasercain ) and Dr. Pamela L. Gay ( @CosmoQuest ) Streamed live on Dec 15, 2025. Atomic hydrogen is the raw material for stars, but there's a problem. It's cold & dark, but it can do a very rare trick, releasing a photon in a very specific wavelength, known as the 21 centimeter line. And thanks to this wavelength astronomers have mapped out star forming regions across the Milky Way, the Universe and into the Dark Ages! This forbidden transition of Hydrogen has led to the mapping of galaxy rotation, a cool classroom…
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