Ep. 775: The Hydrogen 21-cm Line

Astronomy Cast

Science, Natural Sciences, Astronomy

4.8 • 3.4K Ratings

🗓️ 22 December 2025

⏱️ 35 minutes

🔗️ Recording | iTunes | RSS

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Summary

Astronomy Cast Ep. 775: The Hydrogen 21-cm Line By Fraser Cain & Dr. Pamela Gay 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 application of quantum mechanics, and weirdly no Nobel prize. In this episode, Fraser and Pamela take a look at this line's out-of-proportion awesomeness! This show is supported through people like you on Patreon.com/AstronomyCast In this episode, we'd like to thank: Andrew Poelstra, BogieNet, Brian Cagle, Burry Gowen, David, David Rossetter, Ed, Gerhard Schwarzer, Jason Kwong, Jeanette Wink, Michael Purcell, Sergey Manouilov, Siggi Kemmler, Sérgio Sancevero

Transcript

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0:00.0	I'm sorry. I'm going to Astronomycast Episode 775, the 21-centimeter line.
0:52.9	Welcome to Astronomycast, our weekly facts-based journey through the cosmos,
0:56.4	where we help you understand not only what we know, but how we know what we know. I'm Fraser Kane.
1:00.2	I'm the publisher of the universe today. With me, as always is Dr. Pamela Gay, a senior scientist for the
1:05.8	Planetary Science Institute and the director of Cosmo Quest. Hey, Pamela, how you doing? I am doing well enough.
1:14.2	I am currently finding. I have new technology, which is absolutely amazing, but nothing works.
1:20.5	So everyone, thank you for your patience as there is ludicrous hacking that went into putting together this episode.
1:29.4	Now, normally, after the amount of time that I've been gone, I would say, like, hey,
1:34.8	it's great to be back and did you all miss me? But because you, in your infinite wisdom,
1:41.3	said, let's just record all these shows, get them in the can, and then you don't have to
1:44.1	think about this anymore.
1:47.1	We record all those shows, we got them in the can, and I didn't have to think about them
1:50.2	anymore, and I am so glad.
1:52.5	You were so smart.
1:54.2	You were so right.
1:55.7	Because it is, you know, being on the road, it's a gone show.
1:58.7	And trying to then set up internet of different time zones.
2:02.9	It was so nice to go, oh, yeah, all those astronomy casts are done.
2:08.6	So, and now we continue uninterrupted, which was just great.
2:13.2	And so I'm back.
2:15.0	And you escaped.
2:16.2	You escaped the Cosma Quest Hangoutathon this year.
	...

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