Hydrogen
Spectral line
H I · 21 cm line
The quiet glow of cold hydrogen gas. Tune to 1420.405 MHz and you are listening to the galaxy itself.
05 / 06
- Catalogue
- H I · 21 cm line
- Type
- Spectral line
- We listen at
- 1420.405 MHz
- Period
- No period
- Distance
- Everywhere in the Milky Way
- Discovered
- Predicted 1944 · detected 1951
- By
- Hendrik van de Hulst (prediction); Harold Ewen & Edward Purcell, Harvard
- Position
- 20h 20m 00s
+40° 00′ 00″ - From Stilledal
- Always. We point along the Cygnus arm.
Once every eleven million years or so, a single hydrogen atom flips its electron and gives off a photon with a wavelength of 21 centimetres. There are enough atoms that the whole Milky Way glows with it.
The gas is moving, so the line arrives Doppler-shifted. Each bump in the spectrum is a spiral arm, coming towards us or going away. From these bumps astronomers drew the first map of our own galaxy.
It has no rhythm to hear. In the headphones it is a hiss that changes colour as the dish sweeps along the sky.
{What it sounds like}
A hiss, like surf heard through a wall.
{Headphones help. The sound is synthesized from the real period and profile.}
{Next station/} 06 / 06 · 20.100 MHz