[syhprum (OP)]

It is well known that clouds of neutral hydrogen emit radiation at approximately 1420 MHz, what is the source of this energy ? how is it replenished ?

[syhprum (OP)]

Time error the time stamped on my messages is about 66 minutes ahead of UTC ??

[daveshorts]

It should probably be 60... I will try and fix it next time I hear some pips on the radio.

[daveshorts]

Bascially the cloud is just glowing thermally, like the sun or in fact you do (in the infra-red). 1420MHz is a very low energy radiation so a cloud would have to be almost at absolute zero not to radiate at it. The Cosmic microwave background radiation peaks at about 160GHz so if there was no other source of incoming radiation this would be able to excite the hydrogen, but the hydrogen could also be heated up slightly by a nearby star or whatever is going.

[syhprum (OP)]

I guess I am radiating at about 300,000 GHz, quite a worry to those frightened by their mobile phones.

[daveshorts]

You will be radiating much more strongly at about 10 000GHz though - in the thermal infra-red.

[lightarrow]

At about 30,000 GHz, to be more precise, assuming a body temperature of 300 °K, and using the relation: λ_(max) ≈ 3•10^-3/T

[syhprum (OP)]

I am glad to say I am closer to 310°K, sorry about the miscalculation!

[Soul Surfer]

The hyperfine hydrogen line at 1.4 Ghz or 21 cm is a spectrum line emitted from neutral hydrogen atoms when the spin of the electron changes from being in the same direction to the spin of the nuclear proton to being in the opposite direction as the nuclear proton.  It is a vitally important spectral line because as you say it allows neutral hydrogen to be identified throughout the universe.  It has been observed by radio astronomers for more than fifty years and it is one of the most important pieces of information that allow us to identify the structure of our own galaxy.  It can be observed in both emission and absorption and the Doppler broadening and shift of the spectral lines allows both the temperatures and velocities of large clouds of neutral hydrogen to be measured.