Why can’t 21-centimeter radiation be used to probe the interiors of molecular clouds?

Answer 1

I believe they may be too dense. A hydrogen atom takes about 11million years on average to spontaneously drop to the lower energy state and emit the 21cm photon. Unless the space is very cool, diffuse and quiet, atoms will bang into each other and disturb each others energy levels too frequently to have a chance to emit this particular signal.

Edit: Reading up some, it also appears that being locked up with another hydrogen atom in a hydrogen molecule impedes the ability to emit this signal. Only unbound neutral atoms emit it.

Answer 2

The source of the 21-cm line is neutral, non-molecular hydrogen (e.g. regular, unbound, free hydrogran atoms). So, a molecular cloud (e.g. a cloud formed out of material in which most of the atoms are bonded to other atoms) wouldn't have so much non-molecular hydrogen (it might still have some, but not so much as just a cloud of non-molecular hydrogen), which means the amount of 21-cm radiation emitted by the cloud would be relatively limited. This is coupled with the fact that the mean time for a hydrogen atom to emit a 21-cm photon is hundreds of years, so it's a relatively rare occurance.

Answer 3

We get 21 cm (radio) emission from neutral hydrogen in the universe - it is a from a forbidden (spin-flip) transition. As to why we can't use it to probe molecular clouds - I guess it's really only useful if the cloud is mostly hydrogen. Otherwise, I dunno. I've used it to measure the rotation rates of spiral galaxies; so it's useful for finding star-forming regions.

Answer 4

21-cm radiation is in the UHF frequency band (L-band microwaves), and would get a lot of interference from radio and TV broadcasts here on earth.

And I think it has a tendency to bounce/reflect back off the troposphere, so it wouldn't get very far into space.

Answer 5

I don't know for sure, sorry!!!!!!!!!!!!!!!