Are giant molecular clouds, of a million solar masses or more, the most important sites for star formation?
They certainly are. An important aim at which I have been working hard to understand for several years is: what are the best molecular probes of the star formation process? One of the big problems in star formation is the transition from an interstellar cloud that has relatively low density and is cold, to a protostar, in which the nuclear fusion processes can commence. And if you try to follow the physical conditions theoretically you find you get to a stage where the gas is dense but still cold—10 K. At those temperatures everything sticks to the dust on timescales of hundreds of years, much shorter than the time scale of star formation. Molecules such as carbon monoxide or ammonia, which astrochemists and radio astronomers use to probe physics, have frozen out and have ceased to be applicable as probes. With the molecules frozen out the only components of the gas are hydrogen, deuterium, and helium, and they form very few observable species, such as H2D+ and D2H+, but in surprisingl
