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The Interstellar Medium, Part II: Dense Molecular Clouds

© Wesley Colley

This is the second in a series of articles which describe the stuff between the stars, the interstellar medium (ISM). In Part I, I discussed dust, which is comprised of grains of an order of a micron in size. Next up are the dense molecular clouds, comprised of simple to complex molecules as well as dust. While dust is fairly ubiquitous, although splotchily distributed, molecular clouds are generally small and concentrated in distribution. Perhaps most importantly, molecular clouds are the places where stars are born.

Molecular clouds are so called because they're principally comprised of -- you guessed it -- molecules. Isn't everything made of molecules? No. Above 2500 degrees Kelvin (K) or so, molecules generally break apart into atoms due to the violent collisions which occur at such temperatures. Also, stars emit photons which dissociate molecules easily. However, in cooler, denser parts (T ~ 100K, n ~ 10/cc) of the galaxy, molecules can form. Most of these form the "molecular gas" component of the ISM, which I'll discuss next time. Molecular clouds are something like a condensate out of this still quite tenuous molecular ISM.

Because the galaxy is a very dynamic physical system, turbulence, collapse and collisions of gas clouds are regular occurrences. In the simplest case, gravitational collapse of a gas cloud occurs when the gravitational forces become stronger than the pressure (acoustic) forces. This is called the Jeans criterion. One can derive this balance dimensionally (very crudely) by setting the energy in sound waves equal to the gravitational energy. For sound's speed v_s, temperature T, total mass M, mean molecular mass m, density d, Boltzmann's constant k and Newton's constant G, we have the following approximate balance.

v_s² ~ GM/r (ignoring factors like 2 and 3/2)
kT/m ~ G/r * r³d (ignoring factors of 4/3 pi)
r² = kT / Gdm

where r is the "Jeans length." Therefore, for a given density, if a cloud has sufficient size and low enough temperature, it can collapse under its own gravity. The question is: how does a cloud get into this situation under which it can collapse? That's where turbulence and collisions come in.

As a cloud orbits the galaxy it encounters various environments -- other clouds, supernova remnants, spiral arms -- any of which can push or pull on the cloud. If the cloud is pushed in such a way that its Jeans Criterion is met, it can begin to collapse and become much denser. We have excellent evidence for such effects in that on the leading edges of spiral arms we see concentrations of molecular clouds, suggesting that when clouds encounter the gravitational kick

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