What Is a Molecular Cloud? The Cold Dark Places Where Stars Begin

Most interstellar gas is atomic or ionised hydrogen, and molecules form only where conditions protect them. In these clouds the density is high enough and dust grains are abundant enough that ultraviolet starlight, which would break molecules apart, is absorbed in the outer layers, shielding the interior. On the surfaces of those dust grains, hydrogen atoms meet and combine into molecular hydrogen, which is the dominant constituent. The same shielding keeps the interior extremely cold, typically ten to twenty degrees above absolute zero, because there is no radiation reaching it to warm it and any heat is radiated away efficiently by molecules and dust. That cold matters enormously, because gravity can only collapse a cloud if thermal motion is too weak to resist it, so a warm cloud stays diffuse and a cold one can contract. Molecular hydrogen itself is nearly impossible to detect at those temperatures, so carbon monoxide is used as a tracer instead, and essentially every map of these clouds is really a map of that molecule.

The structure is hierarchical and chemically far richer than the vacuum of space suggests:

• •Giant clouds tens to hundreds of light years across, containing hundreds of thousands to millions of solar masses, which are the largest structures in a galaxy's gas
• •Filaments and clumps within them, since the gas is turbulent and highly structured rather than smooth, with filaments now recognised as the dominant morphology
• •Dense cores, small regions where a single star or small group is on the point of forming
• •Dust grains of silicate and carbon, coated in ices of water, carbon monoxide, methanol and ammonia, which are chemical factories on their surfaces
• •Over two hundred identified molecular species including water, ammonia, formaldehyde, alcohols and complex organics, detected by their radio emission lines
• •Magnetic fields threading the whole structure, which resist collapse and channel material along field lines, and which are among the harder things to measure

A cloud collapses when gravity overcomes everything resisting it, and several things resist. Thermal pressure is weak at these temperatures. Turbulence supplies motion that supports the cloud on large scales while simultaneously creating the dense regions where collapse begins locally, which is why the same turbulence both delays and triggers star formation. Magnetic pressure resists compression across field lines. Rotation resists collapse and is shed through discs and outflows. The process is inefficient, with only a small percentage of a cloud's mass converted into stars, and the rest is dispersed by the stars that do form. That dispersal is the feedback loop that regulates galaxies: newly formed massive stars emit intense ultraviolet radiation, drive powerful winds and eventually explode, and those processes heat, ionise and blow apart the cloud that produced them, ending star formation locally and returning material to the wider medium. Clouds therefore last only tens of millions of years, which is brief on galactic timescales.

Being cold and dark, these regions are invisible to optical telescopes except as silhouettes, so nearly everything known about them comes from longer wavelengths. Radio and millimetre observation detects rotational transitions of molecules, which is how carbon monoxide maps are made and how the inventory of interstellar chemistry was compiled, and it works precisely because cold molecules emit at those wavelengths. Infrared observation penetrates the dust and reveals the young stars forming inside, which optical light cannot reach, and this is one of the main reasons space infrared observatories are built. Submillimetre observation detects the thermal emission from cold dust itself, mapping the density structure directly. Interferometers combining many antennas achieve the resolution needed to see individual forming stars and their discs, and the results have shown structured discs with gaps at an early stage, which is evidence that planets begin forming far sooner than models assumed.

Dust absorbs the ultraviolet light that would destroy molecules, so the interiors stay cold at ten to twenty degrees above absolute zero, which is what allows gravity to overcome thermal motion. Molecular hydrogen dominates and is nearly undetectable, so carbon monoxide is mapped instead. Turbulence both supports clouds and creates the dense spots where collapse starts, and the stars that form then blow the cloud apart within tens of millions of years.