A molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2). Unfortunately, molecular hydrogen is virtually undetectable, and therefore astronomers rely on tracers of molecular hydrogen, such as the carbon monoxide (CO). However, it is known that the ratio between CO luminosity and H2 mass is not constant.
Most molecular gas in the interstellar medium (ISM) is in giant molecular clouds (GMCs), with masses of ~105 solar masses, sizes of tens of pc, and average gas densities of ~102 cm-3. GMCs are however very inhomogeneous, with filamentary structures and densest regions called clumps. Clumps are characterized by masses of the order of 103 solar masses, sizes of ~1 pc, and densities of 103 cm-3. Clumps can be inhomogeneous too, and can contain regions with even higher densities, 104 cm-3, and smaller sizes, ~0.1 pc, called cores. Cores are believed to be the sites of the formation of individual stars or small multiple systems.
Optically, molecular clouds appears as "holes" in the sky, and indeed originally they were mistaken for real holes in the distribution of stars. This happens because the dust present in dense molecular clouds absorbs completely photons in the optical wavelength. However, as we move to longer wavelength, the light is able to penetrate the cloud, and therefore we can see through it. Stars observed through the cloud appears red, a phenomenon called extinction.
Optical image of Barnard 68 (courtesy of J. Alves)
Near-infrared image of Barnard 68
The color of a background star provides a measurement of the integrated dust density along the line of sight of the star, and since the gas-to-dust ratio is believed to be constant, the color of a star is also proportional to the cloud gas column density. This is the essence of the Near-Infrared Color Excess (NICE) technique to study the molecular cloud structure. Its refinements, NICER (Lombardi & Alves 2001) and NICEST (Lombardi 2009) provides the best known ways to map the structures of molecular clouds.
Here is a list of a few review articles on molecular clouds and star formation:
- The Structure and Evolution of Molecular Clouds: from Clumps to Cores to the IMF, by J.P. Williams, L. Blitz, L., and C.F. McKee (2000)
- Cold Dark Clouds: The Initial Conditions for Star Formation, by Edwin A. Bergin and Mario Tafalla (2007)
- The Gould Belt, star formation, and the local interstellar medium, by Isabelle A. Grenier (2004)
- Theory of Star Formation, by Christopher F. McKee and Eve C. Ostriker (2007)
- Embedded Star Clusters in Molecular Clouds, by Charles J. Lada and Elizabeth A. Lada (2003)