Molecular Cloud

An interstellar gas cloud that is dense enough to allow the formation of molecules and comprised of a cold dense complex mixture of interstellar gas and dust roughly 75% hydrogen and 21-24% helium. Clouds contain trace amounts of other molecules, of which well over 100 different types have now been discovered in space. Dust grains make up ~1 % of the mass of a cloud. The relatively high density of dust particles plays an important role in the formation and protection of the complex molecules. The emission of molecular lines often shows several distinct intensity peaks, each representing individual clumps or clouds of gas and dust in a region that characteristically extends for 50 light-years. Two distinct types of molecular cloud are known, both associated with star formation: giant molecular clouds (GMCs) and dwarf molecular clouds. GMCs are the coolest (10 to 20 K) and densest (106 to 1010 particles/cm3) portions of the interstellar medium. They typically stretch over 150 light-years and contain several 100,000s of Msun of material, making them are the largest gravitationally bound objects in the Galaxy.

Molecular clouds are the only places where star formation is known to occur. Star formation occurs when deeply embedded clumps of interstellar gas and dust collapse under self-gravitation or due to impinging shock waves. Young stellar objects (YSOs) – newborn stars or stars in the process of forming – are obscured from direct optical view, and the only source of information from inside these clumps is provided by radio and infrared waves emitted by molecules and dust.

Some or all content above used with permission from J. H. Wittke.