Molecular Clouds

About one-tenth of the mass of our Galaxy is interstellar gas, about half of it atomic hydrogen and helium and most of the rest molecules-mainly molecular hydrogen but over a hundred other compounds as well. Because molecular hydrogen has no strong transitions at radio or millimeter wavelengths, the molecular component was largely unsuspected until about 35 years ago when the trace molecule carbon monoxide (CO) was first detected in space. Since then, large-scale CO surveys of the Galaxy have shown that the molecular gas is mainly concentrated in large, cold, relatively dense clouds, some containing up to a million times the mass of the Sun. All star formation takes place within the denser cores of these clouds, where the process is largely obscured optically by trace amounts of dust. Astronomers in the R&G Division play a leading role in star formation research because the radio and infrared telescopes they employ can penetrate the dust to reveal both the initial conditions and early stages of star formation.