Molecular Cloud

Molecular clouds are the coldest, densest regions of the interstellar medium — vast agglomerations of gas and dust where hydrogen exists primarily in molecular form (H₂) rather than as atomic or ionized gas. These clouds are the exclusive sites of star formation in the modern universe, and their structure — filaments, cores, and hubs — is shaped by a complex interplay of turbulence, magnetic fields, gravity, and feedback from young stars. The largest molecular clouds, known as giant molecular clouds, can contain millions of solar masses of gas and span hundreds of light-years, yet they are so tenuous that they would qualify as laboratory vacuums. ALMA and other millimeter-wave observatories probe the interior chemistry and kinematics of these clouds by detecting the rotational emission lines of carbon monoxide and other molecules, transforming them from inert astronomical backgrounds into active laboratories for the physics of condensation and collapse.