A molecular cloud is a large, cold interstellar region of gas and dust where stars and planets are born.Molecular clouds are the main reservoir of cold interstellar gas. Their diameters reach 600 light-years, and masses—millions of solar masses. They are opaque to visible light but transparent to radio waves, which allows radio telescopes to study their structure. More than 200 types of molecules have been found in them, including organic ones (ethanol, glycolaldehyde). The clouds live for tens of millions of years, after which the radiation of young hot stars disperses the remaining gas.
The first indication of the existence of molecules in the interstellar medium: detection of CH and CN in optical spectra (1937). In 1970, with the discovery of CO emission at a wavelength of 2.6 mm, systematic mapping of molecular clouds began. In 1975, Fred Hoyle and colleagues published a model of chemical evolution of clouds. Large surveys of the 1980s–2000s (FCRAO, Bell Labs 7-m) revealed thousands of clouds in the Milky Way. Modern interferometers (ALMA) with high resolution showed a complex filamentary structure and disks around protostars.
Dynamics are controlled by the balance of gravity, thermal pressure, magnetic field, and turbulence. Stability is determined by the Jeans criterion: if the mass of a gas clump is greater than the Jeans mass M_J ∝ T^{3/2} ρ^{-1/2}, it collapses. Compression accelerates, the cloud fragments into many clumps—a star cluster forms. During compression, gravitational energy is released, part of it turns into heat, part into magnetic field energy and kinetic energy of outflows. When the temperature in the center of the protostar reaches ~10⁷ K, nuclear burning of hydrogen begins. The newborn stars ionize the surrounding gas (creating H II regions) and through stellar wind and radiation disperse the remnants of the cloud.
Knowledge of these clouds is necessary for predicting star formation and galactic evolution. Spectral analysis of clouds allows searching for complex organic molecules—possible precursors of life.
In the molecular cloud Sagittarius B2, enormous quantities of ethyl alcohol have been found—enough to theoretically 'fill' an uncountable number of bottles, but in reality it is extremely cold and ionized gas.



















