Any molecule that occurs naturally in clouds of gas and dust in space. Nearly 200 such molecules and molecular ions have already been identified, chiefly by their emission or absorption of radio waves at particular wavelengths. The simplest molecule, H₂, is also the most abundant. Other known molecules include simple groupings such as carbon monoxide (CO), ammonia (NH₃), and water (H₂O); simple organic molecules such as ethyl alcohol (CH₃CH₂OH), formaldehyde (H₂CO), and acetic acid (CH₃COOH); and a variety of ions and radicals which are unstable on Earth, such as hydroxyl (OH), sulphur monoxide (SO), and the formyl ion (HCO⁺). Many isotopic variants are also found, for example HDO, a form of water in which one hydrogen atom has been replaced by an atom of deuterium, a heavy form (isotope) of hydrogen. The largest molecules yet identified include so-called buckyballs, near-spherical structures of 60 or 70 carbon atoms, and the sugar-like molecule glycolaldehyde (C₂H₄O₂). Interstellar molecules regulate the temperature of the gas clouds by radiating away energy at radio wavelengths. This energy loss allows some of the densest regions of the cloud to collapse into stars. Molecules are also formed in the envelopes of gas and dust around old stars. These are usually termed circumstellar molecules because they have formed from material that came from the star, whereas interstellar molecules are formed from general interstellar matter.