Molecules between the stars

Thin, cold clouds of gas and dust drift between the stars. Radio telescopes have found hundreds of kinds of molecules in them, most of them built on carbon.

The Taurus Molecular Cloud, about 430 light-years away, is one of the nearest. Each cubic centimetre of it holds about ten thousand molecules, nearly all hydrogen, H₂, at about −263 °C. The same space in the air around you holds about 25 billion billion. Dust shades the inside of the cloud from starlight, so molecules there last a long time.

Spinning molecules send out radio waves

A molecule in a cloud spins, and the speed it spins at can change only in fixed steps. A molecule with uneven charge, a δ+ side and a δ− side, gives out radio waves of one exact frequency at each step down. The steps depend on the molecule's mass and how it is built, so every molecule has its own set of frequencies, a fingerprint, much as each bond absorbs infrared of its own wavenumber. Chemists measure a molecule's fingerprint in the lab, and astronomers search the sky for it.

Carbon monoxide, CO, sends out radio waves 2.6 mm long. After hydrogen it is the most common molecule in space. Hydrogen's two identical atoms share their electrons evenly, so H₂ gives out almost no radio waves, and astronomers map the hydrogen in a cloud by the carbon monoxide that glows with it.

By late 2026 the Cologne Database for Molecular Spectroscopy listed 349 molecules found between the stars or in the gas round them. Many form on the icy coats of dust grains. There, hydrogen atoms add one at a time to frozen carbon monoxide, making methanal and then methanol.

Rings in the dark

Benzene holds its charge evenly all round its ring, so it sends out no radio waves, and the same goes for coronene, the flat sheet of seven benzene rings. Swap one hydrogen for a nitrile group, -⁠C≡N, a carbon triple-bonded to a nitrogen, and the molecule gains a δ− end. In 2018 astronomers using the Green Bank Telescope in West Virginia found benzonitrile in the Taurus Molecular Cloud, the first single benzene ring molecule picked out by radio. In 2025 the same project found cyanocoronene there.

Flat sheets of benzene rings like coronene are called polycyclic aromatic hydrocarbons, or PAHs. Infrared telescopes see the glow of their C–⁠H and C–⁠C bonds across our galaxy and in others, and astronomers estimate that PAHs hold a tenth to a fifth of all the carbon between the stars.