Stars are born in cold clouds whose main gas is almost invisible
Molecular hydrogen fills the dark clouds where stars form, yet its symmetrical molecules barely emit anything a telescope can catch. Astronomers found the clouds by chasing a stand-in: carbon monoxide, far easier to spot. In 1970 a team led by Arno Penzias used it to detect the first molecular cloud, Sagittarius B2.
The hunt began with radio. During the Second World War, Henk van de Hulst worked out that neutral hydrogen atoms should give off a detectable radio signal: when the spins of the proton and electron flip from parallel to opposite, the atom sheds a little energy at 1420.405 MHz, a wavelength of 21 centimetres. After the war Dutch astronomers converted German coastal radar dishes into radio telescopes to look for it. In 1951 Ewen and Purcell reported the line in March, and Muller and Oort confirmed it from Kootwijk in May. By 1958 the Leiden-Sydney survey had produced the first hydrogen map of the galactic disc, revealing its spiral arms.
Molecules followed. Alan Barrett and Sander Weinreb at MIT caught hydroxyl in the supernova remnant Cassiopeia A in 1963, the first radio detection of an interstellar molecule. Ammonia turned up in 1968, formaldehyde the next year, and George Carruthers identified molecular hydrogen itself in 1969. Then Penzias, Keith Jefferts and Robert Wilson found carbon monoxide in the Omega Nebula, and CO became the standard tracer. A 2022 paper counted more than 10,000 molecular clouds catalogued since.
These clouds fill less than one percent of the Milky Way's interstellar volume but are its densest part, and they cluster along the spiral arms. Besides hydrogen and CO they hold methanol, ethanol, benzene rings and large polycyclic aromatic hydrocarbons. Dust shields the molecules from starlight that would otherwise split them apart. Inside, gas gathers into clumps roughly a parsec across, the seeds of star clusters, and much smaller, denser cores that collapse under gravity into stars.
They do not last. After about 10 million years a cloud is torn apart or transformed, which is why older massive stars have shed their surrounding gas. Only around 2 percent of a cloud's mass becomes stars, so to keep up the galaxy's star formation of roughly 3 solar masses a year, some 150 solar masses of gas must gather into new clouds annually, most likely pulled together by gravitational instability rather than collisions.
Source: Molecular cloud
