interstellar dust grain

Mixed in with the gas of every molecular cloud is a faint haze of solid specks called interstellar dust. These are not dust bunnies from under a bed but microscopic motes of soot and sand-like minerals, far smaller than you can see, drifting between the stars. They are a tiny fraction of a cloud's mass yet utterly disproportionate in their effect on how stars are born.

A typical dust grain is roughly the size of a particle of cigarette smoke, often around a ten-thousandth of a millimetre across, built of carbon (soot-like) and silicates (the stuff of rock and sand), sometimes coated in ices of water, ammonia, or methane out in the cold. By mass, dust is only about one percent of the interstellar material, but because the grains are solid they block and redden starlight (we see distant stars dimmed and made redder, an effect called extinction), and they absorb that starlight and re-emit it as infrared, which is why young stars buried in dust shine in the infrared. Crucially, the cold surfaces of grains act as tiny workbenches where atoms meet and combine: most of the hydrogen molecules H2 in the universe were assembled atom by atom on the surfaces of dust grains.

Dust matters far out of proportion to its small mass. It lets clouds cool by radiating heat away in the infrared, which is what allows them to get cold and dense enough to collapse into stars in the first place. It shields the fragile molecules deep in a cloud from being torn apart by starlight. And the same grains later clump together in the disk around a young star to begin building planets. The atoms that make up dust were themselves forged inside earlier generations of stars, so dust is, quite literally, recycled stardust.