interstellar dust
/ in-ter-STEL-er /
When astronomers say 'dust,' they do not mean the fluff under your bed. Interstellar dust is a haze of microscopic solid grains — tiny specks of soot and sand, far smaller than the dust in your home — floating in the gas between the stars. There is not much of it (only about one percent of the ISM's mass), yet its effects on what we see are out of all proportion to its tiny share.
A typical dust grain is astonishingly small, around 0.1 micrometers across — about a thousandth the width of a human hair, comparable to the wavelength of visible light. The grains are made mostly of carbon (graphite and soot-like material) and silicates (the stuff of rock and sand), often coated in ices in the coldest clouds. They form in the cool outer envelopes of dying stars and in supernova ejecta, then drift into the ISM. Because their size is close to the wavelength of light, they are very effective at scattering and absorbing it — blue light most of all — which is why they dim and redden the stars behind them. Having soaked up that starlight, the grains warm to a few tens of kelvin and re-radiate the energy as infrared.
Dust is far more than a nuisance that obscures our view. Its surfaces are the workbenches of interstellar chemistry, where lone hydrogen atoms meet and combine into molecular hydrogen, the seed of star formation. Dust grains are also the literal raw material of planets — they stick together into pebbles, then boulders, then worlds like Earth. A common misconception is that dust is rare and incidental; in fact, without it, neither the molecules of life nor rocky planets could readily form.
The dark rifts that split the glowing band of the Milky Way are not gaps where there are no stars — they are clouds of interstellar dust blocking the light of the countless stars behind them.
Dark lanes across the Milky Way are dust clouds, not empty gaps.
Interstellar dust is chemically and physically nothing like household dust: the grains are mostly carbon and silicates only a fraction of a micron across, formed in dying stars. Despite being just one percent of the ISM's mass, dust dominates how starlight reaches us.