When stars are born in a galaxy, they make hydrogen glow, like a neon sign, but on a cosmic scale. This light could tell us a lot about how many stars are forming, but on its way to us, it passes through cosmic dust. Dust is like a fogged-up window: it dims the glow and alters its color.
Astronomers gathered data on 260 galaxies where at least one such "hydrogen beam" is visible. For 85 of them, they were able to compare two different beams — the hydrogen lines Hα and Hβ. The difference in their brightness reveals how much dust lies in the way. It turns out dust behaves differently: sometimes it's like a light haze, sometimes like a thick curtain. For 14 galaxies where three beams are visible, the researchers fitted "glasses"—a precise formula that explains how dust distorts light in each case.
These calculations will come in handy for the new SPHEREx space telescope, which will search for galaxies by their hydrogen beams. Knowing how dust hides light will allow us to count stars in the far corners of the universe more accurately, and not mistake a dim galaxy for a small one.
Dust in galaxies isn't dirt: it's tiny particles of carbon and silicon. A single particle is less than a thousandth of a millimeter across, but together they can dim the light of an entire galaxy by a factor of tens.