interstellar extinction

Imagine driving toward a streetlamp on a foggy night: the lamp looks dimmer than it would in clear air, because the fog is taking some of its light away. The same thing happens to starlight crossing the dusty galaxy. Interstellar extinction is the overall dimming of a star's light caused by dust grains in the way absorbing some photons and scattering others off in random directions before they reach us.

Extinction is measured in magnitudes — the same scale astronomers use for brightness — and is usually written A. One magnitude of extinction means the star looks about 2.5 times fainter than it really is; toward the crowded center of the Milky Way the dust can pile up to tens of magnitudes in visible light, dimming stars by factors of billions. Crucially, extinction is wrong-footing in a specific way: it steals far more blue light than red, because the dust grains are roughly the size of a visible-light wavelength and interact most strongly with the shorter blue waves. Two effects ride together: the star is dimmed (extinction proper) and it is also made redder (reddening).

If you ignore extinction, you systematically misjudge the cosmos — a dimmed star looks farther away than it is, throwing off every distance you build on its brightness. Correcting for it is therefore one of the routine, essential chores of astronomy. The escape route is to observe in the infrared or radio, whose long wavelengths slip past the small dust grains almost untouched, letting us see straight through clouds that are opaque to the eye.