extinction and reddening
/ ek-STINK-shun /
The space between the stars is not perfectly empty. Scattered through it are vast, thin clouds of microscopic dust grains, like a faint cosmic fog. When starlight passes through this fog, two things happen, much as they do to a car's headlights in mist: the light gets dimmed, and it gets reddened. The dimming is called extinction; the color shift toward red is called reddening. Together they are the chief nuisance — and a useful clue — in measuring the universe with light.
Dust grains absorb and scatter light, but not equally at all wavelengths. They block shorter (bluer) wavelengths more strongly than longer (redder) ones, just as Earth's atmosphere scatters blue sunlight to make sunsets red. So a star seen through dust looks both fainter (extinction) and redder (reddening) than it truly is. The amount depends on how much dust lies along the line of sight; toward the crowded plane of our galaxy it can be enormous, dimming distant stars by many magnitudes, while looking out of the galactic plane the sky is far clearer.
Extinction and reddening matter because, if ignored, they corrupt nearly every measurement made with light. A reddened star can masquerade as a cooler one, inflating its inferred color index; an extinguished star looks farther away than it is, since the inverse-square law would attribute the extra dimming to distance. Astronomers correct for it by comparing how much a star is reddened (which reveals the dust column) to how much it is dimmed, using the fairly steady relationship between the two. The same dust that obscures visible light glows in the infrared, which is why infrared sees straight through it.
Toward the center of our galaxy, dust dims visible starlight by roughly 30 magnitudes — a factor of a trillion — so the galactic core is utterly hidden to the eye. Yet in the infrared, which slips past the dust, the same crowded star field blazes into view.
Dust both dims (extinction) and reddens starlight — a bias that must be removed before distances and temperatures are trusted.
Reddening by dust is not redshift: dust changes a star's color by filtering its light, while redshift stretches the wavelengths of every line. Confusing them, or ignoring extinction, makes stars look cooler and farther than they are.