interstellar reddening
/ RED-en-ing /
Have you noticed how the setting Sun glows orange-red? Its light is passing through a long, dusty stretch of atmosphere, which scatters away the blue and lets the red through, so the Sun looks redder than at noon. Interstellar reddening is exactly this effect on a galactic scale: dust along the line of sight scatters away a star's blue light more than its red, so the star reaches us looking redder than it truly is.
Reddening happens because dust grains are roughly the size of a wavelength of blue light, and short blue waves are scattered and absorbed more strongly than longer red ones. Astronomers measure it as a color excess — the difference between the color a star actually shows and the color it ought to show for its type. Reddening and dimming go hand in hand: a column of dust that reddens a star also faintens it (extinction), and the fixed ratio between the two is one of the steadiest tools in the trade, since how much a star is reddened tells you how much dust lies in front of it.
Reddening is both a problem and a gift. It is a problem because it disguises a star's true color and temperature, and you must correct for it before trusting any measurement of either. It is a gift because, by measuring the reddening of many stars, astronomers map where the dust lies in three dimensions across the galaxy. Be careful not to confuse interstellar reddening with cosmological redshift: reddening makes light look redder by removing the blue, while redshift stretches the wavelength of every color as the universe expands. They are entirely different physics.
A hot, intrinsically blue-white star seen through a thick dust cloud can appear orange or even red — not because the star changed, but because the intervening dust stole most of its blue light along the way.
Dust can make an intrinsically blue star look orange or red.
Reddening is not redshift. Reddening removes blue light from a beam, changing its color; cosmological redshift stretches every wavelength as space expands. Confusing the two leads to badly wrong conclusions about distance and motion.