Interstellar Reddening and Extinction Interstellar Reddening and Extinction Extinction is the technical term for the dimming of starlight by interstellar dust.
Long wavelengths pass unhindered.
Dust grains in the interstellar medium are about 1 micron in diameter or less. Because blue light is blocked more effectively than red, interstellar clouds tend to make distant stars appear redder than they actually are. This effect is called interstellar reddening . Because infrared and radio light have much longer wavelengths than visible light interstellar clouds are nearly transparent to these types of light. We can use infrared and radio observations to study distant regions of our galaxy which are hidden from view at visible wavelengths.