Interstellar reddening Although we think of interstellar space as a vacuum, it is in fact filled with tenuous gas and dust. Like a smoke-filled room, the gas and dust along the line of sight to a star dim the starlight by absorbing and scattering the light. We call this effect interstellar extinction . If we do not account for this extinction, we will overestimate the distance to the star. Extinction is stronger at shorter wavelengths, as shorter wavelengths interact more strongly with dust particles.
Red light passes through gas and dust more easily than blue light.
The more gas and dust between you and the source, the stronger the reddening. When the sun and moon are near the horizon, you are viewing them through more atmosphere than when they are overhead. That is why the sun and moon look reddish when they rise and set. The reddening of starlight due to the interstellar extinction is known as interstellar reddening . Astronomers often used the terms extinction and reddening interchangeably.