Reddening

Reddening is the alteration of an object's apparent color toward red wavelengths as light is attenuated in a wavelength-dependent way. The term is used in several contexts, most notably atmospheric reddening of the Sun and sky, and interstellar reddening caused by dust in space. In both cases shorter wavelengths are more strongly scattered or absorbed than longer wavelengths, yielding a redder observed spectrum than the intrinsic one.

Atmospheric reddening occurs as sunlight traverses the Earth's atmosphere. Rayleigh scattering by molecules preferentially scatters blue light, while aerosols and other particulates contribute additional wavelength-dependent extinction. The effect is enhanced near the horizon, where the optical path length is longest, making sunsets appear reddish. Correction for atmospheric reddening, called extinction, is routine in ground-based astronomy and meteorology.

Astronomical reddening, or interstellar reddening, arises from dust grains in the interstellar medium. Dust absorbs and scatters shorter-wavelength light more efficiently than longer wavelengths, making stars and galaxies appear redder and dimmer. Reddening is quantified by color excess, E(B-V) = (B−V)observed − (B−V)intrinsic. The total extinction A_V relates to E(B-V) through R_V = A_V / E(B-V), with a typical Milky Way value near R_V ≈ 3.1. The wavelength dependence is described by extinction laws, such as the Cardelli, Clayton, and Mathis formulation.

Practical implications include the need to correct observations for reddening to recover intrinsic properties such as temperature, luminosity, and distance. Reddening maps and three-dimensional dust models aid in this process, and reddening analysis is central to studies of star formation, galactic structure, and cosmology.