Extinction of starlight

Optical photons traveling in the plane of the Galaxy are heavily scattered by clouds of dust (interstellar grains) which limit the range of visibility to, very roughly, 5000 LY, a distance that varies considerably for different viewing directions. (The Galaxy is ~ 100 000 LY in diameter.) Very distant galaxies can be seen if one looks away from the plane of the Milky Way where the dust along the line of sight is minimal.

The grains reduce, by scattering, the amount of light coming to us directly from a given star. The blue photons are preferentially scattered so the residual light is also noticeably redder. This diminution of brightness and reddening is called interstellar extinction, interstellar reddening or interstellar absorption. The latter phrase is rather misleading because the photons are largely scattered, not absorbed (See Fig. 2a). This is a large and important effect that optical astronomers must take into account for almost all of their observations. It also is a good diagnostic of the dust and magnetic field content of the interstellar medium (ISM).

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