Interstellar extinction

Abstract

Star counts reveal that the observed space density of stars decreases with increasing distance from the Sun. Since it is clear that the Sun is not at the center of a concentration of stars, the only explanation for the phenomenon is the assumption that light is being absorbed by an interstellar medium. The most convincing argument in support of that assumption came from Trumpler (1930) who found that the diameter of open clusters, calculated from the angular diameter of the cluster and from the distance moduli of the member stars, seemed to increase with distance. Obviously, the clusters are not larger, but they are closer than what the derived distances suggest, and the excess in inferred distance is caused by interstellar absorption not being taken into account. Trumpler derived a mean interstellar extinction coefficient of 0.7 magnitude per kpc in the photographic domain.