Transcript: In the early part of the twentieth century, astronomers calculated the distances to stars by assuming that interstellar space was perfectly transparent. But eventually comparisons of distance to clusters in different directions in the sky yielded inconsistent results, and in 1930 Robert Trumpler showed that interstellar extinction or obscuration dims the light from all stars, groups, and clusters, that are larger than a distance of a few dozen parsecs. What this means is that the intensity of light falls off more rapidly than would be predicted by the inverse square law. We see a star as dimmer than it truly is, and we overestimate its distance. Without taking into account interstellar obscuration it’s impossible to correctly measure distances to stars, groups, and clusters, and map out the Milky Way galaxy.