How does interstellar reddening affect measurements of globular cluster distances?
Most of the brightest stars are older yellow stars, but a few young blue stars are also visible. This picture is a composite of three images taken by the Hubble Space Telescope.
Interstellar reddening is a significant factor that must be accounted for when determining the distance to a globular cluster. This phenomenon is caused by intervening interstellar matter—specifically gas and dust—located between the observer and the cluster, which absorbs and scatters starlight, making the stars appear redder and dimmer than they intrinsically are.123
Because astronomers rely on methods such as comparing the location of a cluster's main sequence on a colour-magnitude diagram or using the apparent magnitudes of RR Lyrae variables to calculate distance, failing to correct for this reddening would lead to inaccurate distance estimates. The correction factor required is substantial for many clusters, particularly those located near the plane of the Milky Way Galaxy, where dust density is higher. Conversely, the effect is relatively small for clusters situated at high galactic latitudes, away from the galactic plane.13