... But when stars are in a stable star cluster whose physical size is not changing, like the Pleiades , then the apparent motions of the stars within the cluster can be used to determine the distance to the cluster. ... Because a time interval of 100 years can be used to measure d(theta)/dt, precise distances to nearby star clusters are possible. ... The globular cluster luminosity function can be used to estimate the distance to a galaxy from the observed brightness of its globular clusters. ...