Groups of Single Stars

Groups of single stars offer calibration possibilities that enable measurement of astrophysical parameters well beyond what is exposed by the measurements of individual stars alone. This is made possible primarily by exploiting parameters common to groups of stars, such as parallax and proper motion for members of a star cluster, or period-luminosity relations for groups of pulsating stars. In this chapter techniques used to extract such parameters, and thereby exploring the Hipparcos data well beyond the accuracy levels for individual stars, are described, extensively illustrated by means of actual applications.