Annu. Rev. Astron. Astrophys. 1989. 27: 139-59 Copyright © 1989 by Annual Reviews. All rights reserved

5.15 Ursa Minor

The variable stars of the Ursa Minor dwarf elliptical are mainly RR Lyrae stars plus a few anomalous Cepheids (123). Its CMD, first examined in uncalibrated form by van Agt (123), has been investigated by Schommer et al. (112), who found (after eliminating foreground stars by means of proper-motion measurements), that the stars fit a metal-poor globular cluster CMD quite well; the giant branch is blue and steep. However, there is a minor component (approximately 25% of the giants) that scatters to the redward side of the giant branch. Among these is a relatively metal-rich ([Fe/H] = -1.4) star for which spectra were obtained by Zinn (135). The giants on the main part of the giant branch showed a lower metals abundance ([Fe/H] = -2.5, according to Zinn). The peculiar property of Ursa Minor's CMD is its very blue horizontal branch; otherwise, it is very much like Draco (see below). While there is no evidence that an extended period of star formation went on after most of Ursa Minor's stars first formed, the apparent spread in its abundances gives us reason to believe that its history has not been entirely uneventful since that time. Olszewski's discovery of obviously recently determined structure in the system provides further intriguing evidence of this.

Ursa Minor contains at least one carbon star (6).