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

4. CRITERIA FOR THE PRESENCE OF 15-GYR-OLD STARS

There are three reliable criteria that can be used to establish the presence of very old stars in Local Group galaxies. Here I call the oldest stars ``15- Gyr-old stars'' on the basis of the recent results of globular cluster main sequence fitting, which indicate that most of these stars have ages of 14-17 Gyr (53).