Annu. Rev. Astron. Astrophys. 1996. 34: 461-510 Copyright © 1996 by Annual Reviews. All rights reserved

Reprinted with kind permission from Annual Reviews, 4139 El Camino Way, Palo Alto, California, USA

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ABSTRACT. A careful assessment of current uncertainties in stellar physics (opacities, nuclear reaction rates, equation of state effects, diffusion, rotation, and mass loss), in the chemistry of globular cluster (GC) stars, and in the cluster distance scale, suggests that the most metal-poor (presumably the oldest) of the Galaxy's GCs have ages near 15 Gyr. Ages below 12 Gyr or above 20 Gyr appear to be highly unlikely. If these 2 limits are increased by ~ 1 Gyr to account for the formation time of the globulars, and if standard Friedmann cosmologies with the cosmological constant set to zero are assumed, then the GC constraint on the present age of the Universe (t₀ 13 Gyr) implies that the Hubble constant H₀ 51 km s^-1 Mpc^-1 if the density parameter = 1 or 62 km s^-1 Mpc^-1 if = 0.3.

Key words: globular clusters, stellar structure, stellar evolution, ages, subdwarfs, RR Lyrae stars, chemical abundances, cosmology

Table of Contents

INTRODUCTION

THE STELLAR EVOLUTION CLOCK

Uncertainty in L^TO Due to Basic Stellar Physics Inputs

Uncertainty in L^TO Due to Additional Physics Usually ignored

Uncertainty in L^TO Due to Unconventional Physics

Uncertainty in L^TO Due to the Assumed Chemistry of Stars

Tests of Stellar Models

The First Estimates of Globular Cluster Ages

GLOBULAR CLUSTER DISTANCES

Subdwarf-Based Distances

Distances Based on RR Lyraes

SUMMARY

REFERENCES

¹ Killam Research Fellow. Back.