The age of the universe
Introduction
A direct estimate for the minimum age of the universe may be obtained by determining the age of the oldest objects in the Milky Way. This direct estimate for the age of the universe can be used to constrain cosmological models, as the expansion age of the universe is a simple function of the Hubble constant, average density of the universe and the cosmological constant. The oldest objects in the Milky Way are the metal-poor stars located in the spherical halo. There are currently three independent methods used to determine the ages of these stars: (1) nucleochronology, (2) white dwarf cooling curves and (3) main sequence turn-off ages based upon stellar evolution models. In this review I will summarize recent results from these three methods, with particular emphasize on main sequence turn-off ages as they currently provide the most reliable estimate for the age of the universe.
Section snippets
Nucleochronology
Conceptually, the simplest way to determine the age of a star is to use the same method which have been used to date the Earth – radioactive dating. The age of a star is derived using the abundance of a long lived radioactive nuclei with a known half-life (see, e.g., the review [1]). The difficulty in applying this method in practice is the determination of the original abundance of the radioactive element. The best application of this method to date has been on the very metal-poor star CS
White dwarf cooling curves
White dwarfs are the terminal stage of evolution for stars less massive than ∼8M⊙. As white dwarfs age, they become cooler and fainter. Thus, the luminosity of the faintest white dwarfs can be used to estimate their age. This age is based upon theoretical white dwarf cooling curves 3, 4, 5. There are a number of uncertainties associated with theoretical white dwarf models, which have been studied in some detail. However, the effect of these theoretical uncertainties are generally not included
Main sequence turn-off ages
Theoretical models for the evolution of stars provide an independent method to determine stellar ages. These computer models are based on stellar structure theory, which is outlined in numerous textbooks 8, 9. One of the triumphs of stellar evolution theory is a detailed understanding of the preferred location of stars in a temperature–luminosity plot (Fig. 1).3
Summary
A direct estimate for the minimum age of the universe can be obtained by determining the age of the oldest objects in the galaxy. These objects are the metal-poor stars located in the halo of the Milky Way. There are currently three independent techniques which have been used to determine the ages of the metal-poor stars in the Milky Way: nucleochronology, white dwarf cooling theory, and main sequence turn-off ages. The best application of nucleochronology to date has been on the very
Acknowledgements
The author was supported for this work by NASA through Hubble Fellowship grant number HF-01080.01-96A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract NAS 5-26555.
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