Globular clusters such as M92 above contain the oldest stars we can easily identify near the Milky Way. Here is what a page at the Students for the Exploration of Space has to say about it:
According to newer sources, M92 is about 26,000 light years distant, only little more than its brighter apparent neighbor M13. From its HRD (or CMD), it may be a bit younger than M13 as its turnoff point is shifted to the brighter and bluer end. A semi-recent estimate of M92's age has given a value of about 16 billion years (anyway more than 14 billion years), see e.g. the diagram in Sky & Telescope, January 1996, p. 22 (text on p. 20). However, this value is now again under discussion because of the general modifications of the distance scale of the universe, implied by results of ESA's astrometrical satellite Hipparcos: These results suggest that M92, as well as most other globular clusters, may be at a 10 per cent larger distance; therefore, the intrinsical brightness of all their stars must be about 20 % higher. Considering the various relations which are important for understanding stellar structure and evolution, they should also be roughly 15 % younger, in a preliminary off-hand estimate.
If you have read any of the related questions in my Cosmology Questions area, you will see that there is now a controversy over whether we can properly date really old stars. We thought we knew how to date the age of a star cluster by looking at the point in the main sequence where the upper main sequence for the cluster comes to an end and 'turns up' to the giant branch. The mass of the stars at this turn off point indicate an age because massive stars evolve faster than less massive ones further down the main sequence.
Detailed evolutionary models for some globular clusters have consistently suggested that in our own galaxy, the oldest stars are from 12 - 16 billion years old. But this now [as of 1996] contradicts the 'expansion age' of the universe based on the value for the Hubble Constant being determined by the Hubble Space Telescope. There is much that can go wrong in the observational arena, so we should not be too worried YET about this disagreement. But if it continues a few more DECADES as new data come in, we may have to rethink how to date very old stars; or to rethink how the Hubble Constant relates to an age for the universe. Either solution will be a tremendous discovery, confronting one great set of theories or another.
[Note added July, 2000] The above answer was the best we could do back in 1996 what the above answer was provided, but as a result of recent studies of distant supernova, astronomers have now begun to detect that the expansion of the universe is accelerating. This means that the Hubble Constant does not give the full story about the age of the universe, and that the universe is OLDER than the estimates based solely on the local value of the Hubble Constant. This also means that the universe is comfortably OLDER than the oldest stars in the globular clusters.
Copyright (C) 1997 Dr. Sten Odenwald
Return to LOCAL version of Ask the Astronomer.