An absolute age of about ten billion years is determined for the globular cluster 47 Tucanae, which is about two billion years younger than the cluster NGC 6397 is thought to be, suggesting that metal-rich clusters like 47 Tucanae formed later than metal-poor clusters like NGC 6397.
- B. M. S. Hansen
- J. S. Kalirai
- P. B. Stetson
