Mathematics – Logic
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........12m&link_type=abstract
Thesis (PH.D.)--BOSTON UNIVERSITY, 1995.Source: Dissertation Abstracts International, Volume: 56-08, Section: B, page: 4374.
Mathematics
Logic
2
Globular Clusters
Scientific paper
An observational survey of 19 globular star clusters (chosen to have a wide range in properties) was undertaken to probe the fine details of current stellar evolution theory, derive improved values for the properties of galactic globular clusters and constrain possible models for the formation of our galaxy. Using the 0.9-meter telescopes at the Kitt Peak National Observatory and Cerro Tololo InterAmerican Observatory, equipped with wide-field digital detectors (CCD's), it was possible to measure the magnitudes and colors of stars across the entire area of almost all the clusters, yielding photometry averaging in excess of 10,000 stars per cluster. The resulting color-magnitude diagrams show extremely well-defined sequences, necessary for detailed comparison with theoretical models. The analysis included the development of computer software to determine, in an unbiased fashion, fiducial lines through the principal sequences of the clusters, taking into account the effects of observational errors and stars not belonging to the clusters. Other procedures were developed for counting the numbers of stars along the fiducial sequences and at various evolutionary stages. The same analysis was made of the theoretical models, thereby providing a series of tests of critical stages of the theory. The analysis yields a mean age for the 19 clusters of 16.0 +/- 1.84 billion years, with the range in ages being approximately 5 billion years. No significant variations in the helium abundances were found from cluster to cluster; the mean helium abundance (fraction by mass) was found to be 0.214 +/- 0.005, close to the value expected from current cosmological theory. The existence of a significant range in cluster ages supports theories of the formation of the galaxy in which at least some of the globular clusters formed in a chaotic pre-galactic era. The dispersion in cluster ages and in their compositions (taken from published sources) are not sufficient to explain the variations found in the character of the observed color-magnitude diagram sequences. At least one additional physical property must vary from cluster to cluster.
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