New Science from an Old Telescope:
Photographing Star Clusters with the Great Lick Refractor

Although most of the telescopes on Mount Hamilton are used for research, the Great Lick Refractor, once the world's most powerful telescope, is now primarily a tool for education and public access, and an awe-inspiring landmark in the history of astronomy. However, in fall 2000, astronomers began a new program, photographing clusters of stars in our Milky Way Galaxy with the Great Refractor.

Stars are formed in dense clouds of gas and dust called nebulae. As star formation proceeds, the gas and dust are used up or blown away, leaving behind groups of newborn stars. Galactic or open clusters, as they are called, contain from a few dozen to thousands of stars.

There are many galactic clusters in the Milky Way, most too faint to be seen without a telescope. The best known galactic cluster is the familiar grouping of stars called the Pleiades, or Seven Sisters, seen in the winter sky in the constellation Taurus.

Star clusters are important for the study of stellar evolution, but it is necessary to distinguish their members from stars in the foreground or background that happen to lie in the same direction. To do so, astronomers take advantage of the fact that stars in a cluster move through space together, setting them apart as a group. A star sharing the cluster's motion is thus established as a member of that cluster.

A star's motion across the sky is very small as viewed by a distant observer on Earth. For most stars, decades must pass before their motions can be measured. With this in mind, a number of clusters were photographed in the 1970's using the 36-inch refractor and an instrument called the Automatic Camera.

Now, a generation later, enough time has elapsed to reveal the motions of the cluster stars. New photographs are now being made, using the same telescope and camera to facilitate comparison. New star positions will be measured and compared to their positions on the earlier photographs, revealing which stars belong to the cluster.

This program will continue for several years, until all the clusters photographed a generation ago have been repeated. The new data will provide astronomers with better cluster membership information, making studies which rely on individual observations of cluster stars more accurate and reliable.

The photograph of the Orion nebula, courtesy Hubble Space Telescope, photograph of the Pleiades, courtesy Jason Ware (