Star motions may alter view of galaxy.

Star motions may after view of galaxy

The motions of the stars in our galaxyyield information relevant to many astrophysical and cosmological questions, particularly those involving the structure and evolution of the galaxy itself. In astronomers' long history of studying such motions, the latest entry is a particularly large one, the just completed Lick Northern Proper Motion (NPM) program of the University of California's Lick Observatory (headquartered on the university's Santa Cruz campus). The NPM will catalog the proper motions of thousands of stars, intending to provide an abundant statistical basis for studying a wide range of questions.

Already the first study done with informationfrom the NPM has found anomalies in the rates at which certain faint blue stars rotate around the center of the galaxy; this in turn raises questions about astronomers' conventional view of the kinematics and evolution of the galaxy.

Proper motion is a star's motion acrossthe sky as viewed from earch. To determine proper motions, astronomers photograph the same part of the sky at intervals of years and compare the photos to see what has moved and by how much. The NPM, which is currently directed by Burton F. Jones and Arnold R. Klemola, consists of two sets of photographs of the northern sky. C. Donald Shane of the Lick Observatory took the first set between 1947 and 1954; the second series began in 1971 and is now 97 percent complete. The more recent observers used the same photographic emulsion Shane used 40 years ago to ensure comparability of the plates. A companion study of the southern sky is being done by Yale University Observatory and the National University of San Juan, Argentina.

Traditionally, two plates of the samefield of the sky--from different years-- are put into a machine called a blink comparator, which shifts a human observer's vision rapidly between them. The observer notes which images "move' and marks them down for measurement. Although a blink comparator is still used to select objects for study, the measuring is done by an automatic machine developed by Stanislavs Vasilevskis of Lick. Motions of the stars are measured against a background of distant galaxies, 40,000 galaxies being used as a reference for the motions of 300,000 stars. The final catalog will list stars according to many classes of interest to astrophysicists. Klemola reads the literature to determine such interest and enters the appropriate classes into the program.

In what he calls "a first scratching ofthe surface' of the information, Lick Associate Research Astronomer Robert B. Hanson used the proper motions of 60,000 stars to study the rotation of the galaxy. The sun rotates around the center of the galaxy, and so do other stars in the flat disk of the galaxy. As the sun moves along, the proper motions of nearby stars show a streaming effect: They move toward us from the direction to which the sun is going and away from us in the direction from which the sun has come. Hanson found that for a group of 16th-magnitude blue stars lying somewhat above and below the disk, the streaming effect seems wrong: Either the sun is not going where astronomers think it is going or these stars are lagging behind the general rotation. Because the sun's motion is confirmed by other studies, Hanson concludes that these blue stars are lagging.

Astronomers have believed that thegalaxy consists of two main components, the central sphere or bulge and a flat disk outside it. The stars in the sphere are old and do not rotate--presumably they formed before the galaxy began to rotate. The stars in the plane do rotate. Hanson suggests that either something happened to the blue stars during their development that altered their kinematics, or they are a third component between the other two, and the simple two-component model of the galaxy needs adjustment.