The University Record, April 18, 1994

Big Dipper example of a slowly evolving universe

Lovers who declare their devotion to be as constant as the stars above had better beware, according to U-M astronomer Richard G. Teske.

Stars continually move around, causing familiar constellation patterns to change slowly. Careful study of these motions gives astronomers important clues to the history of our 15-billion-year-old Milky Way Galaxy.

One example of a changing constellation is the Big Dipper, Teske explains. Michigan sky watchers should look for the Dipper high in the northeast after dark, where it can be seen with its handle pointing down. Around midnight it passes nearly overhead, upside down. The Dipper’s seven stars form the hindquarters of an even larger classical constellation, the Great Bear, or Ursa Major. Bear and Dipper can be seen from Michigan every night throughout the year, circling round and round the north pole of the heavens with the rotation of the Earth.

“The inner five of the Dipper’s stars are the main members of a sparse star cluster that lies 75 light-years away,” Teske says. “Called the Ursa Major moving cluster, it is the closest one in space to the sun. This self-gravitating group has only 17 identified members, a very small number as star clusters go. All of them drift along together through space, partly approaching the sun while also moving off toward the southeast.”

Teske says two stars—the star at the end of the Dipper’s handle and the star at the lip of its bowl—are not members of the moving group.

“These two stars are bound on an independent journey across the sky in a direction opposite to that of the cluster. Consequently the shape of the constellation is being distorted. The relative motion of these stars is what caused the present-day Dipper to slowly assemble itself in the first place, and it will also eventually destroy the star pattern we see today.”

Since the movements of the Dipper’s stars are leisurely and fairly average for those in space around the sun, the constellation’s form hasn’t changed much during recorded human history.

“It will take about 20,000 years for its members to traverse a distance equal to the full Moon’s width on the sky, so the Dipper will remain recognizable for thousands of years yet to come,” Teske says.

Teske explains that star movements are detected and measured by comparing photographs taken many years apart. Because celestial photography has been practiced for almost 150 years, a great deal of accurate material has been accumulated.

When the star movement measurements are combined with other information, they provide crucial evidence for astronomers’ research on the origin and history of the Milky Way spiral galaxy in which the sun and its planets reside.

“A thorough examination of star motions in the vicinity of the sun gives a detailed picture of the swirling rotation of our galaxy, the Milky Way. This information helps to provide an estimate for the total amount of material in the galaxy, and suggests the total number of stars that inhabit it—around 200 billion,” Teske explains.

“Most of these stars are gathered in a dense disk of bright stars—something like a frisbee—with spiral arms and a fat bulge at its center. A few of the galaxy’s stars inhabit a huge spherical region that surrounds the disk, like a basketball around the frisbee.”