Star clusters shine in Michigan’s winter sky
ANN ARBOR—In January, Michigan’s evening sky features a fine view of two glittering star clusters called the Hyades and Pleiades, says University of Michigan astronomer Richard Teske.
“The Hyades and Pleiades clusters are groups of stars that remain together for hundreds of millions of years under the influence of their mutual gravitational attractions,” Teske said. “Both clusters have been known and regarded with reverence since antiquity when early Greeks began to tell myths about them. Both have an important place in modern astronomy—the Hyades supplying a crucial touchstone in programs to measure distances in the Universe, the Pleiades helping to confirm mathematical theories about how stars age and die.”
At only 140 light-years distance from Earth, the Hyades stars are so close to us, it is difficult to perceive them as a distinct group against the sky, because they seem to spread out so much. Well over 250 stars populate the cluster in Taurus, the Bull. The brightest of the Hyades form the V-shape that outlines the Bull’s head, Teske explained. Most of the other stars are too faint to be seen without a telescope. All of the visible Hyades are much brighter than our sun. Indeed, if the sun were placed in the cluster it would be too faint to view.
Within the V-pattern of Taurus’s head, brilliant red Aldebaran marks the glaring Bull’s eye taking a bead on Orion, the Hunter, before beginning to charge. Aldebaran is an interloper, not a true cluster member, lying halfway between us and the real Hyades.
The Pleiades cluster looks small against the sky because of its greater distance from us, 410 light-years. To the average human eye, it seems to contain six or seven stars; but, like the Hyades, has well over 250 members. Most of them are faint. “Observed with binoculars on a quiet, frosty night, the Pleiades become a dazzling jewel basket of stars,” Teske said.
The Hyades play a central role in development of methods for determining celestial distances, a distinction gained from careful measurement by astronomers of the movement of the cluster and its member stars. The cluster’s motion is accurately gauged because it is so near to us in space. In turn, this knowledge permits a very good determination of the distance to its stars.
“Comparison of the brightnesses of these stars of known distance with brightnesses of identical stars of unknown distance makes it possible to gauge distances to the unknowns. All knowledge of celestial distances, even to the farthest visible galaxies in the Universe, depends heavily, although not entirely, upon astronomers’ knowledge of how far off the Hyades are,” Teske explained.
The Pleiades are of lesser scientific importance. Astronomers use observations of stars within clusters like it to check theories of how stars grow old and die. Each cluster’s members were all formed at the same time and have stayed together ever since. Study of such populations displays how the aging process works, and the Pleiades stand out as one of the best groupings for these investigations. Its population conforms to theoretical predictions and so helps give astronomers confidence in the validity of their mathematical theories and calculations, Teske said.
The Greeks identified seven Hyades as the daughters of Atlas, who was compelled by the Gods to support the heavens on his shoulders. Zeus, the King of Gods, entrusted the care of the infant Bacchus to the seven daughters, and afterwards rewarded them for their faithfulness with a place in the heavens.
According to Greek myth, the seven Pleiades were also daughters of Atlas and half-sisters to the Hyades. They were pursued by Orion, but Zeus interfered and saved them by transforming the sisters into a group of celestial doves. “Because of its striking appearance, the Pleiades cluster appears in legends from nearly all cultures around the world from ancient Mayan and Aztec tradition to tales told among the peoples of the South Pacific islands, as well as in Western lore,” Teske said.
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