As earth hurtles through space, on its annual journey around the sun, we are treated to an ever-changing view of the night sky. For any given time of night, the frame shifts gradually from east to west and, due to the earth's rotation, the stars appear to move from east to west throughout the night.
One of the more striking features of the Northern Hemisphere's winter sky is the Pleiades cluster; also known as "The Seven Sisters," due to the prominant, hot blue stars within the group, this bright, compact cluster is "M45" to astronomers. Visible to the naked eye, the Pleiades begin to rise above the eastern horizon in late October and are now high in the eastern sky by 9PM. Viewed with binoculars, this cluster is seen to contain many more stars that the Seven Sisters and has a hazy, nebular appearance; this, in fact, is not a star factory but, rather, a hydrogen cloud through which the Pleiades are currently moving. The cluster is actually composed of nearly 1000 stars, many of which are brown dwarfs.
Perhaps the most interesting feature of the Pleiades cluster is its youth, having formed just 100 million years ago, during the reign of Tyrannosaurus rex! Currently some 440 light years from Earth, this brilliant star cluster is (from our perspective) moving toward the Orion constellation and will eventually be torn apart by gravitational forces. For now, unlike most of the star patterns that man has named, the Pleiades are truly grouped in space, not merely an optical illusion produced by earth-based viewing.
