dipper@utastro.UUCP (Debbie Byrd) (10/01/84)
Most stars in constellations aren't really connected -- but some stars in the Big Dipper are. We'll tell you in what way -- right after this. September 25: The Ursa Major Group Most stars in constellations aren't really neighbors to each other. They're located at vastly different distances from Earth -- bound only by our line of sight -- and by the imaginations of the early constellation-makers. But the most famous star pattern is the Big Dipper -- and five of the seven Big Dipper stars do belong to a single family. They were born around the same time, and they all move together through the galaxy. They're members of a moving group of stars called the Ursa Major group. If you could study the group with a telescope, you'd find that it contains as many as 15 stars. These stars occupy a volume of space approximately 30 light-years long, and 20 light-years wide. The Ursa Major group is fairly sparse. But it's the nearest group of its kind to the sun, with its center located only 80 light-years away. Again, these stars are distinctive because they move together through space. Late in the 19th century, astronomers noticed something else -- many other stars that we see from Earth have approximately the same space motion as the stars in the Ursa Major group. Our sun is surrounded on all sides by this much larger grouping of stars, which is known as the Ursa Major STREAM. Bright stars in all parts of our sky belong to the stream, including Sirius, the brightest star in our sky. Their motion is a clue to some common bond or origin -- whose exact history has yet to be revealed. Script by Deborah Byrd. (c) Copyright 1983, 1984 McDonald Observatory, University of Texas at Austin