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The Ursa Major Moving Group, also known as Collinder 285, is the closest moving group to Earth, that is, a set of stars with common velocities in space, thought to have a common origin. Its core is located roughly 80 light years away. It is rich in bright stars, including most of the stars of the Big Dipper. Discovery and constituents All stars in the Ursa Major Moving Group are in roughly the same location in the Milky Way Galaxy, are moving in roughly the same direction at roughly the same speed, contain roughly the same mix of metals, and, based on stellar theory, appear to be roughly the same age. This evidence suggests to astronomers that the stars in the group share a common origin. Based on the ages of its constituent stars, the Ursa Major Moving Group is believed to have once been an open cluster, having formed from a protostellar nebula approximately 500 million years ago, which is fairly young. Since that distant time in the past, the sparse group has been scattered over a region about 30 by 18 light-years, whose center is currently some 80 light-years away, making it the closest cluster-like object to the Earth. The Ursa Major Moving Group was discovered in 1869 by Richard A. Proctor, who noticed that, except for Dubhe and Alkaid, the stars of the Big Dipper asterism all have proper motions heading towards a common point in Sagittarius. Thus, the Big Dipper, unlike many constellations or asterisms, is actually largely composed of related stars. Some of the brighter stream members include Alpha Coronae Borealis (α CrB or Alphecca or Gemma), Beta Aurigae (β Aur), Delta Aquarii (δ Aqr), Gamma Leporis (γ Lep) and Beta Serpentis (β Ser). More bright and moderately bright stars which are currently believed to be members of the group are listed in two sections below: Core stars and Stream stars. Group members Current criteria for membership in the moving group is based on the stars' motion in space. This motion can be determined from the proper motions and parallax (or distance) to the stars and radial velocities. The Hipparcos satellite has recently greatly improved both the proper motion and parallax estimates of nearby bright stars, refining the study of this and other moving groups [1]. Based on their distances (measured with Hipparcos) and apparent magnitude, the absolute magnitude can be used to estimate the age of the stars. The stars in the moving group appear to have a common age of about 500 million years. Core stars The core of the moving group consists of 14 stars, of which 13 are in the Ursa Major constellation and the other is in the neighboring constellation of Canes Venatici. The following are members of the moving group closest to its center. These stars are all in Ursa Major except where indicated. * 37 Ursae Majoris (HD 91480) * Beta Ursae Majoris (Merak) (HD 95418) * Gamma Ursae Majoris (Phecda) (HD 103287) * Delta Ursae Majoris (Megrez) (HD 106591) * HD 109011 * HD 109647 (in Canes Venatici) * HD 110463 * Epsilon Ursae Majoris (Alioth) (HD 112185) * 78 Ursae Majoris A (HD 113139A) * Gliese 503.2 (HD 115043) * Zeta Ursae Majoris (Mizar) A (HD 116656) * Zeta Ursae Majoris (Mizar) B (HD 116657) * 80 Ursae Majoris (Alcor) (HD 116842) Stream stars There is also a "stream" of stars which are likely members of the Ursa Major Moving Group, scattered more widely across the sky (from Cepheus to Triangulum Australe). Only stars with Bayer designations or Flamsteed designations are listed here. * Delta Aquarii * Beta Aurigae (Menkalinan) * Zeta Boötis * 18 Boötis * Chi Ceti * Zeta Crateris * 29 Comae Berenices * Alpha Coronae Borealis (Alphecca or Gemma) * 59 Draconis * 21 Leonis Minoris * Gamma Leporis * 16 Lyrae * Gamma Microscopii * Chi1 Orionis * 89 Piscium * Beta Serpentis * Tau-6 Serpentis * Omega Serpentis * 6 Sextantis * 66 Tauri * Zeta Trianguli Australis * Pi1 Ursae Majoris * 41 Virginis Non-members The bright, nearby star Sirius was long believed to be a member of the group, but may not be, according to research in 2003 by Jeremy King et al. at Clemson University. This research seems to indicate that it is too young to be a member. Our Solar System is in the outskirts of this stream, but is not a member, being about 10 times older. Our Sun merely drifted in along its 250 million year galactic orbit, and 40 million years ago was nowhere near these stars. Links
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