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Constellation: Ursa Major Designation Proper Name (alpha) (beta) (gamma) (delta) (epsilon) (zeta) (eta) Dubhe Merak Phekda Megrez Alioth Mizar Alkaid Apparent Magnitude m 1.81 2.34 2.41 3.32 1.76 2.23 1.85 Which star appears brightest? Which star is hottest? Absolute Magnitude M -1.09 +0.41 +0.36 +1.33 -0.22 +0.33 -0.6 Distance lyrs 124 79 84 81 81 78 101 Dimmest? Coolest? Which star is the most luminous? Least luminous? Why does Alkaid appear so dim given that it is very luminous? Which stars are main sequence stars? Why does Duhbe appear so bright? (two reasons) Spectral Type Luminosity Class K0III A1 V A0 V A3 V A0 p A1 V B3 V Single or Multiple Star System visual binary single single single spec. triple visual binary single The Ursa Major Moving Cluster, Collinder 285 Open Cluster Collinder 285, the Ursa Major Moving Cluster, in and around Ursa Major Right Ascension 12 : 03: (h:m) Declination +58: (deg) Distance 0.076 (kly) Visual Brightness 0.4 (mag) Apparent Dimension 1400 (arc min) http://www.seds.org/messier/Pics/Jpg/bigdipper.jpg Brighter stars known pre-historically. Cluster nature by R.A. Proctor 1869 and Huggins 1872 100,000 AD discovered Most of the stars making up the Big Dipper show a common proper motion, as R.A. Proctor has found as early as 1869 (see e.g. Burnham). When W. Huggins, in 1872, determined their radial velocities from their spectra, it became apparent that they move approximately in the same spatial direction, and thus drift commonly through their cosmic neighborhood - a property typically found for members of a physical star cluster. The cluster is currently approaching us at 10 km/sec, and its spatial velocity relative to our Sun is about 46 km/s. This cluster is centered at a distance of about 75 light years from us (i.e., our solar system). As it is spread over a volume of 30 light years length and 18 light years width, it covers an enourmous portion of the sky, and probably includes the outlying member Alpha Coronae Borealis, which is 30 degrees off. The stars are similar to those found in the Hyades and Praesepe (M44), indicating that this cluster is of roughly the same age (700-800 million years) as the other two. Studies of the motions of nearby stars have revealed that a considerable number of conspicuous stars in our neighborhood show motion in about the same direction in space, i.e. drift together with the Ursa Major cluster, although they are spread over the whole sky. These stars include Sirius (alpha Canis Majoris), Alpha Ophiuchi, Delta Leonis, and Beta Aurigae, together with about 100 fainter stars. It seems that these stars are lost "former" cluster members which have their origin in the Ursa Major cluster, but escaped due to mutual encounters, tidal forces of the Milky Way, or encounters with large interstellar clouds and other clusters. Now as they have left the cluster, their orbits around the Milky Way Galaxy's center is still similar to that of the cluster so that they have a common motion. All these stars are sometimes referred to as the Ursa Major Stream, which reaches out to more than 100 light years from the cluster's center. Our Solar System is currently located in the outskirts but within the extent of this stellar stream, but it is clearly not a member of this stream. Instead, it is a chance encounter, because 1. the cluster stars are much younger than the solar system's age of about 4.6 billion years, 2. the stars have a common proper motion, which our Sun doesn't share, as they pass us at about 46 km/s. Very Cool Movie of the changing shape of the Big Dipper at http://www-astronomy.mps.ohiostate.edu/~pogge/Ast162/Movies/proper.html