Ursa Major Moving Group

The Ursa Major Moving Group, also known as Collinder 285 or Ursa Major association, is a nearby stellar moving group – a set of stars with common velocities in space and thought to have a common origin in space and time. In the case of the Ursa Major group, all the stars formed about 300 million years ago.[1] 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 moving in roughly the same direction and at roughly the same speed, containing 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 numbers 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. Since then, the sparse group has 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 Earth.

The Ursa Major Moving Group was discovered in 1869 by Richard A. Proctor, who noticed that, except for Dubhe and Alkaid (Eta Ursae Majoris), the stars of the Big Dipper asterism all have proper motions heading towards a common point in Sagittarius. Thus, the Big Dipper, unlike most constellations or asterisms, is 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. A study published in 2003 using data gathered by the Hipparcos satellite (1989-1993) 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.[1] These stars are all in Ursa Major except where indicated.

Name Constellation B F HD HIP vis.
mag.		 Dist. (ly) Sp. class Notes
ε UMa Ursa Major ε 77 112185 62956 1.76 81 A0p Alioth
ζ UMa A Ursa Major ζ 79 116656 65378 2.23 78 A2V Mizar, Mizat, Mirza, Mitsar, Vasistha
(quadruple system)
β UMa Ursa Major β 48 95418 53910 2.34 79 A1V Merak, Mirak
γ UMa Ursa Major γ 64 103287 58001 2.41 84 A0V SB Phad, Phecda, Phegda, Phekha, Phacd
δ UMa Ursa Major δ 69 106591 59774 3.32 81 A3V Megrez, Kaffa
ζ UMa B Ursa Major ζ 79 116657 3.95 Part of Mizar quadruple system
80 UMa Ursa Major g 80 116842 65477 3.99 81 A5V SB Alcor, Saidak, Suha, Arundhati
(binary system)
78 UMa Ursa Major 78 113139 63503 4.93 81 F2V
37 UMa Ursa Major 37 91480 51814 5.16 86 F1V
HD 115043 Ursa Major 115043 64532 6.82 84 G1V Gliese 503.2
HD 109011 Ursa Major 109011 61100 8.10 77 K2V NO UMa
HD 110463 Ursa Major 110463 61946 8.28 76 K3V NP UMa
HD 109647 Canes Venatici 109647 61481 8.53 86 K0 DO CVn

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).

Name Constellation B F HD HIP vis.
mag.		 Dist. (ly) Sp. class Notes
β Aur Auriga β 34 40183 28360 1.90 82 A2V Menkalinan, Menkalina
α CrB Corona Borealis α 5 139006 76267 2.22 75 A0V Alphecca, Alphacca, Alphekka, Gemma, Gnosia, Gnosia Stella Coronae, Asteroth, Ashtaroth
δ Aqr Aquarius δ 76 216627 113136 3.27 159 A3V Skat, Scheat, Seat, Sheat
ζ Leo Leo ζ 36 89025 50335 3.43 260 F0III Adhafera, Aldhafera, Aldhafara
γ Lep A Lepus γ 13 38393 27072 3.59 29 F7V
β Ser Serpens β 28 141003 77233 3.65 153 A3V Chow
ζ Boo A Boötes ζ 30 129246 71795 3.78 180 A3IVn
χ1 Ori Orion χ1 54 39587 27913 4.39 28 G0V
ζ Boo B Boötes ζ 30 129247 4.43 180 A2III
21 LMi Leo Minor 21 87696 49593 4.49 91 A7V
χ Cet A Cetus χ 53 11171 8497 4.66 77 F3III
γ Mic Microscopium γ 39 199951 103738 4.67 223 G8III
ζ Crt Crater ζ 27 102070 57283 4.71 350 G8III
ζ TrA Triangulum Australe ζ 31 147584 80686 4.90 39 F9V
16 Lyr Lyra 16 177196 93408 5.00 128 A7V
66 Tau Taurus r 66 27820 20522 5.10 396 A3V
59 Dra Draco 59 180777 94083 5.11 89 A9V
89 Psc Pisces f 89 7804 6061 5.13 220 A3V
HD 75605 Pyxis 75605 43352 5.19 229 G8III
18 Boo Boötes 18 125451 69989 5.41 85 F5IV
HD 109799 Hydra 109799 61621 5.41 113 F0V
π1 UMa Ursa Major π1 3 72905 42438 5.63 47 G1.5Vb Muscida
HD 220096 Sculptor 220096 115312 5.65 329 G5IV
29 Com Coma Berenices 29 111397 62541 5.71 402 A1V
HD 18778 Cepheus 18778 14844 5.92 202 A7III-IV
HD 165185 Sagittarius 165185 88694 5.94 57 G3V Gliese 702.1
6 Sex Sextans 6 85364 48341 6.01 200 A8III
HD 171746 Hercules 171746 91159 6.21 112 G2Vv comp
HD 26932 Taurus 26932 6.23 69 G0IV
HD 129798 Draco 129798 71876 6.24 139 F2V DL Dra
41 Vir Virgo 41 112097 62933 6.25 199 A7III
χ Cet B Cetus χ 53 11131 8486 6.72 78 G0 EZ Cet
HD 71974 A Lynx 71974 41820 7.51 94 G5
HD 59747 Lynx 59747 36704 7.70 64 G5 DX Lyn
HD 28495 Camelopardalis 28495 21276 7.76 90 G0 MS Cam
HD 173950 Lyra 173950 92122 8.08 121 G5 V595 Lyr
HIP 66459 Canes Venatici 66459 9.06 36 K5 Gliese 519
HD 71974 B Lynx 71974 41820 9.09 94
HD 95650 Leo 95650 53985 9.68 38 M0 DS Leo, Gliese 410
HD 238224 Ursa Major 238224 65327 9.72 82 K5 Gliese 509.1
HD 13959 Cetus 13959 10552 9.76 124 K2 Gliese 91.1
HD 156498 Ophiuchus 156498 84595 9.98 271 V2369 Oph

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 the Ursa Major group.

References

  1. 1 2 3 King, Jeremy R.; et al. (April 2003), "Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group", The Astronomical Journal, 125 (4): 1980–2017, Bibcode:2003AJ....125.1980K, doi:10.1086/368241

External links

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