Ursa Major moving group

The Ursa Major Moving Group, also known as Collinder 285 and the Ursa Major association, is the closest 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 and part of the Local Bubble. It is rich in bright stars including most of the stars of the Big Dipper.

Map of stars and open clusters within 100 parsecs of the Sun. The Ursa moving group is near the center at 120° galactic longitude.

Discovery and constituents

All stars in the Ursa Major Moving Group are moving in roughly the same direction at similar velocities, and also have similar chemical compositions and estimated ages. 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 about 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 below.

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 average apparent magnitude of all 14 core stars is approximately 4.42. None of these stars are hotter than spectral class A, but stream stars may be more massive and hotter.

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 UMaUrsa Majorε77112185629561.7681A0pAlioth
ζ UMa AUrsa Majorζ79116656653782.2378A2VMizar, Mizat, Mirza, Mitsar, Vasistha
(quadruple system)
β UMaUrsa Majorβ4895418539102.3479A1VMerak, Mirak
γ UMaUrsa Majorγ64103287580012.4184A0V SBPhad, Phecda, Phegda, Phekha, Phacd
δ UMaUrsa Majorδ69106591597743.3281A3VMegrez, Kaffa
ζ UMa BUrsa Majorζ791166573.95Part of Mizar quadruple system
80 UMaUrsa Majorg80116842654773.9981A5V SBAlcor, Saidak, Suha, Arundhati
(binary system)
78 UMaUrsa Major78113139635034.9381F2V
37 UMaUrsa Major3791480518145.1686F1V
HD 115043Ursa Major115043645326.8284G1VGliese 503.2
HD 109011Ursa Major109011611008.1077K2VNO UMa
HD 110463Ursa Major110463619468.2876K3VNP UMa
HD 109647Canes Venatici109647614818.5386K0DO 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 AurAurigaβ3440183283601.9082A2VMenkalinan, Menkalina
α CrBCorona Borealisα5139006762672.2275A0VAlphecca, Alphacca, Alphekka, Gemma, Gnosia, Gnosia Stella Coronae, Asteroth, Ashtaroth
δ AqrAquariusδ762166271131363.27159A3VSkat, Scheat, Seat, Sheat
ζ LeoLeoζ3689025503353.43260F0IIIAdhafera, Aldhafera, Aldhafara
γ Lep ALepusγ1338393270723.5929F7V
β SerSerpensβ28141003772333.65153A3VChow
ζ Boo ABoötesζ30129246717953.78180A3IVn
χ1 OriOrionχ15439587279134.3928G0V
ζ Boo BBoötesζ301292474.43180A2III
21 LMiLeo Minor2187696495934.4991A7V
χ Cet ACetusχ531117184974.6677F3III
γ MicMicroscopiumγ391999511037384.67223G8III
ζ CrtCraterζ27102070572834.71350G8III
ζ TrATriangulum Australeζ31147584806864.9039F9V
16 LyrLyra16177196934085.00128A7V
66 TauTaurusr6627820205225.10396A3V
59 DraDraco59180777940835.1189A9V
89 PscPiscesf89780460615.13220A3V
HD 75605Pyxis75605433525.19229G8III
HD 27022Camelopardalis27022202665.27340G4III or G5IIb
18 BooBoötes18125451699895.4185F5IV
HD 109799Hydra109799616215.41113F0V
π1 UMaUrsa Majorπ1372905424385.6347G1.5VbMuscida
HD 220096Sculptor2200961153125.65329G5IV
29 ComComa Berenices29111397625415.71402A1V
HD 18778Cepheus18778148445.92202A7III-IV
HD 165185Sagittarius165185886945.9457G3VGliese 702.1
6 SexSextans685364483416.01200A8III
HD 171746Hercules171746911596.21112G2Vv comp
HD 26932Taurus269326.2369G0IV
HD 129798Draco129798718766.24139F2VDL Dra
41 VirVirgo41112097629336.25199A7III
χ Cet BCetusχ531113184866.7278G0EZ Cet
HD 71974 ALynx71974418207.5194G5
HD 59747Lynx59747367047.7064G5DX Lyn
HD 28495Camelopardalis28495212767.7690G0MS Cam
HD 173950Lyra173950921228.08121G5V595 Lyr
HIP 66459Canes Venatici664599.0636K5Gliese 519
HD 71974 BLynx71974418209.0994
HD 95650Leo95650539859.6838M0DS Leo, Gliese 410
HD 238224Ursa Major238224653279.7282K5Gliese 509.1
HD 13959Cetus13959105529.76124K2Gliese 91.1
HD 156498Ophiuchus156498845959.98271G5V2369 Oph

Non-members

The Big Dipper stars Dubhe (α UMa) and Alkaid (η UMa) are not members of the group, both being somewhat further away and moving in very different directions.

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, and is moving in the same direction as the group by mere coincidence.

The Solar System is in the outskirts of this stream, but is not a member, being about 15 times older. The Sun drifted in along its 250-million-year galactic orbit, and 40 million years ago was not near the Ursa Major group.

See also

[2]

References

  1. 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
  2. Yadav, Durgesh. {{cite book}}: Missing or empty |title= (help)
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