The Ursa Major Moving Cluster, Collinder 285

Open Cluster Collinder 285, the Ursa Major Moving Cluster, in and around Ursa Major

[Big Dipper]
Right Ascension 12 : 03: (h:m)
Declination +58: (deg)
Distance 0.076 (kly)
Visual Brightness 0.4 (mag)
Apparent Dimension 1400 (arc min)

Brighter stars known pre-historically. Cluster nature discovered by R.A. Proctor 1869 and Huggins 1872.

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. In case of this cluster, the motion is eastward and south, toward a convergence point about 130 degrees away in eastern Sagittarius, approximately at RA 20:24 and Dec -37. 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. However, this assumption is in discord with the age of 160 million years given in the Sky Catalogue 2000.0.

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.
A table of some brighter cluster members follows (from Burnham, p. 1946, precessed to 2000.0):
HD num Star        Con  RA (2000.0) Dec  mag  abs.M  Sp     Notes

87696 21 LMi 10:07.4 +35:15 4.47 +2.2 A7 91480 37 UMa 10:35.3 +57:05 5.16 +2.9 F1 95418 48 Beta UMa 11:01.8 +56:23 2.37 +0.3 A1 Merak 103287 64 Gamma UMa 11:53.8 +53:42 2.44 +0.2 A0 Phecda 106591 69 Delta UMa 12:15.4 +57:02 3.30 +1.2 A3 Megrez 109011 HD 109011 UMa *12:28.8 +55:24 8.1 +6.1 K2 110463 HD 110463 UMa *12:39.5 +56:01 8.4 +6.3 K3 111456 GC 17404 UMa 12:48.7 +60:19 5.87 +3.7 F5 112185 77 Epsilon UMa 12:54.0 +55:57 1.79 -0.3 A0p Alioth; Spectrum var 113139 78 UMa 13:00.7 +56:22 4.89 +3.0 F2 Close binary 115043 GC 17919 UMa 13:13.6 +56:42 6.74 +4.7 G1 116656 79 Zeta (A) UMa 13:23.9 +54:56 2.40 +0.3 A2 Mizar A; binary with B 116657 79 Zeta (B) UMa 13:23.9 +54:55 3.96 +2.0 A7 Mizar B; binary with A 116842 80 UMa 13:25.2 +54:59 4.02 +1.9 A5 Alcor 124752 GC 19195 UMa *14:11.3 +67:49 8.2 +6.8 K0 129798 Struve 1878 Dra 14:42.5 +61:16 6.17 +2.9 F2 Binary 139006 5 Alpha CrB 15:34.7 +26:43 2.23 +0.4 A0+dG6 Alphecca; Uncertain member
  • Image of Ursa Major from the International Space Station, by Donald R. Pettit, March 2003 - the "bowl" part of the dipper

  • SIMBAD Data of Collinder 285
  • Publications on Collinder 285 (NASA ADS)
  • Observing Reports for Collinder 285 (IAAC Netastrocatalog)

    Hartmut Frommert
    Christine Kronberg

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    Last Modification: April 26, 2003