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)

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.

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 (400 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 located in the outskirts but within the extent of this stellar stream.

A table of some brighter cluster members follows (from Burnham, p. 1946):
HD num Star RA Dec mag abs.M Sp Notes
112185 Epsilon UMa 12:51.8 +56:14 1.79 -0.3 A0p Spectrum var
95418 Beta UMa 10:58.8 +56:39 2.37 +0.3 A1
116656 Zeta UMa (A) 13:21.9 +55:11 2.40 +0.3 A2 Mizar A; binary with B
116657 Zeta UMa (B) 13:21.9 +55:11 3.96 +2.0 A7 Mizar B; binary with A
103287 Gamma UMa 11:51.2 +53:58 2.44 +0.2 A0
106591 Delta UMa 12:13.0 +57:19 3.30 +1.2 A3
116842 80 UMa 13:23.2 +55:15 4.02 +1.9 A5 Alcor
97696 21 LMi 10:04.5 +35:29 4.47 +2.2 A7
124752 GC 19195 UMa 14:11.3 +67:49 8.2 +6.8 K0
113139 78 UMa 12:58.6 +56:38 4.89 +3.0 F2 Close binary
91480 37 UMa 10:32.0 +57:20 5.16 +2.9 F1
111456 GC 17404 UMa 12:46.5 +60:36 5.87 +3.7 F5
129798 Sigma1878 Dra 14:40.8 +61:29 6.17 +2.9 F2 Binary
115043 GC 17919 UMa 13:11.6 +56:58 6.74 +4.7 G1
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
139006 Alpha CrB 15:32.6 +26:53 2.23 +0.4 A0+dG6 Uncertain member

Hartmut Frommert (
Christine Kronberg (

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Last Modification: 1 Aug 1999, 21:50 MET