|Right Ascension||12 : 03: (h:m)
|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):
|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 |
|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 |
|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|
Last Modification: 1 Aug 1999, 21:50 MET