The Ursa Major Moving Cluster, Collinder 285

the Ursa Major Moving Cluster,

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

Last Modification: 1 Aug 1999, 21:50 MET