Certain open clusters, for which the individual stars have a pronounced motion toward a convergent point, are known as Moving Clusters. The Hyades is one of the earliest known and the nearest of these clusters. Except for small peculiar motions, all stars that are members of a moving cluster move in space along more or less parallel paths, much like meteors in a meteor stream. The individual proper motions of these stars seem to converge toward or diverge from a common point in the sky in the same way that meteors in showers appear to diverge from their radiants (see Meteors). The point of divergence marks the direction in space toward which the Sun is moving with respect to the cluster. The point of convergence of the proper motions (opposite the point of divergence on the celestial sphere) marks the direction toward which the cluster is moving with respect to the Sun.
The Ursa Major cluster is of particular interest to us since it occupies the same volume of space as our Sun. In fact, it is moving through our space, although the Sun is not a member of this cluster. The Ursa Major cluster is composed of two subgroups which consist of a moderately compact cluster of 14 stars with the same proper motion and an extended stream of stars which has approximately the same motion. The nucleus of this cluster is located about 23 parsecs from the Sun and occupies (roughly) ellipsoidal region 4x6x10 parsecs in diameter. The shortest diameter is perpendicular to the galactic plane, while the longest is in the direction of the motion of the cluster. The motion of the local centroid is 29 km/sec.
Another moving cluster of great interest is the Scorpio-Centaurus or Southern Stream cluster. The Sco-Cen moving cluster is a relatively ancient one as indicated by its rather elongated shape. It is also part of the local system of stars (which see) of which our Sun is a member. The total annual proper motions for the Sco-Cen cluster range from O.02" to O.05." The group formed about 70 million years ago in a region 2200 parsecs distant, in the direction of galactic longitude 59 degrees.
The seven most conspicuous open-type clusters are the Pleiades, Hyades, Praesepe, Coma Bereneces, IC 2602, NGC 3532, and Messier Objects 6 & 7. The Pleiades, perhaps the most photographed object in this universe, has been recorded in ancient times by the Chinese, Hindus, Chaldeans, and the Greeks and is mentioned in the Bible. Called the seven sisters or pigeons, only six stars can be seen today with the unaided eye. It is thought that one of the original stars may have faded since ancient times. There is somewhere between 300-500 stars in the Pleiades cluster with the center 6' west of Alcyone, the lucida, at R.A. 56° 01'01" and Declination of +23°57126." The brightest Pleiades are late B-type, an indication of the youth of this particular cluster. The Pleiades has always received much attention from astrologers and it is more than interesting to note that the position of the cluster in the Zodiac coincides with the intersection of the Galactic and Supergalactic equators, as projected onto the ecliptic.
Notes & Legend for Moving Clusters (Listed with O-Associations)
The listed position for the Moving Clusters is that of the CONVERGENT POINT relative to the Sun. This is the point toward which the cluster seems to be headed. The physical positions for the various clusters, that is, their celestial positions, are listed in the various star cluster categories.
S = The velocity of the cluster relative to the Sun
C = The velocity of the cluster corrected for the Sun's velocity
Copyright (c) 1997-99 Michael Erlewine