Although overlooked for its famous neighbor, the double cluster, M34 is a unique and beautiful open cluster in its own right. Its 100 or so members are middle aged for an open cluster at about 250 million years old, midway between the younger Pleiades (100 million years) and the older Hyades (700 million years). The more massive white and blue stars stand out among their lower mass brethren. A few cluster members have left the main sequence and are conspicuous as red giants. The cluster spans about 10 light years. Its metal abundances and rotational velocities are also midway between the younger and older clusters.
Open cluster systems are diverse and methods of classifying them are equally diverse. One of the better known systems was devised by the American astronomer Robert Trumpler (1886-1956) and published in 1930. It employs three criteria, the degree of central concentration from I (most) to 4 (least), the range of brightness of member stars from I (narrow range) to 3 (wide range), and total number of stars in the cluster from p (<50) to m (50 to 100) to r (>100). The addition of "n" indicates associated nebulosity. M34 has is a Trumpler type II3r designation.