Messier 3 is one of the most outstanding globular clusters, containing an estimated half million stars. It is famous for the large number of variable stars discovered in it.
This cluster was the first 'original' discovery by Charles Messier when he logged it on May 3rd, 1764. Perhaps the discovery of this object eventually caused Charles Messier to start a systematical search for these comet resembling objects, and not just catalog chance findings as in the previously.
At a distance of about 33,900 light years, M3 is further away than the center of our Galaxy, the Milky Way, but still shines at magnitude 6.2, as its absolute magnitude is about -8.93, corresponding to a luminosity of about 300,000 times that of our sun. M3 is thus visible to the naked eye under very good conditions - and a superb object with the slightest optical aid. Its apparent diameter of 18.0 arc minutes corresponds to a linear extension of about 180 light years. It appears somewhat smaller in amateur instruments, perhaps about 10 minutes of arc. But its tidal radius, beyond which member stars would be torn away by the tidal gravitational force of the Milky Way Galaxy, is even larger: About 38.19 minutes of arc. Thus, this cluster gravitationally dominates a shperical volume 760 light years in diameter.
On the other hand, M3 has a compressed, dense core measuring 1.1' in diameter, or linearly, 11 light years, comparatively large for a globular.
The age of globular cluster M3 has been estimated from its color-magnitude diagram on various occasions; historically, early values have been given at 5 billion years (Baade), 11.4 billion years (Woolf), 20 billion years (Arp) and 26 billion years (Sandage). This stellar swarm is approaching us at 147.6 km/sec.
Situated in the Galactic halo, out about 40,000 light-years from the Galactic Center, M3 is moving on a box-type orbit of approximate excentricity 0.55, which takes it out up to 66,000 light-years apogalactic distance and up to 49,000 light-years above and below the Galactic plane (currently it is about 33,000 light-years above - i.e., north of - that plane).
To find M3, either prolong the line from Gamma Comae Berenices near the Comae Berenices Cluster over Beta Comae by about 2/3 and look slightly north to have M3 in the low-power field: it is about 6 degrees north-northeast of Beta Comae.
While M3 is visible to the naked eye only under very good conditions and stays just below the limit of visibility under more average conditions, it can be easily seen with the smallest instrument. In binoculars, it appears just like a hazy, nebulous patch. A 4-inch shows its bright compact core within a round and mottled, grainy glow, which fades slowly and uniformly to the outer edges; it doesn't resolve the cluster, but shows just some of the brightest stars under good conditions. A 6-inch resolves the about outer two thirds into faint stars on a background glow formed by the unresolved fainter member stars of the cluster. An 8-inch shows stars throughout the cluster but in the very core, which is resolved into stars by larger telescopes (about 12-inch).