Discovered 1764 by Charles Messier.

M3 is one of the most outstanding globular clusters, containing an estimated half million stars. At a distance of about 33,900 light years, it 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; Kenneth Glyn Jones mentions an estimate of even 20 arc minutes from deep photographic plates, corresponding to about 200 light years linear diameter. 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. Its half-mass radius is 1.12', or about 11.2 light years, so that half of this clusters mass is contained in a volume of only 22 light years in diameter.

The cluster's brightest stars are of mag 12.7, while the so-called Horizontal Branch giants are of mag 15.7, and the 25 brightest stars have an average brightness of 14.23 mag. 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). Sandage (1954) counted 44,500 stars brighter than mag 22.5 within a radius of 8 arc min; the total mass has been estimated at 245,000 solar masses (Sandage and Johnson). Helen Sawyer Hogg has given M3's overall spectral type as F2, and a color index -0.05, rather blue for a globular, while the Sky Catalogue 2000.0 gives its spectral type at F7, and W.E. Harris lists it as F6. Its color index was determined as B-V=0.69. This stellar swarm is approaching us at 147.6 km/sec.

Globular cluster M3 is extremely rich in variable stars: According to B. Madore (in Hanes/Madore, Globular Clusters, 1978), 212 variables have been found, 186 periods determined, more than in every other globular cluster in our Milky Way galaxy (and thus the most ever observed); at least 170 RR Lyrae variables (sometimes called "cluster variables") were discovered. These stars have served as "standard candles" to determine the cluster's distance. The first variable star was discovered by E.C. Pickering in 1889, the next 87 were found by S.I. Bailey in 1895 (see Pickering and Bailey 1895).

M3 contains a relatively large number of so-called Blue Stragglers, blue main-sequence stars which appear to be rather young, much younger than the rest of the globular's stellar population would suggest. These were first discovered by Alan Sandage (1953) on photographic plates taken with the 200-inch Hale telescope on Mt. Palomar. A mystery for a long time, these stars are now thought to have undergone dramatic changes in stellar interactions, getting their cooler outer layers stripped away in close encounters, which occasionally occur when stars are passing through the dense central regions of globular clusters.

This cluster was the first `original' discovery by Charles Messier when he logged it on May 3rd, 1764. At that time it was the 73rd deep sky object ever observed by human eyes (and apparatus), although at that time, it was only the 56th known nebulous object, while 17 objects had been forgotten again, according to the sources and current knowledge of the present author (see the Deep Sky Object Discovery Table). It was also apparently the discovery of this object which eventually caused Charles Messier to start a systematical search for these comet resembling objects, and not just catalog chance findings as in the previous cases M1 and M2, as is demonstrated by the fact that in 1764, he found and measured all the objects M3-M40.

When the final object of the catalog, M107, a globular cluster in Ophiuchus, was discovered by Messier's friend Pierre Méchain in 1782, 18 years later, a total of at least 140 objects were known, more than double the number, and 110 of them described by Messier (who discovered 42 or 43) and Méchain (27 or 28) -- the doubty counting being a result of the dubious circumstances concerning the discovery of M102.

M3 was first resolved into stars and recognized as cluster by William Herschel around 1784.

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).