Deep Sky Object of the Month - M3

by Stephen Jones

Objects: M3
Constellation: Canes Venatici
Type: Globular Cluster
Magnitudes: 6.2
Discoverer: Charles Messier, 1764
Equipment necessary: Should be fairly easy to detect as a nebulous object in binoculars or a small telescope; naked-eye detection has been reported, but should be exceedingly difficult. Resolution into stars begins in a 4-inch, resolved completely in a 12-inch.

It is a common misconception, generally by folks who have spent more time looking at photographs than doing visual observing, that globular clusters all look the same. Not so! Photos of globulars tend to overexpose the core to bring out the outer filaments, blurring much of the distinctiveness of the individual cluster, but visually we can see these differences easily. M3 is perhaps my favorite among the bright globulars for its distinctive shape.

M3, while bright, can be somewhat of a challenge to locate, just because it’s in a very isolated area with few bright stars very close to its position. One way of locating it is to imagine it as one vertex of a large near-equilateral triangle with Arcturus and ρ Boötis. Another way is to follow a line from γ Comae Berenicis to β Comae Berenicis and continue East

One of the more interesting historical observations of M3 comes from the 19th century British amateur astronomer Admiral William Smyth, whose “Bedford Catalogue” was the very first book ever written in the genre that we visual observers can’t get enough of: the observing guide. In his lengthy description of M3, he compares it to a jellyfish, and illustrates his impression with the sketch seen here.

Smyth’s asymmetrical description of M3 appears to be at odds with popular astronomy works of the 20th century; the revered Burnham’s Celestial Handbook mentions nothing of this feature of M3, and Kenneth Glyn Jones in his work Messier’s Nebulae and Star Clusters even mentions Smyth’s description and actively disagrees with it, stating that he saw the stars to be very evenly distributed. Having read these descriptions before I ever observed M3, I expected the newer sources would be closer to what I would see, so imagine my surprise as I first turned my scope to it and saw Admiral Smyth’s jellyfish staring me in the face! Even more interestingly, on subsequent observations it looked much more round. Could it be the stars on the side that Smyth observed as more compressed (top left in the sketch) are fainter, so that the cluster assumes the jellyfish shape with smaller scopes or poorer transparency, but looks rounder under conditions where those stars are more easily seen? What do you see when you look at M3?

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