Deepsky Top-100 (9): M 3

M 3 (NGC 5272, Mel 109) in Canes Venatici is one of the brightest globular clusters visible on the Northern Hemisphere. Charles Messier discovered it in 1764. With a visual magnitude of 6.3, M 3 should be visible to the naked eye, but the conditions have to be very good. The diameter of M 3 is 18' and it's brightest star shines at magnitude 12.7.

M 3 lies in a star-poor region of the night sky, almost halfway between Arcturus (Alpha Bootis) and Cor Caroli (Alpha2 Canum Venaticorum). If you want to find M 3, first centre your finder scope on Arcturus. Then try to move the telescope in a straight line to Cor Caroli, while you keep looking into the finder scope. When your are almost halfway between these two stars, you will notice a very faint smudge of light in your finder scope. This is M 3. It looks more or less like a "nebulous star". If you have trouble finding it, first try to locate it with binoculars. The map below should give you an idea where to look for M 3. The coordinates are RA 13h.42m.11s and DEC 28deg.42'.32"

Click here to download a printable finder chart (PDF)


In binoculars you cannot resolve M 3. In my 8-inch telescope under light polluted skies (limiting magnitude 4.5) only the stars in the outer parts of the clusters halo are resolved. Using a higher magnification (166x) and averted vision helps to resolve the halo a little more. I estimated the size on 10' to 12'. The 5' core is very bright but not resolved into individual stars.

A few days ago I observed M 3 through a 12 inch Newton from a fellow observer. We observed it from my own backyard. It was really beautiful. The halo was completely resolved and there where several chains of stars visible. The very bright core still remained unresolved but there where stars superimposed on it when observing it at 170x. This cluster is definitely one of my favourite globulars, and it rivals M 13.