Resolving Messier 3’s swarming stars

Messier 3 is a great globular cluster of the late-spring sky. Image: Adam Block.

During late spring or early summer—whichever term you choose, likely depending on how fine the weather has been—is the best time to seek out and observe globular clusters, which are among the most striking and impressive categories of deep-sky objects.

Globular clusters are densely-packed, near spherical collections of ancient stars that populate mainly the extended outer halo of our galaxy. They are believed to have formed in the very early life of our Galaxy, over 11 billions years ago; Messier 3 is thought to be 11.4 billion years old.

Astounding star densities exist inside even run-of-the-mill globulars; on average, 0.4 stars per cubic parsec (a parsec is equal to 3.26 light years), rising to 100 to 1,000 in the denser core.

Messier 3 is one of the largest globular clusters known, with a physical diameter of at least 180 light years, though some sources raise that to 200–220 light years. One thing’s for sure, its tidal radius is a lot more, extending its sphere of gravitational influence by over three times. At its distance of around 34,000 light years, its imposing size gives it an impressive apparent diameter of 18 arcminutes.

Messier 3 lies in the south-eastern corner of Canes Venatici. AN graphic by Greg Smye-Rumsby.

Where to look

Messier 3 is located in the south-west corner of Canes Venatici (the Hunting Dogs), near to the boundary with Bootes and Coma Berenices. In early May, M3 is about 90 minutes from culmination as nightfalls, riding high in the southern sky at an altitude of around 60°. It can be observed throughout May’s relatively short nights, remaining on show until early September.

Shining at around magnitude +6.2 Messier 3 is an easy target for 10 x 50 binoculars; sweep for it about six to seven degrees east of magnitude +4.2 beta (β) Comae Berenices. It should appear clearly non-stellar as a fuzzy, unresolved patch of light.

How easy a globular cluster is to see is not solely down to its brightness; its degree of condensation (how densely packed it is) is also a critical factor. A dense, compact, star-rich globular will show a greater contrast with the background sky and therefore be easier to see than one that is diffuse, and tends to get lost in the sky background. Globular clusters are rated according to the Shapley-Sawyer 12-point scale, which indicates the degree of condensation. It ranges from I (very dense and compact) to XII (extremely diffuse with no central concentration). Messier 3 sits around midway on this scale as class VI, indicating medium density.

Messier 3 looking like a sphere of sparking jewels. Image: ESA/Hubble & NASA, G. Piotto et al..

Through the eyepiece

Messier 3’s individual stars at its outer edges of its 19-arcminute form can start to be resolved through a 80–100mm (three- to four-inch) telescope at around 100<M>x<M>. A 200–250mm (eight- to ten-inch) telescope operating on a steadying transparent night can reveal countless stars across M3, mining those deep at its core at high magnification.

As an interesting aside, there are so many great globulars to choose from in May. Indeed, you could conduct your own globular Messier marathon if you’re so inclined, as all but one of the globular clusters listed in the Messier catalogue, Messier 79 in Lepus, is above the horizon at some stage on May nights.

Messier 3 as observed and drawn through an 200mm (eight-inch), f/6 Newtonian telescope at 120x. M3’s myriad stars are seen right to its core. Sketch: Jeremy Perez.