June 21, 2024
1 Solar System Way, Planet Earth, USA

Resolving the Messier 3 star swarm – Astronomy Now

Messier 3 is a large globular cluster in the late spring sky. Image: Adam Block.

Late spring or early summer (which term you choose will probably depend on how good the weather is) is the best time to look for and observe globular clusters, which are among the most striking and impressive categories of deep-sky objects.

Globular clusters are densely packed, nearly spherical collections of ancient stars that primarily populate the extended outer halo of our galaxy. It is believed that they formed at the beginning of our galaxy, more than 11 billion years ago; Messier 3 is believed to be 11.4 billion years old.

There are astonishing densities of stars even within run-of-the-mill globulars; on average, 0.4 stars per cubic parsec (one parsec is equivalent to 3.26 light years), increasing from 100 to 1,000 in the densest core.

Messier 3 is one of the largest globular clusters known, with a physical diameter of at least 180 light years, although some sources put it as high as 200-220 light years. One thing is certain: its tidal radius is much larger, extending its sphere of gravitational influence more than three times. At a distance of about 34,000 light years, its imposing size gives it an impressive apparent diameter of 18 arc minutes.

Messier 3 is located in the southeast corner of Canes Venatici. A graphic by Greg Smye-Rumsby.

Where to look

Messier 3 is located in the southwest corner of Canes Venatici (the Hunting Dogs), near the border with Bootes and Coma Berenices. In early May, M3 is about 90 minutes from peaking at dusk, rising high in the southern sky at an altitude of about 60°. It can be observed during the relatively short nights of May and remains visible until early September.

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

How easy it is to see a globular cluster is not just due to its brightness; Its degree of condensation (how dense it is) is also a critical factor. A dense, compact, star-rich globular will show greater contrast with the sky background and will therefore be easier to see than one that is diffuse and tends to fade into the sky background. Globular clusters are classified according to the 12-point Shapley-Sawyer scale, which indicates the degree of condensation. It ranges from I (very dense and compact) to XII (extremely diffuse without central concentration). Messier 3 falls halfway on this scale as class VI, indicating medium density.

Messier 3 looks like a sphere of sparkling jewels. Image: ESA/Hubble and NASA, G. Piotto et al.

through the eyepiece

Messier 3's individual stars on the outer edges of its 19 arcminute shape can begin to be resolved through an 80 to 100 mm (three to four inch) telescope at about 100x. A 200 to 250 mm (eight to ten inches) telescope operating on a clear, stable night can reveal countless stars across M3, extracting those deep in its core at high magnification.

As an interesting fact, there are so many fantastic balloons to choose from in May. In fact, you could run your own Messier globular marathon if you want, since all but one of the globular clusters listed in the Messier catalog, Messier 79 in Lepus, are above the horizon at some point during the May nights.

Messier 3 observed and drawn through a 200 mm (eight inch) f/6 Newtonian telescope at 120x. The countless stars of M3 can be seen down to their core. Sketch: Jeremy Pérez.

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