It’s exciting to observing the multitude of bright globular clusters, incredibly dense balls of stars that provide striking viewing with even a modest optical aid, that are on show at this time of the year. The largely southern constellation of Ophiuchus can make a good claim to the title of ‘land of the globulars’, hosting as it does seven, big and bright clusters adorned with the coveted Messier designation.
Luckily for observers at mid-northern latitude, the majority of observers rate the most northerly lying M10 (NGC 6254) and M12 (6218) as the best of the bunch (M14, lying to their east, has a similar declination), the pair lying just three degrees apart and visible in the same 10 × 50 binocular field of view. Both globulars are among the finest to be seen in the entire sky, with small- to medium-aperture telescopes yielding splendid views.
How to observe
Around late June, both globular clusters have risen sufficiently high in the south-south-east to be observed by about 11pm BST. M10, the more southerly of the pair is marginally the brighter (magnitude +6.6) and larger (~15 arcminutes) of the two; sweep with 10 × 50s about 10 degrees east of a pair of third-magnitude stars called Yed Prior (delta [δ] Oph) and Yed Posterior (epsilon [ε] Oph) to see M10’s faint smudge.
M10 is a moderately concentrated globular (class XII), for which a modest 80mm (~three inches) refractor will start to pick out (resolve) the outlying individual stars, with full resolution to the core achieved by a 130mm (~five inches) telescope.
M12 is located just over three degrees to the north-west, about a third of the way along an imaginary line strung between M10 and magnitude +3.8 lambda (λ) Oph. It too is easily visible as a nebulous patch in binoculars, glowing at magnitude +6.8. Through a telescope, it’s easy to see M12’s much looser nature (class IX); if you can observe it through a 100mm (four-inch) telescope, then most of its stars should be resolved.