It’s exciting to watch the many bright globular clusters, incredibly dense balls of stars that provide stunning viewing even with modest optical aid, which are displayed at this time of year. To a large extent, the southern constellation Serpent Bearer can claim the title of “land of the globular” by hosting seven large and bright clusters decorated with the desired designation of the Messiah.
Fortunately for mid-latitude observers, most observers rated the northernmost M10 (NGC 6254) and M12 (6218) as the best in the group (the M14 lying to the east has a similar slope), the pair lying just three degrees from each other and visible in the same binocular field of view 10 × 50. Both globules are among the best that can be seen in the whole sky, with telescopes with small to medium apertures that give beautiful views.
How to observe
Around the end of June, both globular cups rose high enough in the south-southeast to be observed around 23:00 BST. M10, the south of the pair is slightly brighter (magnitude +6.6) and larger (~ 15 angular minutes) than both; swinging 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 the weak spot on the M10.
M10 is a moderately concentrated spherical (class XII), for which a modest 80 mm (~ three inch) refractor will begin to select (allow) individual distant stars, with full resolution to the core achieved by a 130 mm (~ five inch) telescope.
M12 is located just over three degrees northwest, about a third of the way along an imaginary line between M10 and +3.8 lambda (λ) Oph. It is also easily visible as a foggy spot in binoculars, illuminating with a magnitude of +6.8. Through the telescope it is easy to see the much freer nature of M12 (class IX); if you can observe it through a 100 mm (four inch) telescope, then most of its stars need to be separated.