The early summer night sky is packed with so many globular clusters that it’s hard to know where to look first. Messier 13 and Messier 5 are the clear favourites, and Messier’s treasures in Ophiuchus catch our eye, but why not head south for a change and visit the powerful southern constellation of Scorpius to explore Messier 4 and Messier 80, a pair that offer interesting and contrasting physical and observing characteristics?
Messier 4: a top-notch globular
Messier 4 (NGC 6121) is the more famous of the two globular clusters. With a magnitude of +5.8 and an area of 8 meters, it is one of the largest globular clusters we see. Messier 4 is also one of the closest clusters, lying about 7,200 light-years away. Its proximity explains its large apparent diameter, as physically Messier 4 is quite small, spanning only 75 light-years across.
Messier 4 lies about 80' west of Antares (alpha (α) Scorpii, magnitude +1.06), the great red supergiant luminary in Scorpius. Scorpius is a horizon-hugging constellation off the coast of the UK, which is a shame as it is packed with a wonderful selection of bright deep-sky quarries.
In early June, Antares and Messier 4 culminate around 12:40am BST at a low altitude of around 12°. Given an uninterrupted view to the south, a pair of 10 x 50 binoculars will show the pair, and a small telescope may begin to resolve some of Messier 4’s individual stars on a clear, stable night, despite the cluster’s unfavourable altitude. It helps that Messier 4 is one of the loosest globulars in the sky, classified as class IX on the 12-point Shapley-Sawyer concentration scale (I to XII).
Messier 80 Compact
Messier 80 (NGC 6093) is located 4.5° northwest of Messier 4 and, at a distance of about 32,600 light-years, is almost five times farther away. It is also one of the densest and most compact globular clusters known; In the Messier catalogue, its rating of II on the Shapley-Sawyer scale is matched only by Messier 2.
Messier 80 is located about four degrees east of magnitude +2.3 of Dschubba (delta (δ) Scorpii) and, at magnitude +7.3, is bright enough to be seen with 10x binoculars. 50. Due to its compact size across its 8.9' sphere and its proximity to the horizon, observers at mid-northern latitudes will find it difficult to distinguish individual stars within Messier 80, but try!
