Head south for a Messier globular duo

Messier 4 is one of largest and loosest globular clusters in the Messier catalogue. Image: Adam Block.

The night sky in early summer is packed with so many great globular clusters that it’s hard to know where to look first. Messier 13 and Messier 5 are clear favourites, and the Messier hoards in Ophiuchus beckon, but why not head south for a change to the mighty southern constellation of Scorpius to explore Messier 4 and Messier 80, a pairing that provides interesting and contrasting observing and physical characteristics.

Messier 4: a top-drawer globular

Messier 4 (NGC 6121) is the more famous of the two globular clusters. Shinning at magnitude +5.8 and covering a substantial 26’, it is one of the largest globulars we see. Messier 4 is also one of the closest clusters, lying about 7,200 light years away. Its proximity accounts for its large apparent diameter, as physically Messier 4 is rather on the small side, spanning just 75 light years across.

Messier 4 lies about 80’ west of Antares (alpha [α] Scorpii, magnitude +1.06), Scorpius’ great red supergiant luminary. Scorpius is a horizon-hugging constellation from UK shores, which is a crying shame as it’s packed with a marvellous selection of brilliant deep-sky quarry.

At the start of June, Antares and Messier 4 culminate at about 12.40am BST with a lowly altitude of around 12°. Given an uninterrupted southern vista, a pair of 10 x 50 binoculars will show the pair, and a small telescope may start to resolve some of Messier 4’s individual stars on a transparent and steady night, despite the cluster’s unfavourable altitude. It helps that Messier 4 is one of the loosest globulars in the sky, rated class IX on the 12-point (I to XII) Shapley–Sawyer concentration scale.

Messier 80 lies over four times further away than Messier 4 and is one of the most dense and compact clusters known. Image: Jim Misti.

Compact Messier 80

Messier 80 (NGC 6093) lies 4.5° north-northwest of Messier 4 and, at a distance of around 32,600 light years, is nearly five times further away. It’s also one of the most dense and compact globular cluster 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 Dschubba (delta [δ] Scorpii) and, shining at magnitude +7.3, is bright enough to be picked up in 10 x 50 binoculars. Owing to its compactness across its 8.9’ sphere, and its proximity to the horizon, observers at mid-northern latitudes will find it tough to resolve individual stars within Messier 80, but by all means have a go!

Messier 4 and Messier 80 are a pair of globular clusters in the spectacular constellation of Scorpius. AN graphic by Greg Smye-Rumsby.