Messier 4 (NGC 6121) is a standout globular cluster, shining at a remarkable magnitude of +5.8 and spanning an impressive 26 arcminutes across the sky. This makes it one of the largest globular clusters visible in the Messier catalogue. Despite its grand appearance, Messier 4 is relatively close, situated at a distance of approximately 7,200 light-years from Earth. Its proximity accounts for its substantial apparent size, though physically it is rather modest, spanning only 75 light-years across.
This globular cluster is conveniently located about 80 arcminutes west of Antares (alpha [α] Scorpii), the brilliant red supergiant that serves as the heart of the magnificent Scorpius constellation. From mid-northern latitudes, Scorpius hugs the southern horizon, which is unfortunate given the wealth of deep-sky treasures it contains.
In early June, Antares and Messier 4 reach their highest point in the sky around 12:40 am BST, culminating at a rather low altitude of around 12 degrees. With a clear view of the southern horizon and a pair of 10×50 binoculars, observers can easily spot this duo. On nights with steady and transparent skies, even a small telescope may begin to resolve individual stars within Messier 4, despite its unfavorable altitude. That’s aided by the fact that Messier 4 is one of the loosest globular clusters in the sky, rated class IX on the 12-point (I to XII) Shapley-Sawyer concentration scale.
In contrast to the prominent and expansive Messier 4, Messier 80 (NGC 6093) is a much more compact and dense globular cluster. Located 4.5 degrees north-northwest of its larger counterpart, Messier 80 lies at a staggering distance of around 32,600 light-years from Earth, nearly five times farther away than Messier 4.
Despite its remote location, Messier 80 shines at a respectable magnitude of 7.3, making it bright enough to be spotted through 10×50 binoculars. Observers can find this compact globular cluster about four degrees east of Dschubba (delta [δ] Scorpii), a notable star in Scorpius that glows at magnitude 2.3.
What sets Messier 80 apart is its extraordinary compactness. Rated as class II on the Shapley-Sawyer concentration scale, it is one of the most densely packed globular clusters known, rivaled by only one other entry in the Messier catalogue – Messier 2. Despite its relatively small apparent diameter of 8.9 arcminutes, the cluster’s dense core and low altitude from mid-northern latitudes make it challenging to resolve individual stars within its boundaries.
While Messier 4 captivates observers with its grand scale and loose structure, Messier 80 presents a contrasting view – a tightly bound, concentrated sphere of stars. Together, these two globular clusters offer a fascinating study in the diversity of stellar systems found within the Messier catalogue and the wonders that await in the rich starfields of Scorpius.
