" Messier 71, sits in the small constellation Sagitta the arrow. This area of the sky is associated with the plane of the Milky Way's disc and is literally awash with stars when viewed with binoculars or a widefield telescope. Messier 71 being a small globular cluster, is therefore easy to miss within this plethora of stars. As I processed the data and watched the above image materialise, I was amazed at how many stars I could see!" - Kurt Thrust current Director of the Jodrell Plank Observatory.
A Comparative Report on the Globular Cluster Messier 71
Messier 71 (M71), located in the small constellation Sagitta, is one of the more unusual globular clusters in the Messier catalogue. At a distance of roughly 13,000 light-years, it appears as a loose, irregular grouping of stars through modest telescopes, and for much of the 19th and 20th centuries it was debated whether M71 was in fact a globular or a rich open cluster. Advances in stellar photometry and spectroscopy have since clarified its nature: M71 is a genuine globular cluster, though a relatively low-mass, metal-rich, and loosely concentrated one. Comparison with the more archetypal clusters Messier 13 (M13), Messier 3 (M3), and Messier 4 (M4) underscores the peculiarities of M71.
Messier 13, the “Great Hercules Cluster,” is perhaps the best-known northern globular cluster. It contains several hundred thousand stars in a compact halo over 140 light-years across, with a dense, bright core that exemplifies the globular class. By contrast, M71 contains perhaps only 20,000–50,000 stars spread across a mere 27 light-years. Its lower concentration and modest size explain why early observers mistook it for an open cluster. Whereas M13 presents a dazzling display of densely packed stars and exhibits a well-defined horizontal branch, M71 has only a stubby red giant branch and no extended blue horizontal branch, making its stellar population appear more subdued.
Messier 3 provides another instructive comparison. Situated in Canes Venatici, M3 is among the richest and most extensively studied globular clusters, hosting over half a million stars and more than 200 known variables, including numerous RR Lyrae stars. Its variable population and prominent horizontal branch have made it a cornerstone for calibrating stellar evolution models. M71, on the other hand, lacks a significant RR Lyrae population and is chemically distinct. Its relatively high metallicity ([Fe/H] ≈ –0.8) is atypical for globular clusters, especially when contrasted with the metal-poor stars of M3. This implies that M71 formed later in the Galactic halo’s history, from interstellar gas already enriched with heavier elements, and therefore represents a younger evolutionary epoch within the globular cluster system.
Messier 4, in Scorpius, provides a comparison from the opposite end of the structural spectrum. Though M4 is one of the closest globular clusters to Earth (about 7,200 light-years), and not especially rich in stars compared to giants like M3 or M13, it nevertheless displays a clear globular morphology with a strong central concentration. M4 is notable for its rich population of evolved stars, a well-populated horizontal branch, and chemical peculiarities linked to multiple stellar generations. In terms of mass and richness, M4 is closer to M71 than are M13 or M3, yet it retains the compact, globular appearance that M71 lacks. The difference lies primarily in concentration: M4 is compact and easily recognisable as a globular cluster, while M71 is diffuse, giving it a hybrid appearance that complicated its classification.
In conclusion, M71 highlights the diversity within the globular cluster family. It lacks the immense stellar density and archetypal morphology of Messier 13, the vast population and chemical simplicity of Messier 3, and even the structural clarity of Messier 4. Instead, it is a relatively loose, metal-rich, and moderately populated globular cluster, whose transitional characteristics blur the distinction between open and globular clusters. Its study provides valuable insight into the chemical enrichment of the Galaxy and the lower-mass limits of globular cluster formation.
Professor G.P.T. Chat visiting astrophysicist at the Jodrell Plank Observatory.