M71, also known as the Angelfish Cluster, is a globular cluster located in the constellation Sagitta, the Arrow. Unlike most globular clusters, M71’s relatively loose and sparse structure has historically led to debates about whether it might instead be an open cluster. However, its characteristics, such as older stars and metal-poor composition, have led astronomers to classify it as a globular cluster. Its unique appearance and intermediate density make it a fascinating object for study.
The cluster was first discovered in 1746 by the French astronomer Philippe Loys de Chéseaux. It was independently rediscovered and catalogued in 1780 by Charles Messier, who listed it as the 71st entry in his catalogue of celestial objects.
Physical Characteristics
M71 is estimated to be approximately 12.7 billion years old, placing it among the older globular clusters in the Milky Way galaxy. It lies at a distance of about 12,000 light-years from Earth and spans a diameter of roughly 27 light-years, making it relatively compact compared to other globular clusters.
The cluster contains several thousand stars, but its density is significantly lower than that of typical globular clusters. This lower density contributes to its open appearance and was a key factor in the past debate over its classification. The stars in M71 are primarily metal-poor, meaning they have low abundances of elements heavier than helium, a hallmark of ancient stellar populations.
The stars within M71 orbit around the cluster’s centre of mass, bound by gravity. However, the cluster’s dynamics are less tightly bound than those of more compact globular clusters, consistent with its more dispersed structure. M71’s unique combination of properties makes it an intriguing object for studying the diverse nature of globular clusters.
Visual Appearance
With an apparent magnitude of 8.3, the Angelfish Cluster is too faint to be seen with the naked eye. Under dark skies, it appears as a faint, hazy patch of light when observed with binoculars or a small telescope. Unlike many globular clusters, M71 lacks a densely packed core, giving it a more open and diffuse appearance.
Through larger telescopes, some of its individual stars can be resolved, particularly at the edges, while the central region retains its hazy, granular look. This unique structure, along with its faint glow, makes it an interesting target for amateur astronomers equipped with suitable optical tools.
Observation
M71 is best observed during the summer months in the Northern Hemisphere, as it is located in the constellation Sagitta, near the Summer Triangle asterism. The cluster becomes visible in late spring, around May, when it appears low in the eastern sky during the early evening hours. Its visibility improves through June, July, during which it climbs higher in the night sky, reaching its best viewing position in August.
By early autumn (September), M71 remains visible but begins to set earlier in the evening. Observers in the Northern Hemisphere will need to plan accordingly, as viewing later into the evening may be required. To observe the cluster effectively, a telescope under dark skies with minimal light pollution is recommended, as its faintness and diffuse structure can make it challenging to detect in suburban settings.
In summary, M71’s unique structure and faint glow make it a rewarding but slightly challenging target for observers, offering both a visual and scientific glimpse into the diversity of globular clusters.
