M71 - NGC 6838

Messier 71 (NGC 6838) is a globular cluster located approximately 13,000 light-years from Earth in the constellation Sagitta. It was discovered by Philippe Loys de Cheseaux in 1745 and independently found by Johann Gottfried Koehler before Charles Messier cataloged it in 1780. M71 has long been a subject of debate among astronomers regarding its classification. For much of the 20th century, it was classified as a dense open cluster rather than a globular cluster because of its loose structure, lack of strong central concentration, and relatively high metallicity. However, modern studies of its stellar population, including the detection of RR Lyrae variables and a well-defined horizontal branch in its color-magnitude diagram, confirmed its identity as a genuine globular cluster, albeit an unusually loose and young one. M71 has a Shapley-Sawyer concentration class of X-XI, making it one of the loosest globular clusters known, and it has an estimated age of 9-10 billion years, younger than typical globulars which are often 12-13 billion years old. The cluster spans about 27 light-years in diameter and has a relatively high metallicity of [Fe/H] = -0.8. M71 is located in a beautiful star field along the Milky Way, between the arrow-shaped asterism of Sagitta. In binoculars, it appears as a fuzzy patch of light, and a 4-inch telescope begins to resolve it into a sprinkling of faint stars. Due to its loose concentration, M71 looks remarkably different from dense globulars like M13 or M2, instead resembling a rich open cluster with a slightly denser center.

M71 spans about 27 light-years in diameter, lies approximately 13,000 light-years from Earth, and is estimated to be about 9-10 billion years old.