NGC 6535

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NGC 6535 is a globular cluster, a tight group of old stars, located in the Serpens constellation. It lies about 22,000 light-years from Earth and sits near the Milky Way’s central region, in a halo position close to the galactic bulge.

The cluster is relatively lightweight for its type, with an estimated mass of about 2.2 × 10^4 solar masses. It has a compact core radius of roughly 2.3 light-years and a tidal radius of about 50 light-years. Its light suggests an apparent brightness around magnitude 9.9.

NGC 6535 is very old, with an age of about 12.75 billion years. It is metal-poor, having a low abundance of elements heavier than hydrogen and helium (iron-to-hydrogen ratio about [Fe/H] = -1.95). Interestingly, the cluster shows multiple generations of stars, which is unusual for such a small globular cluster.

The cluster contains no known RR Lyrae variable stars, but it does host nine blue straggler stars, which are hotter, brighter stars that stand out from the cluster’s older population.

There is evidence suggesting the presence of an intermediate-mass black hole at the cluster’s center, a rare possibility for a cluster of this size.

NGC 6535’s discovery is usually credited to John Russell Hind in 1852, though some researchers argue that William Herschel may have observed it earlier, in 1780. It is also known by other names, such as C 1801-003 and Glc 83.