NGC 6535, Globular Cluster
NGC 6535 is a globular cluster with a brightness of 9.3mag and an apparent size of 3.4 arcminutes, that is located at a distance of around
22,000 light-years in the constellation of the Serpent Bearer (Oph). It was discovered by F.W. Herschel in August 1780 - but independently
a few decades later in 1852 by J.R. Hind with a 7" telescope.
The cluster belongs to concentration class XI - it therefore has no distinct center. The core has a diameter of just 4.5 light-years, while the
"half-light" radius is 5.5 light-years. The total diameter can be calculated from the apparent size and the known distance to 22 light-years.
NGC 6535 contains stars with a total mass of approx. 20,000 solar masses. Unusually, the stars have a rather high metallicity of 22% of that
of the Sun and there appear to be several stellar populations, but no variable stars of the type "RR- Lyrae". The age of the cluster was
determined to be 10.5 billion years.
A special feature of NGC 6535 is, that an IMBH (intermediate black hole) with a mass of 10,800 solar masses is suspected in its center.
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NGC 6535 appears quite small, bright and compact in my 20" f/3 telescope. The brightness of the globular cluster is almost homogeneous,
the center appears only slightly brighter. Overall, the cluster is quite well resolved, both in the core and in the outer regions. About 10 brighter
individual stars are evenly scattered across the cluster.
Some brighter stars in the neighborhood offer a nice contrast.
