NGC 6539, Globular Cluster
With an apparent brightness of 8.9mag and a magnitude of 6.9', the globular star cluster NGC 6539 is located in the constellation of the Serpent Bearer
(Oph). It is approximately 27,000 light-years away from Earth and just under 10,000 light-years from the galactic center. The cluster was first discovered
in 1856 by the Danish astronomer T. Brorsen.
The core of the GC has a diameter of 8.5 light-years, while half of the mass is located in a sphere with a diameter of 27 light-years. Individual stars are still
gravitationally bound up to a distance of 170 light-years. The total mass of NGC 6539 is estimated to be 540,000 solar masses. Many of the stars are quite
metal-rich and have around 22% of the metal content of the Sun. Their total luminosity adds up to 160,000 suns, so that the globular cluster reaches an
absolute magnitude of M= -8.2mag.
It should also be mentioned, that a millisecond pulsar was discovered in NGC 6539 in 1990.
----------------------------------------------------------------------------------------------------------------------------------------------
In my 20" f/3 Dobsonian, NGC 6539 appears as a somewhat larger globular cluster with a low surface brightness, which calls for a dark sky.
The center is only slightly brighter. With good seeing and a magnification of 270x, the background appears slightly grainy - but no individual stars can be
seen yet. However, not much seems to be missing to resolve the cluster.
