Object of the Week March 13th, 2016 – The Spiral Open Cluster NGC 2506

[Uwe Glahn]

NGC 2506 (Mel 80, Cr 170)

Constellation: Monoceros
RA: 08h 00m 01s
Dec: -10° 46’ 12”

Type: Open Cluster, Trumpler Class I2r
Distance: 11.000 ly
Size: 12’
Mag: 7.6V
Brightest Star: 11.0V

Till the beginning of the 20’th century the real nature of all “nebula objects” was not exactly known. Herschel already suspected that all nebula's exist of stars which can not be resolved at all objects.
The situation improves with the large “Leviathan” – the 72-inch telescope of Lord Rosse. He first saw spiral structure in some objects and named them in a new classification. His idea was to resolve as many “nebula's”, also “Spirals”, as the telescope could show. And he seems to win the fight with some objects – one of it our OOTW NGC 2506.
In his second paper 1861 he described the cluster as: “frequently observed, several observers have fancied that the stars exhibit some approach to a spiral arrangement, with cellular center, not unresolved neby”. His son Lawrence wrote for example: “more approach towards spiral appearance than I have seen in other cluster…the spiral appearance is confined to the brightest stars…”

Today we knew that NGC 2506 is a normal, old, metal poor cluster in a average distance.

Sadly the cluster can not be resolved in telescopes of the 4-inch class. With aperture of 8/10-inch+ the spiral structure can be detected. Even in big telescopes the clusters remains interesting because of its star rich appearance. What can you see? And still remember Lord Rosse between his walls and in front of his large telescope.

NGC2506_30b.jpg
DSS blue 30'x30'

NGC2506.jpg
sketch: 14.5", 141x, NELM 6m5+

"Give it a go and let us know!"

[Steve Gottlieb]

Excellent cluster with very interesting structure, described below through my 18-inch.

At 175x, this is a beautifully rich 8' group of stars, roughly circular, with 80-100 stars mag 11-14 over unresolved background haze. The brightest two stars are at the west side and form a wide, 27" double. The richest portion roughly forms a "U" outline, open to the south, as if a chunk of the cluster was missing. The north side of the "U" consists of a string of stars oriented WNW-ESE (north of the two brightest stars) with a pair at the WNW end. At the ESE end of this string a few brighter stars form a right angle heading SSW and forming another side of the "U". A faint string of stars extends out of the cluster to the SE and another string extends out the WNW.