Object of the Week, September 20th, 2015 – The "crowned" open cluster IC 1311

IC 1311 (Cr 414, Lund 928, C 2008+410, Tr 36)

R.A.: 20h10m47s
Dec.: +41°10'19"

Size: 5.0'
Type: I 1 r n
Total mag (V): 13.1
SB (V) 16.6
Number of stars: 60
Mag of brightest star (V): 17.0

The northern Milky Way is full of interesting open clusters. One of the highlights and my personal favorite of all is IC 1311.

With 2.3° (WNW) distance to Sadr (gamma Cyg) the cluster stands high in the sky, also for northern observers and can be observed for several summer – autumn months. Although good placed in the sky, the finding is a little bit tricky because of the lots of stars in the region – maybe the reason of its anonymous status.

The physical parameters of this old cluster are often examined but still not very precise. The distance ranges between 5kpc – 16kpc and the age between 0.8Gys and 2Gys. More important for us are the I 1 r n Trumpler type (strong concentration, most stars of nearly the same brightness, rich, cluster in involved in nebulosity) and the number of stars. More about the magnitudes later.

When we look deeper in the photographs of Rosso or with bigger aperture of the 4m-Mayall telescope, the surrounding of the cluster looks like shells of a large PN. The Capella-team made Ha and [OIII] exposures which shows, that a PN is very unlikely.

When it comes to the practical observation the SB and the faint magnitude of the brightest star act as a deterrent but in reality the cluster can bee seen under good transparency with a 4" telescope. The brighter stars which frames the cluster stands out first – that is why I call it "crowned". From 8" the cluster began to show a mottled structure. With 27" the cluster is fully resolved into dozens of stars. In contrast to the Trumpler classification the visible concentration is not so distinct.
Beside the open cluster itself the neighborhood offers lots of types of objects like the small and faint PN K 4-51 (PNG 077.7+03.7, 25' SE), the poor OC Dolidze 2 (16' NW) or the DN B 342 (16' W).

Martin Germano, 8"
Attachment 1790


As always, give it a go and let us know!

It's interesting that although IC 1311 was discovered by Thomas Espin with a 17-inch reflector (private observatory in England), he described it as an "extremely faint nebula within a circle of bright stars." So, he was unable to resolve it or perhaps used too low of a magnification (he was sweeping for red stars).

With my former 17.5-inch (back in 1993) I noted ~15 stars mag 13.5 to 15 in a 4'x3' oval outline over unresolved haze.

Thanks Steve for the discovery history and your observation.

And you are right - I forgot it to write in the OOTW article, the given magnitude of the faintest star of 17vmag (StarClusters, Achinal/Hynes) seems also to low for me. I noted "mottled structure with some stars popping out" with my 16", so your estimated magnitudes fits better also to my experience.