Sunday, July 20, 2008
A spectral riddle in the Rocking Horse
In the night of July 13th/14th 2008 I had a short observing session with the 300mm Dob. I took a quick peek at a few old friends (M27, M29, the Blinking Planetary and 16 Cygni) trying out my new 35mm Panoptic. While sweeping through Cygnus, I noticed a bright, small clump of stars North-Northeast of Gamma Cygni. It was very easy to spot with the 35mm Panoptic. I increased the power to 230x with the 7mm Pentax XW, and I was looking at a wonderful little asterism of stars that, as a group looked like a little dog or horse. I made a rough sketch of this object, and when I later checked the sketch with my planetarium program (Voyager), it proved to be NGC 6910, an open cluster from the Herschel 400 list. According to some sources on the Internet, NGC 6910 is also called the Rocking Horse cluster. I had never heard about it or observed it before, so my first impression that it looked like a dog or horse, was not that strange. Others thought of it as a little horse as well.

Where can you find NGC 6910? The map below should give you a rough idea. Just center your telescope on Gamma Cyngi and move just about half a degree north-northeast. There you will find this nice clump of stars.


(Image from Voyager 4.5,

When you start looking for NGC 6910, bear in mind that it is a small cluster, only 7' in diameter, but then again, at lowest magnification it jumped out at me in my 12-inch scope. With the 22mm Nagler (fov 68') I already saw the complete outline of the little horse, and with the 7mm at 230x I could see a few dimmer, magnitude 12 stars. NGC 6910 is a Y-shaped cluster oriented northwest southeast. I counted between 20 and 25 stars but its always difficult to tell which do belong to the cluster and which are not included. The two brightest stars looked definitely yellow and are from the 7th magnitude.
There is however something that riddles me about these two yellow suns. According to all planetary programs and Internet databases, the Northernmost of the two stars is SAO 49556, a spectral type K1III, which explains the yellowish color. The southernmost of the two is SAO 49563 (or V2118 Cygni), a variable star of spectral type B1.5Ia. Normally I would think that a B1 star shines Bluish or at least mainly white, and not yellow. I checked other observing reports and I found that Sue French's (Celestial Sampler, page 132) reports:

At 87x, two yellowish stars of 7th magnitude and a pearly, split chain of eight 10th magnitude stars unite in a Y-shaped pattern about 5' long.

Sue sees two yellow stars as well, so I'm very curious why this B1 star seems to appear yellow instead of white. Does it have something to do with the Variable character of the star? Maybe one of you out there can help on this one.

Anyway, visually it is a very interesting group, so I made a sketch. The sketch below was made with the 300mm dob, a 7mm eyepiece (fov 18') giving a magnification of 230x. South is up and west is to the left. The next time when you're in the area observing, check out on NGC 6910 and let me know which colors you could see.


Posted by Math on 07/20 at 12:35 PM | (0) Comments | filed in: Deepsky log | Print