Object: NGC 6910 (The Rocking Horse Cluster)

Magnitude A / B / C
Separation A-BC/ AC
Position angle A-BC / AC
Spectral class A / B / C
Colour A / B / C
: Cygnus
: 20 23.2 / +40 47
: 10
: 7.4
: 66
: 9.61
: 1507
Detail sketch:
Date / Time
Observing Location
Seeing / Transparency
Magnification / Field of View '
: 14/07/08 / 01:30
: Landgraaf
: 4 / 3
: Orion Optics UK 300mm
: 7mm Pentax XW
: 229 / 18
NGC 6910

Observing Report

When you start looking for NGC 6910, bear in mind that it is a small cluster, only 10' 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 out-line of the little horse, and with the 7mm at 230x I could see a few dimmer, mag. 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 really do belong to the cluster and which are not included. The two brightest stars looked definitely yellow and are of magnitude 7.


There was something that riddled me about the 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.
The explanation I got from Jim Kaler: As you know it is in the middle of Cygnus, near Gamma Cygni, not far off the galactic plane. As such it is subject to a great deal of interstellar extinction and reddening from interstellar dust. The dust selectively absorbs and scatters blue light (roughly 1/wavelength), so when you look at a star through dust it will appear redder than it really is (basically the same reason the sun is reddened at sunset). The intrinsic B-V color of a B1.5Ia super giant is -0.2. The observed color is 0.83, about the color of a K0 dwarf or a G5 giant, so the star appears yellowish to the eye. Good observing on your part to notice that. You can see the effect in many other distant clusters near the galactic plane.et.
To illustrate the nebulosity around NGC 6910, I got permission from Dr. Franz Gruber to use his wonderful images of the NGC 6910 region. You can visit his website http://www.steuerhilfe.at/GruberAstro/ for more beautiful images. The first image shows quite a few well-known deepsky objects around NGC 6910. You can also use it as a finder chart. And what about the nebulosity in this region! The second image is a close up of the NGC 6910 area. You see NGC 6910 in the circle. Many stars in the cluster look yellowish, while around NGC 6910 some stars look blue. They are not obscured by the dust, or maybe foreground stars?
NGC 6910 - Regions

Overview of the region where NGC 6910 is situated (Image by Dr. Franz Gruber)

NGC 6910

Detailed view of NGC 6910. Notice the yellow stars in the cluster, caused by interstellar dust, and the bluish-white stars in the vincinity of NGC 6910. These stars are probably foreground stars (Image by Dr. Franz Gruber).