Object: NGC 2266

Constellation
RA / DEC
Diameter in '
Magnitude
Number of stars
Brightest star (magnitude)
Distance in parsecs (3.26Ly)
: Gemini
: 06 43.3 / +26 58
: 5
: 9.5
: 50
: 11
: 3400
Details sketch
Date / Time
Observing Location
Seeing / Transparency
Telescope
Eye-piece
Magnification / Field of View '
: 26-12-08 / 22:28
: Landgraaf
: 4 / 5
: Orion Optics UK 300mm
: 7mm Pentax XW
: 229 / 18

Stacks Image 2091
Observing report
NGC 2266 is an open cluster that can be found a little less than two degrees north of Mebsuta or 27 epsilon Geminorum. It is a very compact open cluster, and not easy to locate. However once you spotted the four brighter components, you can see NGC 2261 as a smudge of light using averted vision. If you really want to resolve this little gem, you have to use higher magnifications. With my 300mm f/5 telescope, the 7mm Pentax XW (229x) provides the ideal magnification. When using this eyepiece, NGC 2266 looks more or less well detached from the background, but you still have to use averted vision to see most of the fainter cluster members.

The four brighter stars lie in crooked line, a sort of very flat bowl-shape. The other cluster members lie south of this bowl. There are only 20 to 30 stars visible, using averted vision, and there is definitely a background glow of unresolved stars. Visually it's a poor cluster in the 12-inch. Most stars are just barely visible. I do not see a real central star. I do not detect any double stars or stars with a color.

Besides the four bright components, there are no real chains of stars in NGC 2266. There are however three small asterisms visible. First there is a small diamond shape made up of 4 stars. Below this diamond I see a kind of fishhook, also composed out of four stars. Last but not least I detect an arrow shaped form, again made up of four very dim stars, resembling the constellation Sagitta, the arrow.
Notes
Stacks Image 2103
NGC 2266 is an open cluster of intermediate age, close to the ages of the Hyades or M44 (Praesepe). As you can see on the HR diagram to the right, the cluster main sequence is terminated at V=13.5 and to the right you see at V=13.7 and B-V=0,93 a group of 9 stars that form a clump of red giants. Research showed that based on the number of Red Giants that were identified in the central part of NGC 2266, it can be concluded that the cluster is a factor three richer in stars than the Hyades. A conservative estimate indicates that there are about 190 cluster members with a visual magnitude < 19.5 or absolute magnitude ≤ 6.5 (From "The Hyades-age Anticenter Cluster NGC 2266" by Janusz Kaluzny and Beata Mazur 1991).

What you also can deduce from the HR diagram of NGC 2266 is that most stars of this cluster are of a visual magnitude 14 or dimmer, so what I have actually observed (and sketched) is just the tip of the iceberg, only a handful of stars compared to the 190+ cluster members and field stars.

As you can see on both the sketch above as well as the image of NGC 2266 below, the cluster looks a bit like a Christmas tree with a bright white star at the top of the tree. However, this bright star, Hipparcos 32194, does actually not belong to NGC 2266. While looking at the image of NGC 2266 (shot by T. Credner & S. Kohle) I noticed the Christmas tree shape at once, just as I did in the past with three other open clusters: M103, M39, and the one open cluster that really is called the Christmas tree cluster, NGC 2264.

On the image you see the red giants woven into the blue-white stars of this celestial gem. I noticed that I plotted quite a few of these red giants into my sketch, but at the eyepiece, I did not detect any orange or red stars in NGC 2266. This is probably because of the very faint magnitude. With my telescope it is impossible to detect color in magnitude 12 or dimmer stars.

The brighter trio, which I plotted downwards and to the left from the brightest star in the sketch, are red giants as well (compare with the image of Crednet & Kohle). They are of magnitude 10-12 and will not be in the clump of red giants in the HR diagram, but probably al little higher to the right.

Stacks Image 2113

Image of NGC 2266 by T. Credner & S. Kohle, AlltheSky.com

Stacks Image 2121

Finderchart of NGC 2266 created with Voyager by CarinaSoft