Object: NGC 2129

Magnitude A / B / C
Separation A-BC/ AC
Position angle A-BC / AC
Spectral class A / B / C
Colour A / B / C
: Gemini
: 06 00.7 / +23 19
: 2.5
: 6.7
: 73
: 10
: 2200
Detail sketch:
Date / Time
Observing Location
Seeing / Transparency
Magnification / Field of View '
: 16/02/10 / 21:45
: Landgraaf
: 4 / 3
: Orion Optics UK 300mm
: 12mm Nagler Type 4
: 133 / 37
NGC 2129

Observing Report

NGC 2129 is easy to locate using the star hopping method, but on the other hand, at low magnifications very hard to recognize as an open cluster. With the 35mm panoptic at 46x only the two brightest members of NGC 2129 jump out of the background. Using the 17mm Nagler at 94x reveals the true nature of this small open cluster.

The optimum magnification for this tiny cluster is achieved with the 12mm Nagler (133x). The bright little cluster looks well detached from its surroundings. NGC 2129 is a relatively poor cluster with not more than 25 stars. The range in brightness is small, only the two brighter stars really stand out from the group. The cluster consists of two chains of stars running more or less east to west. The brightest star just shows a hint of yellow. All the other stars seem to be white.
I see no background glow from unresolved stars. There is no real central star.


NGC 2129 is situated inside the local spiral arm, towards the direction of the Perseus spiral arm. The apparent magnitude of 4.3 (6.7 corrected with 2.6 mags for absorption from interstellar dust) and a distance of 6000 Ly result in an absolute magnitude of -7. If you would place this young open cluster at the same distance as the Pleiades, which is 440 Ly, the visual magnitude would increase from 6.7 to 1.1. The apparent size of NGC 2129 would increase from 6' to 80' or 1.3 degrees.

Nearly two thirds of the light of the cluster comes from the two brightest members, the highly luminous B2 and B3 supergiants at the centre of NGC 2129. B-type stars normally look white, but due to "reddening" by interstellar dust, white stars can appear yellowish. An extreme example of this effect can be found in NGC 6910.

A recent study (Carraro, Chaboyer, Perencevich) shows that NGC 2129 is a true open cluster. However its actual size is only 2.5', and the two brightest members do not seem to belong to the cluster.