NGC 2129: Gemini’s Youthful Star Cluster

NGC 2129 is a small, yet bright open star cluster in the Gemini constellation. It’s easy to pass over a smaller open cluster like this one during the wintertime due to the rich collection of available sky targets. Don’t pass NGC 2129, though, as it’s worth the time to seek out binoculars or a telescope.

Around 20-30 stars make up this young star cluster and its diameter is relatively small with a diameter of 10.4 light years. In the sky, it’s 2.5 arcminutes wide, which is pretty small considering the full moon is 31 arcminutes in size.

Of note, two bright stars lie near the center of the cluster. Both stars are B-Type stars. This means both burn hotter than many stars and appear blue-white in color — not the red color shown in the banner photo above. Also, these two stars follow the same motion in the sky, so it’s believed they may make up a true binary star. ¹

My Observations

A thin sliver of high clouds has moved into the region. As the cloud approach Gemini overhead, I’ve decided on one last-ditch effort to spot NGC 2129. How I wish this night would remain crisp and clear, but the conditions are deteriorating. Still, I aim for this small cluster.

To my surprise, a handful of stars peer through the hazy night. I notice two brighter ones near the center, and it appears a handful of stars (like salt on a plate) are sprinkled around them. I want to study this cluster again on a better night.

Key Stats

Constellation	Gemini
Best Viewing	Winter
Visual Magnitude	+6.7
Absolute Visual Magnitude	-5.0
Distance from Earth	7,200 ly
Milky Way Location	Orion Spur
My Viewing Grade	B

Sources and Notes

• Photo of NGC 2129 provided by the Sloan Digital Sky Survey. This file is licensed under the Creative Commons Attribution 4.0 International license.
  
• ¹ Wikimedia Foundation. (2021, July 1). NGC 2129. Wikipedia. Retrieved May 14, 2022, from https://en.wikipedia.org/wiki/NGC_2129
  
• Sketch by Wayne McGraw.