Sometimes open clusters appear in pairs or groups in our sky. The Double Cluster in Perseus is the most famous of these, partly because the two clusters are actually close to one another in space. Other groupings are simply chance alignments, with one cluster being significantly further than the other.
One such grouping is Messier 35 and NGC 2158 in Gemini. NGC 2158 was once thought to be a globular cluster, which is actually sort of satellite galaxy that orbits large galaxies like our own. It was eventually determined to be an unusually dense open cluster located much further away than Messier 35.
|Messier 35 (left) and NGC 2158 (right) in Gemini|
|Messier 38 (left) and NGC 1907 (right) in Auriga|
|Messier 38 (upper-right) and Messier 36 (lower-left)|
Another interesting pair is Messier 46 and Messier 47 in Puppis. Planetary nebula NGC 2438 appears to be part of Messier 46, but in reality it is between us and the cluster. The image below also contains two smaller open clusters cataloged as NGC 2423 and NGC 2425 (see the labeled image, further down).
|Messier 46 (left) and Messier 47 (right)|
|Open Cluster Family Portrait with Labels|