Messier 35 and NGC 2158 are open cluster in the constellation of Gemini. The two clusters are unrelated and 9,000 light years far awy from each other.
NGC 2158 is believed to be about 2 billion years old, and its stars are strongly concentrated, similar to a globular cluster. Previously, it was mistaken for such due to its concentration; however, due to its age, the identification as an open star cluster is unequivocal.
When NGC 2158 is observed alongside M35 in the telescope's field of view, the contrast in size and star density becomes particularly evident. NGC 2158 is five times farther away but has at least 10 times more stars and a much higher age.