M 35 and NGC 2158 - Two Colorful Open Clusters in Auriga

Messier 35 and NGC 2158 are both open star clusters located in the constellation Gemini. Despite their proximity in the sky, these clusters exhibit striking differences in age and appearance.

Messier 35, also known as M35, is a relatively young open cluster, estimated to be around 150 million years old. It contains hundreds of stars spread across a diameter of about 24 light-years. M35 is characterized by its rich population of blue and white stars, which are indicative of its youthfulness. These hot, luminous stars are formed from the cluster's original molecular cloud and are still in their main sequence phase of evolution. M35 is a prime example of a vibrant stellar nursery, where new stars continue to form from the remaining gas and dust.

In contrast, NGC 2158 presents a starkly different scene. This open cluster is considerably older than M35, with an estimated age of around 1.5 billion years. Located relatively close to M35, NGC 2158 showcases a much denser and more compact core, with a diameter of approximately 12 light-years. The stars within NGC 2158 have evolved significantly since the cluster's formation, with many transitioning into later stages of stellar evolution, such as giant and red giant phases. Consequently, NGC 2158 appears much fainter and more concentrated than its younger counterpart.

The striking contrast in the color compositions of these two open clusters reflects the relationship between the age of a cluster and the spectral characteristics of its constituent stars. Younger clusters, like Messier 35, boast a higher proportion of hot, blue stars, which emit predominantly blue and ultraviolet light. As clusters age, their more massive and luminous stars exhaust their nuclear fuel and evolve into cooler, redder giants and supergiants. Thus, older clusters, such as NGC 2158, exhibit a redder hue due to the prevalence of these evolved stars.

In summary, Messier 35 and NGC 2158 offer a captivating glimpse into the dynamic nature of stellar evolution within open clusters, showcasing the profound impact of age on the color and composition of these celestial communities.

Photographed from Bamberg, Germany under Bortle 5 conditions.