If you look in the direction of the constellation of Gemini you could in fact come across this spectacle of the sky: practically on the same line of sight you can observe two open clusters!
The two clusters of course are not physically connected: M35 (bottom left) is 2800 light years away from Earth, while NGC 2158 (top right) is four times farther away.
As you can easily see at first glance, the two clusters seem very different from each other.
M35 is in fact a very young cluster, with an age of only 150 million years, as we can understand from the bright blue color of its components. M35 is also a very widespread cluster: it contains approximately 2500 stars scattered in a volume with a side of 30 light years.
NGC 2158, on the other hand, is ten times older than M35, with an estimated age of over one billion years.
The age calculation of NGC 2158 was carried out based on the color of the stars that compose it. Blue stars are in fact stars of large mass which consequently have a short life time: therefore they evolve rapidly becoming red giants. Yellow stars, on the other hand, are low-mass stars that live very long.
Looking at NGC 2158 it is easy to see that it contains only yellow/red stars and no blue stars. This means that the cluster must have lived for at least a billion years, a time long enough to allow the blue stars initially present to evolve into red giants.
Credit: CFHT, Coelum, MegaCam, J.-C. Cuillandre (CFHT) & G. A. Anselmi (Coelum).
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