Thursday, January 12, 2023


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Open clusters are groups of relatively young stars formed by the same molecular cloud and bound by mutual gravitational interactions. Because of this, stars belonging to the same open cluster share a lot of their properties, such as age and metallicity.

Open clusters, however, are not all the same: we can actually find near the Earth as far away, scattered as compact or extremely young as older.

Among the best examples for studying the diversity of clusters are M35 and NGC 2158 which, despite being practically on the same line of view for a pure perspective effect, could not be more different.

M35, on the lower left, is located relatively close to Earth, just 2800 light years away. The cluster is very young, with an age of only 150 million years, and relatively widespread, with about 2500 stars scattered in a sphere with a diameter of 30 light years. Young clusters like M35 are characterized by an intense blue color due to the very massive stars that make it up.

NGC 2158, on the upper right, is four times farther from Earth than the M35, ten times older and much more compact. The cluster also appears yellow, in sharp contrast to M35. This is because all the very massive stars that were initially located inside it have already exploded, having finished burning all the nuclear fuel at their disposal. Within NGC 2158, only stars of solar-like mass remain, which live longer and glow instead of yellowish light.

Open clusters are not entities that exist eternally. As time passes, the stars move more and more in space, and as a result the gravitational interactions that bind them together become weaker and weaker, until the cluster is completely dissolved.

Credit: CFHT, Coelum, MegaCam, J.-C. Cuillandre (CFHT), G. A. Anselmi (Coelum).

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