Celestial hemisphere:  Northern  ·  Constellation: Gemini (Gem)  ·  Contains:  M 35  ·  NGC 2158  ·  NGC 2168
M35 and NGC2158, Peter Hannah
M35 and NGC2158
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M35 and NGC2158

M35 and NGC2158, Peter Hannah
M35 and NGC2158
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M35 and NGC2158

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M35, an open cluster in Gemini,  is on the edge of naked-eye visibility at magnitude 5.2.  The small round cluster to the south-west of M35  is NGC 2158, also an open cluster although its shape is similar to a globular, and early observations identified it as such.
The two clusters differ in several respects: M35 is larger on the sky (about the size of the full Moon) but is much closer at about 2,800 light years compared with NGC 2158’s 15,000 light years (this based on a 2020 study of Gaia satellite data).
The bright blue colours of M35 's stars signify its youth – it is only about 100 million years old – whereas the more yellowish colours of 2158 are consistent with its age of  about 2 billion years. After that length of time most of the blue giant and supergiant stars have disappeared, leaving the older orange and yellow middle-aged stars similar to the Sun. Of course both clusters will also have many red dwarfs but most of these will not be visible at the clusters’ distances.
It should also be remembered that the greater distance of 2158 means that its light will be dimmed and reddened more by galactic dust.

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M35 and NGC2158, Peter Hannah