M35 Plus One

M35 is an open cluster in Gemini, near the foot of the western twin – Castor, if we name the twins after the stars that mark their heads in the constellation figure. Interestingly M35 is the only Messier object to be found in Gemini.

It is on the edge of naked-eye visibility at magnitude 5.2, but is easily visible in binoculars.

With the aid of binoculars, or better still a telescope, a small round cloud of stars appears to the south-west of M35. This is NGC 2158, another open cluster although its shape is similar to a globular, and early observations did identify 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 (based on a 2020 study of Gaia satellite data).

The colours of their stars give away another contrast between the two clusters: the bright blues of M35 signify its youth – it is only about 100 million years old – whereas the more yellowish colours of 2158 give away its age – 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 our 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.

Technical details:

Planewave CDK-14 corrected Dall-Kirkham reflector, FLI Proline P9000 cooled CCD camera and filter wheel with Astrodon LRGB filters. Total exposure times were 4 hours through each filter. All processing carried out in PixInsight.