Open clusters abound in the winter sky, offering observers so much choice. One not to miss out on is Messier 35 (NGC 2168), a big and bright (magnitude +5.1) open cluster in Gemini that’s easy to find and may be visible to the naked eye. It comfortably sits alongside the Beehive Cluster (Messier 44) and Messier 37, one of Auriga’s splendid close-lying Messier trio, as the best and most accessible open clusters outside of perhaps only the peerless Pleiades (Messier 45), and the pair of clusters (NGC 869 and 884) that together form the magnificent Double Cluster, that are on show at this time of the year. A humble pair of binoculars easily picks out M35, while a small telescope can show many of its more than 400 stars.
How to observe
Gemini is an easily recognised constellation lying to the upper-left (north-east) of Orion. The Gemini twins, Castor to the north and Pollux to the south, can be visualised as two stick figures standing next to each other. The brightest stars, Castor (alpha [α] Geminorum; magnitude +1.6) and Pollux (beta [β] Gem, +1.1), mark the twin’s heads.
If you follow the line of stars westwards from Castor by around 18 to 20 degrees you will find Tejat (mu [μ], magnitude +2.9), Propus (eta [η], +3.3) and 1 Geminorum (+4.2), the stars that mark Castor’s left leg and foot. From 1 Gem, it’s just a 1.5-degree jump north-east to the middle of M35.
At mid-February, M35 is well up in the south-east by nightfall, at 6.30pm GMT. And culminates (peaks in altitude in the south) at about 8.30pm at a respectable altitude of over 60 degrees.
Once you’ve located Messier 35 with optical aid and you’re well dark-adapted at a dark-sky site and are experiencing a very transparent sky on a moonless night, why not have a go at trying to glimpse it with the naked eye as a mere fuzzy patch of light.
A pair of 10 × 50 binoculars or a small telescope shows M35 to be a large open cluster with an apparent diameter of 28 arcminutes, not far off that of a full Moon. The former can resolve many of its eighth- and ninth-magnitude stars, while you’ll see around 40 members through the latter.
Messier 35 is a relatively young open cluster and is believed to have formed around 100 to 150 million years ago. Comparing it to a much older cluster, albeit one that’s smaller and fainter, is easy: just nudge your 150mm (six-inch) telescope just 20 arcminutes to the south-west to observe NGC 2158. This diminutive cluster (5’ across) looks very yellow and compact – it could be mistaken for a loose globular cluster – but it is one of the most ancient of all open clusters, thought to have formed an amazing two billion years ago.