Astronomy Now - January 2021

Highlights: Observe Gemini’s outstanding

MESSIER 35

When: all night throughout the month

What’s special: 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 nd and may even 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 magnicent 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 or so stars.

How to observe: Gemini is an easily recognised constellation lying to the upper-left (north-east) of
Orion. e Gemini twins, Castor to the north and Pollux to the south, can be visualised as two stick
gures α standing next to each other. e 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 nd 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-month, M35 is well up in the eastern sky by 7pm GMT. It L C E culminates at about
10.30pm 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. e 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 20 arcminutes to the south-west to observe NGC 2158.
is 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.

Magnicent Messier 35 and compact NGC 2158 to its lower right (south-west) are contrasting open clusters.

Messier 35 lies at Castor’s feet in the splendid winter constellation of Gemini.

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Highlights: Observe Gemini’s out...

January 2021

Astronomy Now