64 AUSTRALIAN SKY & TELESCOPE January 2018
NGC 2301: MARTIN GERMANO; NGC 2244: JEREMY PEREZ; NGC 2264: LJUBINKO JOVANOVIC / CC BY-SA 3.0southeast. With keen eyes and dark skies, you may also spot a
fifth cluster, NGC 2252, in the middle of the square.
Just over 5° north-northeast of NGC 2244 lies NGC 2264 ,
the Christmas Tree Cluster. The cluster really does look
like a Christmas tree, about ½° tall, with the tip pointing
south. The base is marked by 15 Monocerotis, another O
giant. Spectroscopically, 15 Mon is about as blue as it’s
possible for a star to be, but the perception of star colours
varies enormously among observers, so have a look for
yourself. Like NGC 2244, NGC 2264 is bound up in a vast
field of subtle nebulosity, but you’ll need crystalline skies
and fanatical dark adaptation to spot any of it through
binoculars or small telescopes.
Three degrees northwest of NGC 2264 is a nice little
Y-shaped asterism, and about 6° further on the same track
you’ll find Xi (ξ) and Nu (ν) Orionis, either the right hand
or elbow of the Hunter, depending on how the stick figure is
drawn. If you imagine Xi and Nu Orionis as eyes, you’ll find
a pretty, boxy little group of stars forming a freckled nose
below and between them. This is NGC 2169, the 37 Cluster,
so named because it seems to spell out the number 37 when
seen through a telescope. The ‘print’ here is too fine to read
through binoculars, so instead look for a symmetrical chain
of six stars arching a degree to the east of the cluster.
After you’ve had a good look at the cluster and the star
chain, scan just over 2° southeast for the wide binocular
pair of 73 and 74 Orionis. This pair of stars is one of my
favourite cosmic odd couples — pairs of objects that show up
well in the same field of view, despite lying at wildly different
distances. The southeastern member of the pair is 74 Orionis,
an F-type yellow-white subgiant. It’s only slightly larger than
our own Sun, and lies only 63 light-years away (hello, 1955!).
The other star, 73 Orionis, is another story entirely. It’s a
blue-white B giant, seven times the diameter of our Sun and
almost 700 times as luminous, lying 1,200 light-years away
(hello, Dark Ages!).
For our last stops we’re going 10° north, to Gemini and
the western foot of the western twin. You’ve probably been
here countless times to look at M35, but don’t skip to that
famous cluster just yet. Instead, take a minute and check in
on Cr 89, a beautiful and often-neglected open cluster. It’s
2° east-southeast of M35, anchored between the foreground
stars 9 and 12 Geminorum. Cr 89 is a little bigger than M35,
a little more spread out, and a little less bright, but it’s an
easy catch and a rewarding view, and most binoculars will
show it in the same field as its Messier neighbour.
And that brings us to our last stop, M35 and NGC 2158,
another cosmic odd couple. M35 is big and bright, naked-
eye visible under good conditions, and a stunning object
through binoculars or telescopes of all sizes. That’s because
it’s close, only 2,600 light-years away. Close is a relative term
here, since the light we see now left M35 around 600 BC,
before the founding of the Roman Republic. NGC 2158 is a
tougher catch, much smaller and dimmer than M35. That’s(^5) SetyourlargerbinosuponatripodforNGC2301.Through8×50s,
theclusterislittlemorethanasmudge,butwhenviewedwithstabilised
15 ×70s,starswillbegintopop.Thebrighteststarinthecluster,
8th-magnitudeG8subgiantTYC148-2862-1,maybeaforegroundstar,
nottruememberofthecluster.
(^6) ThebrilliantopenclusterNGC2244dwellsatthecentreofthe
RosetteNebula,andcloudsofdustandgassurrounditsbrightstars.
You’ll need dark skies to tease out the nebulosity, but even if light
pollutioninterfereswithyourviewoftheRosette,thehot,youngcluster
never disappoints.
(^7) The brighter stars of open cluster NGC 2264 outline the rough form
of a traditional Christmas tree. This cluster is a star-forming region, so
its component stars burn blue and hot, energizing the clouds of
nebulosity surrounding them.
NGC 2301
NGC 2244
NGC 2264
N
BINOCULAR TOUR