Sky and Telescope - December 2015 - 36

Winter Milky Way
If you live far enough south to see to -40°° or -50°,
the stretch of the Milky Way that runs through southern
Puppis and Vela is fantastically rich, and it will reward
patient exploration. But there are wonders to the north as
well, and that is where we're headed next.
The wide triangle formed by Xi, Rho, and 11 Puppis is
an arrowhead pointing north and a bit west. Follow the
arrow about ﬁve degrees and you'll ﬁnd a line of three
equally bright stars trending to the northwest - 16 Puppis, HD 65810, and 6 Puppis. Just over 1° north of 6 Pup
is the ﬁne binocular double Knott 4. With a separation
of 129″ it should be an easy split. Two degrees northwest
of Knott 4 you'll ﬁnd the famous pair of Messier open
clusters, M46 and M47 (Stop 10).
If appreciating celestial objects through a telescope
is enhanced by knowing what they are, enjoying them
through binoculars is about seeing them in context -
and nowhere is that more true than with M46 and M47.
First, compare the clusters themselves. M47 is a bright
shotgun blast of splashy stars, compared to the dense
swarm of dimmer lights that makes up M46. Now, widen
your view. Notice how the pair is bounded to the east and
west by wide, canted pairs of stars. These are 2 and 4 Puppis on the east, and KQ Puppis and HD 60325 on the west.
Another bright star, HD 61722, sits between the clusters
to the south, much closer to M46 than to its sibling. Taken
together, the ﬁve bright stars on the east, west, and south

M46 & M47

M47

TOM WILDONER

M46

of the clusters form a wide "bowl" that seems to hold the
clusters and associations stacked within.
Wait - stacked? Yes! About ½° north of M46 is a
bright association that may outshine the neighboring
Messier cluster. About the same distance north of M47 is
a genuine cluster, NGC 2423. People have spotted NGC
2423 in 70-mm binoculars, but you'll need dark skies
and good transparency to pick up this faint wisp of light.
Taken altogether, there's a lot more going on in the northwest corner of Puppis than just the two Messier objects.
For our last two stops you'll need your astronomical seven-league boots. We're going to take a couple
of long strides east and north to pick up two more
binocular clusters. First, line up M46 with the bright
stars to the east - 2 and 4 Pup, and 9 Pup another 1.5°
out. They don't fall perfectly in line, but if you sweep
east-northeast from M46 about 7° along that rough line,
you'll come to 19 Puppis and the open cluster NGC 2539
(Stop 11). The star is only 185 light-years out, compared
to 4,000 light-years for the cluster. I ﬁnd NGC 2539 a
challenging target. It's almost overwhelmed by the glare
from 19 Puppis - if a 4th-magnitude star can be said
to have "glare"! But spend some time here and see how
many stars you can ﬁsh out of NGC 2539 by steadying
your instrument and using averted vision.
We've come to the end of Puppis, but there's one more
stop, across the border in Hydra. Another 7° sweep, this
time to the north of 19 Puppis, will bring you to M48
(Stop 12). This sprawling expanse of distant suns looks
like a wide chevron to me. Two to three degrees north
on either side are the bright stars C Hydrae and Zeta
(ζ) Monocerotis. Fix these as landmarks and see if you
can spot M48 with your naked eyes. I've never managed
it myself, but other observers have. And when failure
means that you spend a couple of minutes looking at a
pretty part of the sky, there's not much to lose by trying.
There's a lot more to see in this part of the sky - the
whole winter Milky Way from Puppis to Gemini and
beyond is packed with wonders. Many of them are associations and asterisms that don't appear in the famous
observing catalogs or "best of" lists, but they're well worth
seeking out. Realizing how much there is to see beyond
the Messiers was a big step in my binocular observing. I
hope you get swept up in the same exploratory spirit. ✦

10 Visible to the naked eye under dark skies, M47 sparks to life even through
modest binoculars. Under high power, the cluster's central stars jump from
sparks to blazes. Neighboring M46 appears dimmer, a cluster of pinpricks of
light piercing a patch of haze.

December 2015 sky & telescope

Matt Wedel is a mild-mannered paleontologist by day and
an adventurous stargazer by night. He blogs at 10minuteastronomy.wordpress.com.

http://10minuteastronomy.wordpress.com http://10minuteastronomy.wordpress.com

Sky and Telescope - December 2015

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