Sky & Telescope - February 2021 - 28

Winter Deep Sky

HD 250956

Mu Orionis

S FAR, FAR AWAY The open cluster NGC 2141 is packed with at least
300 suns, but it's more than 13,800 light-years from Earth and therefore
challenging to observe in a light-polluted sky. NGC 2141 appears larger
here than it does in backyard telescopes. Thankfully, the pale cluster
isn't difficult to locate, as it's framed by an attractive pentagon-shaped
asterism.

Numerals in Space
To reach my third stop, I followed 74 and 73 Orionis northwestward to 4.5-magnitude Xi (ξ) Orionis and its near twin,
Nu (ν) Orionis, 1.2° farther northwest. Conveniently, Xi and
Nu make an almost equilateral triangle with the 6th-magnitude open cluster NGC 2169, which lies within a degree
of both stars. The triangular threesome is a quick catch in
finderscopes; the cluster itself is a glittery smudge in my 6×30
and 8×50 finders. Indeed, NGC 2169 is the only specimen
in my Club Tour boasting stars bright enough to plot on the
Uranometria charts. Better yet, it's sufficiently prominent to
appreciate in small telescopes. I began investigating it using a
102-mm (4-inch) f/6.5 achromatic refractor.
28

FEBRUARY 2 021 * SK Y & TELESCOPE

NGC 2169 holds maybe 30 stars, most of them arranged
in two clumps on a northwest-southeast slant, with a narrow gap between them. Each clump is about 2½′ wide;
together they span 6′ of sky, gap included. Half of the
cluster members shine at 7th- to 10th-magnitude;
the others are much fainter. Not a deluxe deal.
The refractor at 100× presented a grand total
of six stars in the northwest clump, eight
in the opposite clump. You might not be
impressed until you discover that 6 + 8
= 37. Strange but true: The bare bones
of this odd duck resemble the numeral
37, as eyed in an erect field, which
(thankfully) the refractor's prism
star diagonal gave me. Check it out
for yourself. (In a Newtonian reflector, the 37 will be upside-down.)
My hefty Mak-Cass operating at
90× brightened up the ragged 37,
yet added very few extra stars to the
barren cluster. However, the longfocus scope rewarded me with one
delightful detail: NGC 2169's 7thmagnitude lucida, in the southeastern
clump (the 3 in 37), is a binary star.
Σ848 (Struve 848) is a tight tandem:
Its 7.3-magnitude primary and 8.2-magnitude secondary are separated by a scant
2.6″. When the air was steady, the uneven duo
resolved cleanly at 113×. On turbulent nights,
though, Σ848 rarely obliged me. Give it a try -
Σ848 is a challenge worth attempting.
On one night of perfectly calm seeing, I cranked up my
10-inch reflector to 227× and stared hard at NGC 2169. In
addition to a finely resolved Σ848, the high power picked up
10 very faint suns around the 37 formation. The bigger aperture helped confirm that the population shines blue-white,
with three evident exceptions. A 9.9-magnitude star on the
eastern periphery of the southeast group was yellowish. An
8.4-magnitude star atop the northwest group seemed deeply
yellow. Beside it, a 9.1-magnitude star (northernmost in the
cluster) burned orange. I suspect the orangey outlier isn't
physically part of NGC 2169.

The Low-Power Look
An especially pleasing aspect of NGC 2169 is its picturesque setting, which was fabulous in the wide-field refractor. Reducing the 4-incher to 20×, I nudged northwestward
toward Nu Orionis. Two-thirds of the way there, I noticed the
ruddy hue of 7.1-magnitude HD 41676, a red giant star. West
of it, I spotted two doubles within 10′ of each other. AG 321
comprises 7.8- and 8.8-magnitude stars 36″ apart, while its
nameless neighbor sports 9.4-magnitude twins 63″ apart. The
refractor captured all this near-to-Nu stuff (and the cluster)
in a single field of view.

POSS-II / STSCI / CA LTECH / PA LO M A R OBSERVATORY

NGC 2141



Sky & Telescope - February 2021

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