Sky & Telescope - January 2022 - 22

Treasures in the Sky
1 A Concealed Planetary. Hidden in the glare of 4thmagnitude
Mu (μ) Orionis is the faint planetary nebula
Abell 12, discovered by George Abell on photographic plates
in 1966. In the line of stars representing the Hunter's raised
arm, Mu is the next bright one up from Betelgeuse. The
adjacent planetary, at a distance of about 6,900 light-years,
is much farther away than the star, which is only at around
150 light-years.
Abell 12 is just 50″ northwest of Mu, which makes it quite
difficult to observe - but an O III filter will dim the star
enough to allow the planetary to materialize. In my 18-inch
Dob with filter employed, Abell 12 is quite apparent. Under a
dark sky, I've even detected Abell 12 in my 8-inch SCT using
an Ultra High Contrast (UHC) filter. It appears as a ghostly
disk of mostly even brightness about 30″ in diameter with a
slightly brighter rim.
In optimal observing conditions, if you place Mu Orionis
just outside your field of view, you might even glimpse
Abell 12 without a filter.
2 A Sidekick Cluster. From comic books to the " B " Westerns
of the 1940s and '50s, the sidekick has been an endearing
character: a faithful companion to a more important
personage. The secondary position that the open cluster NGC
2158 takes to the brighter, more famous M35 always puts
me in mind of such a partnership. The two clusters aren't
actually associated at all: NGC 2158 is at least 4½ times more
distant than its flashier neighbor. If they were at the same
distance, the situation would be reversed, since NGC 2158
has more than twice the number of stars than does M35. As
it is, though, M35 appears 25 times brighter than its little
sidekick. NGC 2158 is 5′ in diameter and shines at magnitude
8.6. It appears rather concentrated and is even listed as
a globular cluster in both the Revised New General Catalog of
Nonstellar Astronomical Objects (RNGC) and the Lund catalog
of open clusters. However, Brent Archinal and Steven Hynes
in their book Star Clusters point out that NGC 2158 has none
of the properties of a globular. To find the smaller cluster, aim
a bit less than ½° southwest of the bright Messier object at
the foot of Gemini and look for a dense puffball of stars.
3 Headgear for a Norse God. Imagine a line connecting
the two brightest stars in Canis Major, Alpha (α) and
Beta (β), more familiar to us as Sirius and Mirzam. Then
extend that line eastward about 1½ times its length and point
your scope a little north of that spot. At low power with a
UHC or O III filter, you'll be treated to a remarkable nebula.
NGC 2359 is known as Thor's Helmet due to the two hornlike
extensions that protrude out of a dome-shaped nebula.
In photographs (such as the one on page 24), it truly does
resemble the iconic horned Viking headdress of mythology. In
the eyepiece, I see it more as a snail or a rabbit, as one of the
" horns " is rather faint.
NGC 2359 is a vast emission nebula powered by a WolfRayet
star, an extremely hot giant thought to be in a presupernova
stage. It lies about 12,000 light-years from us and
is approximately 30 light-years across. Unfiltered, NGC 2359
is a faint, barely detectable haze. The view through a narrowband
filter, though, is spectacular, and gets more so with
increasing aperture.
4 The Unicorn's Other Fan. Fan-shaped nebulae are
sometimes called cometary nebulae due to their resemblance
to stubby-tailed comets. Today, the modern term is generally
applied to objects illuminated by T Tauri stars (young variable
stars akin to the prototype, T Tauri). Monoceros is home to
perhaps the most famous cometary nebula, Hubble's Variable
Nebula (NGC 2261), but it also houses another often-overlooked
fan. NGC 2316 is a small emission/reflection nebula
about 1° northwest of the open cluster M50.
According to S&T Contributing Editor Steve Gottlieb's
" Adventures in Deep Space " website (see link on page 27),
William Herschel discovered NGC 2316 in March of 1785.
Irish engineer and astronomer Bindon Stoney observed the
nebula in February 1851 and reported a second nucleus that
John Louis Emil Dreyer subsequently recorded as NGC 2317
in his New General Catalogue of Nebulae and Clusters of Stars
(published in 1888). The RNGC lists NGC 2317 as nonexistent,
and several modern references assume that 2316 and
2317 refer to the same object.
To find the nebula, look for a small, cone-shaped glow
spreading toward an arc of three stars. You'll need at least a
10-inch scope, and a light-pollution filter, such as the Lumicon
Deep Sky, will enhance the view. Use moderate magnification
to separate the nebula from the stars.
HIDDEN PLANETARY Kick off your quest for celestial treasures in
Orion, the Hunter, by playing peekaboo with Abell 12.
22 JANUARY 2022 * SKY & TELESCOPE
5 A Holiday Icon. The open cluster NGC 2264 is also
known as the Christmas Tree Cluster and is associated with
DEREK SANTIAGO
Sky & Telescope - January 2022

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