Sky & Telescope - September 2022 - 21

SKETCHES: CINDY KRACH (2)
and Arzachel are different sizes. Ptolemaeus is the northernmost
and the largest, with a flat, lava-flooded floor broken by
a single craterlet (Ammonius). Just below it (and sharing an
overlapping rim), Alphonsus is a bit smaller and is also mostly
flooded, but its central peak still stands just a smidgen above
the lava. Arzachel is off by itself half a diameter to the south
and has a distinct central peak and slumped walls. It's great to
study these three siblings, each with its own unique features.
The rule of threes says the Moon should provide a third
triplet, but nature doesn't really care about that sort of thing.
It has given us two really good ones (and a bunch of lesser
ones), so let's move on to deeper sky.
Stellar Triplets
How about triple stars? There are dozens of them, way too
many to describe here, so I'll just talk about my favorites.
Observers often consider Iota (ι) Cassiopeiae the most
colorful triple, in that it has a 4.6-magnitude white primary
with 6.9-magnitude yellow and 9.0-magnitude blue companions.
The separations between the primary and its companions
are only 3.0″ and 6.7″, respectively, so you'll need plenty
of magnification and steady seeing to split all three, but
when you do it's a beautiful sight. To find it, extend the line
between Delta (δ) Cassiopeiae and Epsilon (ε) Cassiopeiae to
the northeast the same distance again.
There's another beautiful triplet in the middle of the Pleiades.
Alcyone is the notorious central Pleiad, but right next to
it on its northwest edge is my main reason for looking at the
Seven Sisters on any given night. A delightful triangle of stars
comprising 24 Tauri, HD 23607, and HD 23608 (magnitudes
6.3, 8.3, and 8.7, respectively) form a pleasing trio that
stands out well even at medium-low power.
As long as we're digging into the Henry Draper catalog,
I also think HD 51502 in Gemini is pretty special. It's the
lucida of a triplet of 7th- and 8th-magnitude stars with
enough separation (81″, 107″, and 124″) to just be splittable
in binoculars if you steady them on something solid. It's nice
in a telescope at low power, too. Look for them about 3½°
northeast of Xi (ξ) Geminorum. They make a nearly equilateral
triangle, which I always find pleasing in triple stars,
and they have subtle color differences of white, yellow, and
blue. The stars aren't a true triple, since they lie at different
distances, but it's a neat little asterism.
HD 46867, about 1° east-northeast of the center of the
Rosette Nebula in Monoceros, is the anchor of another nice,
nearly equilateral triangle. These three 8th- and 9th-magnitude
stars split at 30″, 34″, and 40″, so they're pretty tight in
binoculars but easy in a telescope.
What about more intricate triangles? Are there any made
up of more than just three stars? Indeed, there are. One of
uCOPIOUS CRATERS (Top) Theophilus, Cyrillus, and Catharina make
a distinctive triplet on the Moon in more ways than one: Catharina is one
of only a handful of lunar features named after a woman. (Bottom) Ptolemaeus,
Alphonsus, and Arzachel stand out beautifully near first quarter.
The fields of view (FOV) of the two images are 2.5′ and 3′, respectively.
sk yand tele scope .o r g * SEPTEMBER 2022 21
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Sky & Telescope - September 2022

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