Sky & Telescope - September 2022 - 23

POSS-II / STSCI / CALTECH / PALOMAR OBSERVATORY
they're rising in the east, NGC 659 and NGC 663 and the
stars in between them look like a goat. NGC 659 is the tail of
the goat, the arch of stars between clusters is the back, and
NGC 663 is the head. There are even strings of stars sticking
downward to make legs. It's pretty distinctive. Once you see
this one, you can't un-see it.
In the winter sky, you'll find another nice set of open
clusters - 6.1-magnitude M46, 4.4-magnitude M47, and
6.7-magnitude NGC 2423 - about 13° almost due east of
Sirius. This trio spans 1.5°, so you'll need your lowest power
to see them all at once, but it's also fun to move the telescope
around and watch each concentration of stars fill the eyepiece
in turn. They're great in binoculars, with each cluster standing
out well against the background. As you might expect
from their catalog numbers, NGC 2423 is the runt of the litter,
but it's still distinctive enough to fool a lot of people into
thinking they're seeing M46. Nope, M46 is on the other side
of M47, which is the showiest of the trio.
M46 has an Easter egg in it: The planetary nebula
NGC 2438 is superimposed on its northern edge (see, for
example, page 41 in the February issue for an image).
Globular clusters sometimes bunch together, too, but not
tightly enough for any of them to really feel like a triplet.
So, let's look at an interesting triple formation within one of
the sky's most famous globular clusters: M13. You'll find the
5.8-magnitude globular about two-thirds of the way on a
line connecting Zeta (ζ) to Eta (η) Herculis. The Great Hercules
Cluster, as it's also known, has a subtle trefoil of dark
lanes within it called the Propeller. It's hard to spot; you
need high magnification and good transparency to bring out
the contrast, but if you keep looking it will slowly emerge for
you until it becomes quite obvious. It's off-center, southeast
of the core, and only spans about a third of the cluster. It's
surprisingly symmetrical, like a triple-bladed airplane propeller,
hence its name.
Nebulous Trios
How about nebulae? The Trifid Nebula (M20) is of course the
most famous three-part nebula in the sky. Look for it a little
bit more than 6° northeast of Lambda (λ) Sagittarii, or Kaus
Borealis, the topmost star in the lid of the Teapot in Sagittarius.
A dark dust lane cuts the bright emission nebula into three
sections. (Although to be honest, it looks like four sections to
me.) A nebula filter helps considerably with the contrast.
The Trifid is triple in another way as well: It's made up of
three different types of nebula. There's the emission nebula
PORTAL, PERCHANCE? The Stargate - a triangle within a triangle - is one of the
neatest triplets in the sky. Look for it in late spring on the border between Corvus, the
Crow, and the zodiacal constellation Virgo. The sketch's field of view is 30′.
sk yand tele scope .o r g * SEPTEMBER 2022 23
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Sky & Telescope - September 2022

Table of Contents for the Digital Edition of Sky & Telescope - September 2022
