Sky & Telescope - July 2021 - 38

NGC 6633 is ideal for 7×50 binoculars and should be better known. It looks somewhat like a T with a bent upright,
except that the lower (southwestern) end of the upright
broadens into a rich wedge of stars. My 70-mm ﬁnder works
well on the elongated cluster, revealing about 40 nearly equalmagnitude sparks at 17×. NGC 6633 is better separated from
the ﬁeld with the 70-mm ﬁnder than with my 16-inch.
The cluster nicely ﬁlls the 51′ ﬁeld of view of my 16-inch
at 76×, which shows it as very elongated southwest to
northeast, bright, and straggling. However, Star Clusters (by
Brent Archinal and Steven Hynes) gives the diameter as only
20′, which would mean that the northeastern end is likely
not part of NGC 6633. Three doubles and a nice arc adorn
the northeastern end of the true cluster. At this low power
I see about 50 fairly bright stars and as many faint ones. A
Cassiopeia-shaped asterism highlights the western side.
Similarly, IC 4756 is also perfect for 7×50 binoculars. It's a
rich, similar-magnitude group straggling 1° east-northeast to
west-southwest, and is many times the area of NGC 6633. In
my 4.2-inch Astroscan at 16×, the cluster is a formless mass
of eighty 10th-magnitude stellar pinpoints, overlain with a
few brighter ones. It's an enjoyable object but doesn't lend
itself to a compelling description as there are no prominent
arcs or star chains. Even so, I still called it a " beauty " in my
logbook. IC 4756 is well separated in my 70-mm ﬁnder at
17×, while it's far too big for the 16-inch at 76× - but I nevertheless enjoyed using the big scope to sweep through this
loose, equal-magnitude cluster. Two orange stars decorate the
southern side, and a bright yellow one lies on the eastern side.
Some guidebooks label IC 4756 Graff's Cluster, honoring
Polish-German astronomer Kasimir Romuald Graff's independent discovery of it in 1922. But both Steve Gottlieb and
Wolfgang Steinicke point out that in Celestial Objects for Com-

S NGC 6231 Twinkling above Zeta1 (bottom, at right) and Zeta2 Scorpii,
the cluster forms the " head " of the False Comet (see also page 41).

mon Telescopes (published in 1859), British astronomer and
reverend Thomas William Webb wrote: " Between it [NGC
6633] and Theta [(θ) Serpentis], nearer the former, is a beautiful large cloud of stars, chieﬂy 8 and 9 mg., a nearer part,
apparently, of the Galaxy; visible to naked-eye, and requiring
a large ﬁeld. " Therefore, it might be more appropriate to refer
to the object as Webb's Cluster.

Last But Not Least
NGC 7789 is known as Caroline's Rose, a succinct name that
both describes the cluster and hails Caroline Herschel as its

NEAR THE STINGER
M7, the most southerly of
the Messier objects, is so
large and bright that it's an
easy naked-eye target. It's
sublime in binoculars.

J U LY 2 0 2 1 * S K Y & T E L E S C O P E

M7: WOLFG A NG PA ECH / FR A NZ HOFFM A NN / CH A M Ä LEON OBSERVATORY; M7 SK E TCH: GU Y
M ACK IE; NGC 6231: JOSEF PÖPSEL / STEFA N BINNE WIES / CA PELL A OBSERVATORY

Favorite Open Clusters

Sky & Telescope - July 2021

Table of Contents for the Digital Edition of Sky & Telescope - July 2021