Sky & Telescope - July 2021 - 37

BUTTERFLY CLUSTER
Darker areas of the Great
Rift help M6 pop into view.

M 6: WOLFG A NG PA ECH / FR A NZ HOFFM A NN /
CHAM ÄLEON OBSERVATORY; SK E TCH: GU Y M ACKIE

19h

α

18h

17h
+10°

κ
Star magnitudes

many pairs and triplets, which
sparkle in patterns of lines and
small groups. " No wonder Ernst
Johannes Hartung called it a " glorious cluster. "
Charles Messier discovered M23 in 1764. It's not too difficult naked-eye - it's the fuzzy found about 4° west of M24,
the Small Sagittarius Star Cloud. My home observatory's
16-inch Newtonian showcases a bright, magnificent, rich
cluster, arranged in three concentric arcs. The stars, mostly of
the same magnitude, are uncountable.

6633
1
2
3
4
5

IC 4665

γ

OPHIUCHUS
0°

Best Things Come in Threes

M11

The clusters IC 4665 and NGC 6633 in Ophiuchus and
IC 4756 in Serpens Cauda are all large, bright, and very suitable for binoculars. I also find they're not too difficult with
the unaided eye from my dark backyard (located in a lakeside
subdivision without streetlights). Swiss astronomer Philippe
Loys de Chéseaux discovered NGC 6633 in 1745. He's also
credited with finding IC 4665 around the same time, but,
given my independent, naked-eye discovery as an inexperienced teenage observer, I have to believe that many ancient
astronomers must have noted this 4.2-magnitude fuzzy
patch. IC 4665 was the second naked-eye object that I spotted
while admiring the summer Milky Way for the first time at
a dark site (on the same evening that I first noticed M7). But
IC 4665 wasn't plotted on my 1959 edition of Norton's Star
Atlas, so it was many years before I learned what I had found.
IC 4665, sometimes called the Summer Beehive, is large and
prominent in my 7×50s. It's a loose cloud of at least 10 resolved
sparklers - more if outliers are members. In my 8-inch at 61×
it does resemble the Beehive Cluster. There are two touching
circles of jewels at the center, a reddish gem, and lines of stars
marking the cluster boundary. A half degree east of the main
throng of glittering jewels winds a loop of 9th-magnitude stars
(including three pairs) that appear to form a border to IC 4665.
(But they may not even be associated with the cluster.)

M26

M12

η

β
δ

M14

α

M10

ν

ζ -10°
ο
ξ

M16

SERPENS
CAUDA

γ

SCUTUM

M17
M25

ο

σ

β

IC 4756

M18
M24

μ

ξ2
M22

λ

M28

M107

η
M9

M23

-20°
M21
M20
M19

M8

δ

M54
M69

Antares

τ

M62

γ

-30°

M6

ε

M70

ε
SAGITTARIUS

M7

λ

μ

SCORPIUS
θ
CORONA
AUSTRALIS
α

-40°

6231

ζ

θ
α

TELESCOPIUM

α

IC 4651

-50°

ARA

s k y a n d t e l e s c o p e . o r g * J U LY 2 0 2 1

37



http://www.skyandtelescope.org

Sky & Telescope - July 2021

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

Contents
Sky & Telescope - July 2021 - Cover1
Sky & Telescope - July 2021 - Cover2
Sky & Telescope - July 2021 - 1
Sky & Telescope - July 2021 - Contents
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