Sky & Telescope - July 2020 - 62

Big, Bold, Bright, Beautiful

ters just over the border in Scorpius.
They're visible to the naked eye under
a dark sky and are glorious in binocuM11
lars or a wide-ﬁeld telescope.
Wild
Duck
AQL
Cluster
Of the two, M6 is my favorite. It's
α
smaller
than M7, but richer and more
δ
M26
ν OPHIUCHUS
delicate. It's often called the Butterﬂy
-10°
Cluster, for reasons that are obvious
in binoculars but less so in telescopes.
Its stars appear bright blue and conSCUTUM
spicuous, arranged in a wide oblong
6611 M16
Eagle
that suggests the wings of a butterﬂy.
2
SERPENS
γ
Nebula
ξ
C
A
U
D
A
There's even a fairly obvious V of stars
3
-15°
that form the butterﬂy's antennae.
4
M17
5
M7 is bigger and showier than M6
B93
M18
6
B92
and
so bright it's easily spotted with6603
7
M24 Star Cloud
out optics. The Alexandrian astronoM25
The
M23
mer Ptolemy mentioned it as early as
Caribou
-20°
130 AD and described it as "a little
μ
ξ2
cloud following the stinger of Scorpius."
M21
π
Commonly known as Ptolemy's Cluster,
ο
M20
M22
its stars are brighter and more widely
Triﬁd Nebula
spaced than in M6, but similar in numM28
M8
ber (about 80). The reason for that is
Lagoon Nebula
-25°
λ
distance: At 800-1,000 light-years, M7
OPH
σ
φ
is only about half as far as M6.
6520
Galactic
B86
As long as we're in the neighborCenter
τ
(Sgr A*)
hood, we should stop by M4, a globu6522
δ 6528
lar cluster in Scorpius a little more
M54
than 1° west of Antares. At a distance
ζ
M6
-30°
γ Baade's
Butterﬂy
Window
of 7,000 light-years, M4 is one of
Cluster
M70 M69
the nearest and brightest globular
clusters in the sky. But at declinaSAGITTARIUS
tion -26° it's close to the horizon for
ε
M7
Northern Hemisphere observers. M4
-35°
is well worth a look, though, since it's
SCORPIUS
η
probably the easiest globular to resolve
λ υ
CRA
γ
into individual stars all the way to
the core. In fact, there's an obvious
chain of stars running right through it. The cluster is just a
background glow. These two clusters are 11-12 billion years
smidgen bigger than the Moon, so use low to medium power.
old, which make them two of the oldest globulars known.
M4 is disappointing in twilight or in light-polluted sky, but
Lying 2° directly north of Baade's Window you'll ﬁnd
it's glorious in full darkness.
the compact open cluster NGC 6520. While this is a nice
object in its own right, with its young blue stars standing out
against the yellower stars deeper toward the galactic core, the
Globulars Galore
main attraction here is Barnard 86 just a smidgen to the
Sagittarius, Scorpius, and Ophiuchus are all lousy with globuwest. B86 is a dark nebula that's so dense no stars are visible
lar clusters. Not surprisingly, Charles Messier picked all the
through it. The few stars you can see against the blackness are low-hanging fruit, so all the brightest have Messier numin the foreground. It's clouds like this that mask the core of
bers. One of my favorites is M5 next door in Serpens. Why?
the galaxy from our view in all but Baade's Window.
Because I discovered it by naked eye long before I'd tracked
down all the Messier objects. I was looking into Serpens
Caput through a gap in some trees when I saw what looked
Follow the Steam
like a fuzzy star. I got out the binoculars and had a look, and
From here the logical course is to simply follow the "steam"
sure enough, a fuzzy star! In a telescope it resolves into one of
of the Milky Way rising out of the spout of the Teapot. But if
the richest globular clusters in the sky.
you do that you'll miss M6 and M7 - two great open clus19h 00m

18h 30m

Star magnitudes

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

18h 00m

17h 30m
-5°

Sky & Telescope - July 2020

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