Sky and Telescope - July 2017 - 56

JULY 2017 OBSERVING
Deep-Sky Wonders

7 Sgr

9 Sgr
NGC 6530

p The author sketched the Lagoon Nebula as
viewed through her 130-mm refractor at 48×
×.

galaxy's halo. M13 is currently 15,000
light-years above the galactic plane.
M13 is visible as a small misty patch
to the unaided eye in a dark sky or
through binoculars in a suburban sky.
The cluster is beautiful even at 37× in
the 130-mm scope. It seems to sit on
a line between two bright ﬁeld stars:
a 6.8-magnitude, yellow-orange star
to the east and a white star just a half
magnitude dimmer to the south-southwest. The hazy ball of light is ﬂecked
with barely resolved stars and otherwise
appears quite grainy. Its very faint halo
spans about 15′. The cluster becomes
suddenly brighter within a diameter
of about 8′ and is a bright, speckled
haze within 4′. At 102× a large halo of
pinpoint stars surrounds a dense core
adorned with glittering ﬂecks tangled
in the ﬂeecy glow of unresolved suns.
Eye-catching star chains spring mostly
from the southwestern half of the
bright region and droop toward the
white star. The bright region is wonderfully resolved at 164×. It ﬂaunts a
greater range of star magnitudes than
most globulars and evokes a mound of
glittering sugar.
Dusky lines arranged like a threebladed propeller overlay the southeastern part of M13. They're visible through
my 10-inch reﬂector at 219× and
smack-in-the-face obvious through the
15-inch at 192×. Two of the blades cradle the cluster's bright interior, and the
third strikes out away from the cluster,
56

J U L Y 2 0 1 7 * SK Y & TELESCOPE

where it may be invisible if your view
doesn't expose enough stars to backdrop
it. The propeller was ﬁrst brought to
light in a sketch by Bindon Stoney, who
was in charge of Lord Rosse's Birr Castle
observatory in the mid-1800s.
Next we'll visit the gorgeous Lagoon
Nebula (M8), named for the darkling
channel within, like a shadowy bay
lapping on a silver shore. In a fairly
dark sky, this emission nebula is readily
visible to the unaided eye as a misty
patch hovering above the spout of the
Teapot asterism formed by the brightest
stars of Sagittarius. My sketch shows
the view through my 130-mm refractor
at 48×. No ﬁlter was used when placing the stars, but a narrowband ﬁlter
was added to help deﬁne the nebula.
The ebony lagoon divides the nebulosity
enveloping the open cluster NGC 6530
from the section harboring the brilliant but tiny double-knot of nebulosity
called the Hourglass. The two sections
are crowned by fainter bands of mist to
their north. The brightest star within
the nebula is blue-white 9 Sagittarii,
and just off M8's western side, 7 Sagittarii shines pale yellow.
Another captivating nebula in
Sagittarius that begs for a sketch is the
Omega Nebula (M17), though I've
always preferred its alternate nickname,

Omega Nebula

p Sometimes called the Swan Nebula due
to its evocative shape, the Omega Nebula
improves when viewed through an O III or
narrowband filter. The sketch shows the view
through a 130-mm refractor at 63×.

Butterﬂy Cluster

BM Sco

p You may be able to spot the Butterfly Cluster
with your naked eye if you're under dark
enough skies, but this cluster really sparkles
with binoculars or a small scope. The sketch
shows the individual lights as viewed through a
130-mm refractor at just 48×.

the Swan Nebula. One look at its most
prominent feature brings to mind a
celestial swan ﬂoating on the dark
waters of the night. Beyond the ﬁgure
of the swan, large swaths of diaphanous
haze wrap around the swan's body. An
O III or a narrowband ﬁlter makes the
nebula stand out better, and each was
used to reﬁne the nebulosity on my
sketch. Surrounded by its full retinue
of stars, the swan is also enchanting
without a ﬁlter.
We'll wind up this month's tour
with a drawing of the Butterﬂy Cluster
(M6) in Scorpius. It was a faint daub of
light to the unaided eye when I sketched
this lovely pile of jewels with my 130mm refractor at 48×. Its brightest star
is the irregular variable BM Sco, which
looks yellow-orange through the telescope. Its magnitude range is about 5.3
to 6.5. While a sketch can't compare to
a nice personal view or photo, it gives
a good idea of the relative brightness
of the stars. The Butterﬂy Cluster is so
named because many of its stars seem
to outline four wings, with perhaps
even a head and antennae to the northwest. Can you picture our star-spangled
lepidopteran?!
¢ Contributing Editor SUE FRENCH
welcomes your comments at scfrench@
nycap.rr.com.

SUE FRENCH (3)

Lagoon Nebula

Sky and Telescope - July 2017

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