Sky & Telescope - August 2021 - 18

FRANK SACKENHEIM
MILKY WAY CLOUD-HOP by Richard P. Wilds
E
arly observers of nebulae - among them noted American
astronomer E. E. Barnard and German astrophotography
pioneer Max Wolf - noticed that bright and
dark cosmic clouds often cluster together. William Herschel
famously referred to the complexity of such objects when he
described the Rho Ophiuchi cloud as a " hole in the sky! " He
also noted the positional relationship between dark nebulae
and rich star clusters that we now refer to as globular clusters.
The Rho Ophiuchi region is a well-known example of this
juxtaposition, but there are many more. Let's begin our exploration
of collections of nebulae in the summer sky, starting at
southern declinations and gradually moving northward.
First Stop: Corona Australis
Head for the boundary between the Southern Crown and
Sagittarius, a bit less than 1° west-northwest of Gamma (γ)
Coronae Australis. A classic scenario that would have pleased
observers of yore greets us: Not only do we fi nd intertwined
bright and dark nebulae, but also a nearby globular cluster
(NGC 6723) to boot.
German astronomer and geophysicist Johann Friedrich
Julius Schmidt discovered NGC 6726/27 in 1860. The variable
star TY Coronae Australis and the young stellar object
HD 176386 power these overlapping nebulae, while a double
star lurks nearby. A year later, Schmidt identifi ed NGC 6729, a
variable refl ection nebula reminiscent of NGC 6621 (Hubble's
Variable Nebula). The variable binary R Coronae Australis
illuminates the northwestern end, while T Coronae Australis
occupies the southeastern corner. A high-power eyepiece will
bring the double stars in both objects into view. Immediately
to the southwest you'll come to the large and diffuse IC 4812
(discovered by American astronomer DeLisle Stewart in 1899).
A wide-fi eld eyepiece works best for this expansive target.
These refl ection nebulae are all on the northwestern edge
of a huge pair of dark clouds comprising SL 39 and SL 40.
English astronomer Harold Knox-Shaw chronicled his observations
of this region in 1916 while he was superintendent
of the Khedivial Observatory at Helwan, Egypt. Knox-Shaw
encouraged Barnard to focus on dark nebulae, which Barnard
famously did, going on to publish a catalog several years later.
And yet, curiously, in spite of Knox-Shaw's urgings, Barnard
never covered this particular collection of objects.
Corona Australis
Object
Type
NGC 6726/27 Refl ection
NGC 6729
IC 4812
SL 39
SL 40
Refl ection
Dark
Dark
Emission/
Refl ection
Size/Sep
9′ × 7′
1′ × 1′
10′ × 7′
42′ × 42′
15′ × 15′
RA
Dec.
19h 01.7m -36° 53′
19h 01.9m -36° 57′
19h 01.1m
-37° 04′
19h 03.8m -37° 16′
19h 02.9m -37° 07′
Angular sizes are from recent catalogs. Visually, an object's size is often smaller
than the cataloged value and varies according to the aperture and magnifi cation of
the viewing instrument. Right ascension and declination are for equinox 2000.0.
18 AUGUST 2021 * SKY & TELESCOPE
Sliding into Sagittarius
Now let's visit the well-known Small Sagittarius Star Cloud
(M24) to fi nd B92 and B93. The richness of the background
greatly enhances the visibility of these nebulae and makes B92
one of the easiest dark clouds to observe. It has a lone 11thmagnitude
foreground star that's conspicuous in any telescope
under a dark sky, as is the general outline of the nebula itself.
However, if your telescope can go down to 15th magnitude (or
fainter) the nebula will be more defi ned. The 8th-magnitude
star HD 167411 is just 2′ east of B92, and if you continue east
from the star some 10′, you'll arrive at the bottom end of B93.
While also easy to discern, B93 isn't quite as impressive as B92.
Explore Su
Bright and
Nebulae
The Milky Way offers a
profusion of nebulae, more
often than not intertwined.
Sky & Telescope - August 2021

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