68 ASTRONOMY • AUGUST 2020
BINOCULAR UNIVERSE
One of the most amazing
regions to enjoy through
binoculars lies just north
of the Sagittarius Teapot’s spout.
There, the clouds of our Milky Way
billow northward like steam from a
percolating kettle. Aim toward the
star at the spout, Alnasl (Ga mma^2
[γ^2 ] Sagittarii), an optical double
star circumstantially paired with
unrelated 5th-magnitude Gamma^1
(γ^1 ) almost a degree to its north.
While viewing through your
binoculars, hop northeastward
along a series of wonderful deep-sky
targets. You’ll pass a gallery of
showpieces, including the Lagoon
Nebula (M8) and the Trifid Nebula
(M20).
By all means, pause at each, but
then continue northeastward. Some 12° — about two
binocular fields — beyond Alnasl, you’ll arrive at a
large, denser region of the Milky Way measuring about
three Moon diameters across. Charles
Messier included this as the 24th entry in his
fa mou s c at a log , not i ng it s appe a r a nc e i n 176 4
as “a large nebulosity in which there are many
stars of different magnitudes.”
M 24 was later christened the Small
Sagittarius Star Cloud, but that is not accurate.
M24 is not a single object. Nor is it a cloud of
stars at all. Instead, this star-studded area is
an open window into the depths of the Milky
Way. Like other spiral galaxies, the plane of
our Milky Way is heavily polluted with inter-
stellar matter, including vast clouds of opaque dust.
That’s why studying the center of our galaxy is so chal-
lenging. There is just too much local interference —
except toward M24. There, the dust is thin enough that
we can peer through our local arm to a depth of about
16,000 light-years, all the way to the Carina-Sagittarius
arm, halfway between us and the inner Norma arm.
Somewhere along the way, M24 went “missing.” At
least, that’s how some 20th-century references termed
it. That’s probably because those authors were looking
through the narrow fields of their telescopes, which
couldn’t take in the full breadth of the area and thusm issed M24. O t hers sa id t hat M24 mu st be one a nd t he
same as NGC 6603, a densely packed open cluster super-
imposed on the northeastern quadrant of M24. But
NGC 6603 shines faintly at 11th magnitude and mea-
sures only 4'. Compare that to Messier’s original descrip-
tion and it’s obvious that they are not the same object,
though many past sources insisted they were.
While M24 stands out nicely through binoculars,
NGC 6603 is challenging due to its dimness, com-
pounded by its unfortunate position in front of the “star
cloud’s” brightest region. Estimates place as many as 100
individual stars within, although with none brighter
than 14th magnitude, they are far too faint to resolve
through even giant binoculars. Instead, their light
blends to form a softly glowing smudge against a rich
backdrop. Look for it just north of an equilateral triangle
of 7th-magnitude stars. By waiting for that special night
under a clear, dark sky free from moonlight, artificial
light pollution, and summer haze, you just might see
something that Messier missed.
There are several isolated patches of dark nebulosity
silhouetted in front of M24. The most obvious is cata-
loged as Barnard 92 in Edward Barnard’s “On the Dark
Markings of the Sky, With a Catalogue of 182 Such
Objects,” published in The Astrophysical Journal’s
January 1919 edition. Barnard’s one-line description
portrayed it as a “Black spot, 15' north and south, 9' east
and west.” That likely led to its nickname, the Black
Hole, coined long before the expression was applied to
the end result of the most massive stars when they deto-
nate as supernovae.
Barnard’s black spot may be glimpsed with 35- and
50mm binoculars, growing more obvious as magnifica-
t ion a nd aper t u re cl i mb. It doe s i ndeed look l i ke a n ova l
“black hole” oriented north-south through my 16x70s,
appearing about half as large as our Moon.
If you spot B92 , t r y you r luck w it h t he nex t
catalog entry, Barnard 93, about 1/3° east of
B92. But be forewarned; it’s tougher. While
B93 matches B92’s size north-to-south, it
appears less than half as wide east-to-west.
Unlike B92, which has sharp edges that clearly
define it from the surroundings, B93 slowly
diffuses outward into its environs. In photo-
graphs, it looks like the silhouette of a
northward-facing comet with a denser “coma”
and a fainter “tail” trailing southward.
Finally, as a postscript to the history of
M24: While it is obviously not NGC 6603, it is in fact
listed in the second supplemental Index Catalog pub-
lished after the New General Catalogue. There, we find
it cross-referenced as IC 4715 and described as a “most
extremely large cloud of stars and nebulae.” That seems
reasonable.
Questions, comments? Send them to me via my web-
site, philharrington.net. Until next month, remember
that two eyes are better than one.Explore one of the richest areas of the Milky Way.
Explore
Sagittarius
The region known as
M24 contains a vast
cloud of stars, the
compact star cluster
NGC 6603 (left of
center), and two
prominent dark
nebulae, B92 and
B93 (above right of
center). FRED CALVERT AND
ADAM BLOCK, NOAO, AURA, NSFAn amazing
region lies
just north
of the
Sagittarius
Teapot’s
spout.
BY PHIL
HARRINGTON
Phil is a longtime
contributor to
Astronomy and the
author of many books.BROWSE THE “BINOCULAR UNIVERSE” ARCHIVE AT
http://www.Astronomy.com/Harrington