18 ASTRONOMY • JULY 2018I
n the summer of 1764,
the French comet hunter
Charles Messier came
upon a nebulous object
“near the extremity of the
bow of Sagittarius, in the Milky
Way.” He went on to describe
it as “a considerable nebulosity,
of about one degree and a half
extension: In that nebulosity
there are several stars of dif-
ferent magnitudes; the light
which is between these stars is
divided in several parts.” It was
recorded as the 24th entry in
his catalog of nebulous objects.
Messier 24 (M24), visible to
the unaided eye and three Full
Moon diameters in length, was
one of the last members of the
Messier catalog that I officially
notched. Why? It all has to do
with resources that misidenti-
fied it as NGC 6603.
As I’ve mentioned in the
past, my earliest cosmic excur-
sions were made with a little
3-inch f/10 Edmund Scientific
ref lector. Throughout the
1970s, I used this scope exclu-
sively to work my way through
all the entries in the Messier
catalog. Bright Messiers, like
the Andromeda Galaxy (M31),
the Orion Nebula (M42), and
the Beehive Cluster (M44), fell
quickly. Within a few years, I
had moved on to less accom-
modating targets, like the Crab
Nebula (M1), the Pinwheel
Galaxy (M33), and the galaxies
in the Coma/Virgo area.
As the decade drew to a
close, only a handful of Messier
objects had eluded my eye. One
of those holdouts was M24.The problem was that several
of my resources identified M
as NGC 6603, when in fact the
two are not the same object.
M24 is a 1°-by-2° detached
chunk of the Milky Way, also
known as the Small Sagittarius
Star Cloud. On the other hand,
NGC 6603 is a small, faint
open cluster, tucked away in
the northern part of M24, that
was discovered by John
Herschel in 1830. John Louis
Emil Dreyer’s 1888 edition of
the New General Catalogue
correctly described NGC 6603
as a “remarkable cluster, very
rich, very much compressed,
round,” but then incorrectly
cross-identified it as M24.
Thus began the confusion.Messier’s description of M
matches the appearance of the
Small Sagittarius Star Cloud;
Dreyer’s doesn’t. Moreover,
NGC 6603 is likely too faint to
have been visible through
Messier’s telescopes. American
astronomer Robert Burnham
Jr. had said as much in the
third volume of his Celestial
Handbook. Nevertheless, I
decided to withhold planting a
f lag atop M24 until I had
notched NGC 6603. This was
hardly an easy task, as this
5'-wide cluster shines at a feeble
11th magnitude and is more
aptly viewed with a telescopeOBSERVINGBASICS
BY GLENN CHAPLEDemystifying
Messier 24
M24 is often misidentified as NGC 6603, but don’t let that
stop you from seeking out this superb nebulosity.twice the aperture of mine.
Numerous were those summer
nights when I swatted mosqui-
toes while struggling with this
elusive target! Finally, near
midnight on the evening of
July 28, 1978, I glimpsed an
“incredibly faint, but persistent
averted vision haze.” To
remove all doubt that I had
indeed seen NGC 6603, I care-
fully sketched the field to
include stars in the immediate
vicinity of my suspect. Indoors,
I compared my sketch with a
photo of NGC 6603 in the
Celestial Handbook. The match
was perfect! Fist pump!Many sources describe
M24’s brightness as magnitude
4.5. I agree more with Stephen
James O’Meara’s assessment
that it’s about two magnitudes
brighter. M24 is reasonably
visible from my backyard,
where the limiting magnitude
is around 5. Under such skies,
a 4.5-magnitude nebulous
object would be extremely dif-
ficult to see.
Needless to say, the real
Messier 24 (correctly identified
as IC 4715) is best viewed
through binoculars or a rich-
field telescope. Through 10x
binoculars, it appeared to meas a kidney bean-shaped nebu-
losity interspersed with numer-
ous stars. The brightest four
formed a kite-shaped asterism.
As I suspected, the embedded
NGC 6603 wasn’t visible. I’m
sure a skilled observer working
from a dark sky site can pick
out NGC 6603 with a common
2.4-inch refractor and moder-
ately high (75x to 100x) magni-
fication. You’ll need a scope
6 inches or larger in slightly
light-polluted suburban areas.
Larger instruments will resolve
individual stars.
If M24 is on your list of yet-
to-see Messier objects, don’t
torture yourself like I did.
Look for a naked-eye patch
of light hovering high above
Sagittarius’ Teapot asterism, 2°
north and slightly east of the
4th-magnitude star Mu (μ)
Sagittarii. Get a close-up view
with binoculars, take time to
enjoy the sight, and then put a
check mark next to M24 on
your Messier catalog list. If
you’re up for a small-telescope
challenge, try your luck with
NGC 6603 as well. Just remem-
ber to put on some bug spray!
Questions, comments, or
suggestions? Email me at
[email protected]. Next
month: a backyard mission to
Mars. Clear skies!BROWSE THE “OBSERVING BASICS” ARCHIVE AT http://www.Astronomy.com/Chaple.One of the sky’s richest star regions, M24 (central diagonal cloud) makes up part
of the Milky Way’s Sagittarius spiral arm. On the other hand, NGC 6603 — a small,
dim open cluster — is located in the northeastern section of M24. CHRIS SCHURGlenn Chaple has been an
avid observer since a friend
showed him Saturn through a
small backyard scope in 1963.The problem was that several of my resources
identified M24 as NGC 6603, when in fact
the two are not the same object.