101 SKY OBJECTS
50 The Small
Sagittarius Star Cloud
Charles Messier first noted the Small Sagittarius Star Cloud
(M24) on June 20, 1764. He subsequently recalled it as a “large
nebulosity in which there are several stars of different magni-
tudes: the light which is diffused over this cluster is divided into
several parts.” Although it could not be mistaken for a comet,
which was the basis for his catalog, he still added it in as M24.
Later observers went on to misidentify M24 as NGC 6603,
which is a separate, small, tightly packed open cluster. The
confusion wasn’t laid to rest until the 1960s, when British
astronomer Kenneth Glyn Jones suggested that M24 was
actually the larger region known as the Small Sagittarius Star
Cloud, which encompassed NGC 6603.
The name “star cloud” is misleading, though. We are not
looking at a distinct deep-sky object, such as a star cluster or
a nebula. Instead, we are peering through an open window
into the inner depths of the Milky Way. The plane of our galaxy
is littered with cosmic dust, which is so opaque in some spots
that we have no idea what’s going on behind it. But in this 1.5°
by 0.5° rectangular frame, the dust curtains have been pulled
aside, revealing stars more than halfway to the galactic core.
Only appearing as a bright patch of the Milky Way to the
naked eye, the Small Sagittarius Star Cloud blossoms through
binoculars into countless stars. But what about NGC 6603?
This packed but faint open cluster resides in the richest part
of the cloud, northeast of its center. Don’t expect to see many
of this cluster’s individual stars, though, unless you observe it
through at least a 10-inch scope.
A view of the star cloud does not come without a few inter-
vening dust clouds, either. A pair of dark nebulae, Barnard 92
and Barnard 93, create a silhouette along its northern edge.
Barnard 92 is a rectangular ink blot oriented north-south and
measuring about 14' by 8'. To its east, Barnard 93 is a smaller
8' by 3', as well as less defined. — P. H.
While Perseus is best noted for its
wealth of clusters, including the famous
Double Cluster, it also harbors the exotic
and diminutive beauty M76 — one of
only four planetary nebulae in Charles
Messier’s catalog. Commonly known as
the Little Dumbbell Nebula, this expand-
ing shell of gas from a dying Sun-like
star resembles Vulpecula’s Dumbbell
Nebula (M27; see #42) in shape, but not
in brightness or size. 10th-magnitude
M76 is three magnitudes fainter and
three times smaller than M27 — though
it is nearly twice as far away.
Messier’s contemporary Pierre
Méchain discovered the Little Dumbbell
Nebula on Sept. 5, 1780, communicat-
ing to Messier that it is “small and faint”without any stars. To this day, the nebula
still causes visual observers to marvel at
its subtleties.
In 1787, William Herschel found that
M76 consists of two unresolved nebulae
in contact, which became known as
NGC 650 (the south component) and
NGC 651 (the northern component).
In 1891, Isaac Roberts suggested that
the object’s peculiar rectangular
nature is due to observers seeing a
ring edgewise. He was right: M76 is a
bipolar planetary nebula, with a dusty,
doughnut-shaped ring (or torus) that we
see nearly edge on. The butterfly wings
extending from the torus are expanding
bubbles of gas ejected from the central
star’s polar regions.
M76 lies less than 1° north-northwest
of yellowish Phi (φ) Persei. Its oft-quoted
magnitude of 12.2 is much too faint.
Use moderate magnification to seek
it out and expect a 10th-magnitude,
small, diffuse glow (oriented northwest-
southeast) that swells to about 3' with
averted vision. Small to moderate-sized
telescopes will show it as a uniform
rectangle of light separated by a narrow
lane of darkness. Careful scrutiny at
high power will reveal its hollow butter-
fly wings. The central star is a challenge,
shining around magnitude 16.0. — S.J.O.51 The Little Dumbbell Nebula
DAN CROWSON WWW.ASTRONOMY.COM 31CHRIS SCHUR