101 SKY OBJECTS
WWW.ASTRONOMY.COM 4564 The Wild Duck Cluster
The summer sky is for the birds — literally. There is Cygnus the Swan, Aquila the Eagle, the Eagle
Nebula, and even a flock of wild ducks within the small constellation Scutum the Shield.
The Wild Duck Cluster (M11) was discovered in 1681 by German astronomer Gottfried Kirch, who
described it as “a small, obscure spot with a star shining through.” It wasn’t until 1733, however, that
English Clergyman, philosopher, and scientist William Derham resolved Kirch’s obscure spot into
countless stars.
M11 is some 6,100 light-years away and covers a region some 23 light-years across. That places
it in the Sagittarius arm of the Milky Way, along with such deep-sky treasures as the Eagle Nebula
and the Omega Nebula. Discovering M11 for yourself is most easily done by first locating Aquila the
Eagle’s diamond-shaped body and then its tail-feather stars, Lambda (λ) and 12 Aquilae. These team
up with Eta (η) Scuti to form a three-star arc that curves right toward M11.
The Wild Duck Cluster takes its nickname from the V-shaped pattern formed by its brighter stars.
Describing the appearance in his 1844 classic book A Cycle of Celestial Objects, Admiral William
Smyth wrote that the cluster “somewhat resembles a flight of wild ducks in shape.” Although this
analogy might be true through smaller telescopes, it’s lost through scopes much larger than 6
inches in aperture.
M11 is one of the richest and most compact open clusters found anywhere in the sky. By some
counts, it contains more than 2,900 stars. While most stars in M11 shine at magnitude 10 and below,
the single star that Kirch noted shines at 8th magnitude. This star is a hot blue main sequence
star — as are many others in the cluster. There are also several red and yellow giants sprinkled
throughout, adding splashes of color to the scene. Based on studying those giants, astronomers
have pegged M11’s age at around 220 million years. — P. H.Like its companion, the Large Magellanic
Cloud (LMC; see #80), the Small Magellanic
Cloud (SMC) is a remarkable spectacle
enjoyed by Southern Hemisphere astrono-
mers. It is visible to the unaided eye as a
hazy glow. But at 4° by 3° in size, it is fainter
and smaller than the LMC. The neighboring
clouds are located 22° apart in the sky, with
the SMC the more southerly of the pair. In
fact, the SMC is so far south that it remains
invisible from all of North America except
southernmost Mexico.
Studies tell us the SMC is about 200,000
light-years away, about 7,000 light-years
across, and hosts several hundred million
stars. Even through modest binoculars, the
SMC’s unusual shape is evident. Some have
likened it to a fishhook or a comma, with
a broader, brighter portion of the satellite
galaxy extending toward the south. That
appearance has also led astronomers to
suggest that the SMC may have once been
a modest barred spiral whose structure
was morphed by tidal forces from both the
Milky Way and the LMC.
Both Magellanic Clouds played anMessier, Méchain was an astute observer but had no interest in cataloging
new nebulae and clusters. He did, however, communicate them to Messier,
who, in turn, checked their positions, and added them to his catalog with
credit. William Parsons, Earl of Rosse, discovered its spiral structure, making
it one of 14 “spiral nebulae” discovered by 1850. He had no idea, however, of
their true nature.
At 100,000 light-years across, M63 rivals our Milky Way in size, but its
structure is different. M63 has no central bar and only moderate to loosely
wound arms. The galaxy displays strong spiral structure from its nucleus
out to around 10,000 light-years; after that, there is a significant lack of
cohesion. Within its lenticular nuclear region lies an active galactic nucleus
and several young star clusters. A dim system of arches and “plumes” blos-
som from the galaxy’s periphery, suggesting the galaxy consumed a dwarf
companion in the last few billion years. Recent studies have shown about
a dozen additional dwarf galaxies gravitationally bound to M63 — perhaps
tempting, appetizing morsels for this extragalactic cannibal.
To find M63, look about 1½° north-northwest of 20 Canum Venaticorum,
which shines at magnitude 4.5. Moderate-sized telescopes will show this
milky white glow sporting an intense starlike nucleus within a mottled
disk, which appears to form a ring around the lenslike nuclear region.
The galaxy’s inner spiral arms are best left to larger telescopes and
astrophotographers. — S.J.O.FERNANDO OLIVEIRA DE MENEZESimportant role in our understanding of the
size and structure of the universe. During
her studies of variable stars in the clouds
more than a century ago, Henrietta Swan
Leavitt of Harvard College Observatory
found that several showed a relationship
between their period of variability and
energy output, or luminosity. These stars,
called Cepheids, all share a distinctive
characteristic: the longer the period, the
greater the star’s luminosity. By knowing
the brightness and period of a Cepheid,
astronomers can calculate its distance.
Cepheids have been found in galaxies as
far away as 60 million light-years away,
making them a fundamental tool for gaug-
ing cosmic distances.
Decades ago, astronomers proposed
that the SMC was splitting in half. This
was more recently confirmed thanks
to the Gaia spacecraft. The stars in the
southeastern portion of the SMC are
slowly being pulled toward the LMC. And
any of the SMC's remnants will likely
be absorbed into the Milky Way itself.
Eventually, the SMC will be no more. — P. H.JOHN CHUMACKJEFFREY WEISS63 Small Magellanic Cloud