101 SKY OBJECTS
WWW.ASTRONOMY.COM 2334 M109
The third-largest constellation in the sky, Ursa
Major, contains seven Messier objects. Five are
galaxies. The least observed of these — based on
the number of books and articles featuring it — is
M109, also known as NGC 3992. Messier’s original
catalog topped out at 103, but today, astronomers
recognize 109 objects.
Discovered in 1781 by Pierre Méchain, M109
is the most distant Messier object, located a
whopping 83.5 million light-years away. (The
closest is M45, the Pleiades open cluster, at only
445 light-years.) Although difficult to observe
visually because of their low surface brightness,
the galaxy’s arms are complex and somewhat
symmetrical. M109 is a barred spiral, but unlike the
more classic S-shaped galaxies of this type, its bar
ends in short counterclockwise arms that wrap so
closely on one another that they form what looks
like an encompassing ring. Both large, clockwise
(regular) arms split into two, giving the galaxy a
modified S shape. As a result, M109 is classified as
an SBbc(rs) galaxy, with rs indicating it has both
ring and S-shaped structure. Only one supernova,
which went off in 1956 and reached magnitude 12.4,
has been documented in this galaxy.
For a non-naked-eye object, M109 is easy to
locate, only ¾° southeast of Phecda (Gamma [γ]
Ursae Majoris). Under low power, the star can
interfere with seeing M109 at its best. The galaxy’s
visual magnitude is 10.6, bright enough to be seen
in small scopes. Its dimensions of 7.6' by 4.7' make
it larger than many galaxies, but it is one of the
smallest Messier galaxies in Ursa Major. (Only
M102 [NGC 5866] is smaller.) As galaxies go, it’s
unimpressive with small instruments. All you will
see is the condensed core and the brightest part of
the bar. Even larger scopes have trouble with the
low-contrast detail unless the night is free of light
pollution.
M109 is the brightest member of the M109
Group, which consists of about 33 mostly spiral
galaxies spanning 11° in declination and 40' in right
ascension. The closest bright member is NGC 3953,
a 10th-magnitude SBbc spiral with a tiny bar and
multiple arms, which sits roughly 1° southeast of
M109. — A.G.The Southern Pleiades (IC 2602) in the
constellation Carina the Keel is a daz-
zling open cluster. It lies 550 light-years
away and occupies a region 50' in
diameter. That’s 2.6 times as much area
as the Full Moon covers.
IC 2602 contains about 75 stars sur-
rounding blue, magnitude 2.7 Theta (θ)
Carinae, so it’s sometimes referred
to as the Theta Carinae Cluster. More
commonly, however, observers call
it the Southern Pleiades because its
discoverer, French astronomer Nicolas
Louis de Lacaille, compared it to the
Pleiades (M45; see #8) in the northern
constellation Taurus the Bull.
Astronomers originally thought
IC 2602 was a young object, on the order
of 15 million years old. Recent studies
at the Anglo-Australian Observatory,
however, have placed its age closer to
45 million years — which is still pretty
young for an open cluster.
With a total magnitude of 1.9 (which
makes it 52 percent as bright as M45),
PATRICK WINKLERMARCO LORENZIthe Southern Pleiades ranks as the
fifth-brightest open cluster in the sky.
Most observers agree that, as with the
northern Pleiades, IC 2602 looks better
through binoculars. That’s because
telescopes, though they provide
increased magnification, have a more
limited field of view and spread out the
stars too much. If, however, you can
use a short-focal-length scope with an
eyepiece that gives a 1½° field of view,
this collection of stars will knock your
socks off.
Through such an instrument, it looks
like you’re viewing two clusters with a
0.3°-wide gulf between them. The west-
ernmost group includes Theta Carinae
and two curving lines of stars that start
at Theta. One heads north and the
other south.
Some observers think that the
eastern half of IC 2602 looks like a
tiny Orion, whose stars have different
relative brightnesses than those in the
winter constellation. — M.B.33 The Southern Pleiades