101 SKY OBJECTS
WWW.ASTRONOMY.COM 5581 NGC 253
The Silver Dollar or Sculptor Galaxy is the bright-
est deep-sky object in the diminutive constellation
Sculptor. NGC 253 is highly inclined at 78° from
face-on, and doesn’t have distinct arms across its
broad, 27.5' by 6.8' disk. Recent imaging shows a
poorly developed bar and two concentrated arms
amidst a disk rich in dark and emission nebulae.
At 8th magnitude, NGC 253 can be seen in
binoculars and is about 7° south of Beta (β) Ceti.
Slightly larger optics may allow you to note the small
nuclear brightening, and even larger apertures will
reveal some granulation from the abundance of dust
clouds. Without distinctive arms, the mottled disk is
worthy of scrutiny.
NGC 253 is located 11 million light-years from us
and is the largest in the Sculptor Galaxy Group, one
of the closest galaxy groups to ours. It is so closeM55 is one of the most unsung
yet beautiful globular star clusters
in the night sky. It’s also one of
the most southerly globulars in
Charles Messier’s catalog. Abbe
Nicolas Louis de Lacaille discov-
ered it in 1751 while surveying the
southern stars from the Cape of
Good Hope. Through his small
telescope (inferior to today’s
binoculars) he recorded it as
looking like the “faint nucleus of a
large comet.” Messier’s report of it
was not much different: “a whitish
patch about 6' across.”
Today we know this close
(about 17,600 light-years distant)
and ancient (roughly 12.5 billion
years old) relic bristles with up
to perhaps 100,000 suns spread
loosely across 100 light-years
of space. This senior swarm of
gravitationally bound starlight
is approaching us at more than
100 miles per second (62 kilome-
ters per second) on an elliptical
orbit that arcs through the distant
halo of our galaxy.While all globular clusters are
richly populated with metal-poor
aged stars, M55 is exceptional in
this regard: On average, its stars
have only about 1 percent of the
fraction of heavy metals found
in our Sun, making M55 one of
the most metal-poor globular
star clusters known. It also has
a possible tidal extension — not
visible to amateur equipment —
which could be caused by the
tidal shocks that buffet the cluster
when its orbit takes it swooping
through the disk of the Milky Way.
Under dark skies (which
are required to see this cluster
well), keen-eyed observers have
spied its 6th-magnitude glow
without optical aid about 8° east-
southeast of Zeta (ζ) Sagittarii.
Binoculars will present the cluster
well, as a uniformly hairy star.
The telescopic view becomes
increasingly more magnificent as
power is added. At low power, the
cluster’s uniform glow splinters
into teasing whispers of structure.The cluster begins to blossom
at 70x when some of its brighter
members (11th magnitude) pulse
in and out of view across the
globular’s 20'-wide face. Higher
magnifications bring out more
stars. Those in M55 on the
horizontal branch — the stage
of stellar life that red giants
evolve into — shine at an average
magnitude of 14.4, meaning that
moderate-sized telescopes can
resolve the cluster reasonably
well. — S.J.O.82 M55
KFIR SIMONDAN CROWSONthat other members are scattered in other constel-
lations, including NGC 247 in Cetus and NGC 625
in Phoenix. This group is dominated by low-mass
irregular galaxies similar to the Large and Small
Magellanic Clouds; its members are best seen in
large optics because they range from 10th to 16th
magnitude.
The Silver Dollar Galaxy is about as far from the
plane of the Milky Way as possible, located around 2°
from the South Galactic Pole. That means we are look-
ing through a minimum amount of interstellar dust.
The nearest bright deep-sky object (1.8° southeast)
is the magnitude 8 globular cluster NGC 288. It’s
worth noting that although most globulars are close
to the Milky Way’s galactic plane, at 37' northeast of
the South Galactic Pole, NGC 288 is about as far as a
globular cluster can get in apparent position (but not
physical distance) from the plane. — A.G.FERNANDO OLIVEIRA DE MENEZES