Sky & Telescope - May 2020 - 58

MAY 2020 OBSERVING
Going Deep

My incipient interest in obscure and
esoteric things in the sky was ignited
by their contagious passion. I recall
one object in particular that piqued my
curiosity: a perfectly round galaxy with
a gap between the core and outer ring. I
had glimpsed Hoag's Object.

Hoag's Objects
Also known as PGC 54559, Hoag's
Object is located in northwestern
Serpens Caput, about 6.5° southwest
of Alphecca, or Alpha (α) Coronae
Borealis. The nearest "bright" star - at
magnitude 5.6 - is 1° south-southeast
of the galaxy. It's relatively isolated
from nearby deep-sky objects, as the
closest is 14th-magnitude NGC 5910
(one of two Hickson Compact Group
galaxies) about 1° to the southeast.
Hoag's Object as a whole has a B
magnitude of 15.8 and a V magnitude
of 15.1. Since the core is yellow and the
fainter ring very blue, color must be
taken into account when describing the

q BRIGHTER CORE, FAINTER RING At 15th-magnitude, Hoag's Object can be a challenging
target, and the fainter ring even more so. This 10′ × 10′ POSS-II blue-plate image enhances the
ring. Magnitudes of nearby stars are marked.

16h 00m

15h 30m

+30° COR.

BOR.

15h 00m 14h 30m

137

α
δ γ
SERPENS
CAPUT
Hoag's Object
+20°
ι
κ
Arcturus
BOÖTES
β
γ
ζ
+10°

109

Star magnitudes
0 1 2 3 4 5 6

M AY 2 0 2 0 * S K Y & T E L E S C O P E

127

102

galaxy. Observations indicate that this
face-on ring galaxy is likely inclined to
our line of sight by 19°. The outer circumference of the ring spans 45″, which
corresponds to a diameter of some
130,000 light-years at a distance of a
little more than 600 million light-years.
The core spans 6″ in diameter. Most
of the visible light from the galaxy is
concentrated in this inner core, which
appears 2 to 3 magnitudes brighter than
the ring - this is especially evident in
red photographic plates.
Coincidentally, another ring galaxy
(SDSS J151713.93+213516.8) peeks
through the dark gap between the core
and the ring of Hoag's Object. This
may be a "cartwheel"-type galaxy, as
Hubble Space Telescope images hint
at faint spokes inside its ring. What a
remarkable tableau!
When I started observing this galaxy
in the spring of 1993, it was one of the
most exciting things I'd ever seen and
sparked an abiding interest in unusual
objects. I viewed it half a dozen times
in the 25-inch throughout the following decade, noting the brighter center
and hazy, outer ring. In 2004, the dark
"gap" separating the ring from the core
eluded me even at 908× and with the
galaxy positioned near the zenith. It
took the increased power of my 32-inch
f/4 scope to be certain of seeing that
feature. Despite the larger aperture, the
background "Baby Hoag" ring galaxy at
the one o'clock position within the gap
remains elusive. I plan to keep trying.
Arthur Hoag researched many
things, but his other eponymous ﬁnds,
as noted, were three closely clustered
quasars southeast of M82 in Ursa Major.
The brightest is Hoag 1 with a V mag-

CA RT WHEEL G A L A X Y: CURT STRUCK / PHILIP A PPLE TON
/ K IRK BOR NE / R AY LUCAS / N ASA / ESA; ZOO M IM AG E:
POSS-II / STSCI / CA LTECH / PA LO M A R OBSERVATORY

in the deep sky. Attendees are drawn to
the event to share their fascination for
remarkable celestial objects with fellow
observers. In the early 1990s I was still
new to serious deep-sky observing and
brought a large, heavy, and boxy 18-inch
reﬂector to the 1992 TSP. I quickly
learned of higher quality optical and
mechanical options. So the following
winter I traveled to Pennsylvania to pick
up a 25-inch f/5 Tectron reﬂector housing a ﬁne Galaxy Optics mirror. Then
at the 1993 TSP, I was introduced to the
ﬁne art of visual sleuthing and to a host
of intriguing objects, thanks to one of
the best observers I've ever met.
Larry Mitchell of Houston, Texas,
has been an avid observer of the deep
sky for more than three decades. In the
late 1980s he spent almost every clear
night viewing with his 24-inch reﬂector,
during which time, among many feats,
he bagged all 2,400+ objects observed
by William Herschel. Now best known
for his challenging Advanced Observing
Program at the TSP, a quarter century
ago he and his observing partner Barbara Wilson were "sirens of the sky,"
calling out the names of unusual and
seldom-observed (or never-observed)
objects back and forth, drawing
unsuspecting initiates into their lair.

COSMIC CARTWHEEL The Cartwheel
Galaxy, lying some 500 million light-years
away in Sculptor, likely formed after a smaller
galaxy plunged into a larger spiral around 200
million years ago. These events trigger spurts
of star formation that can be seen in the ringlike structure surrounding the core.

Sky & Telescope - May 2020

Table of Contents for the Digital Edition of Sky & Telescope - May 2020