Sky and Telescope - November 2017 - 40

Hickson Compact Groups
* Visit https://is.gd/HicksonGroups for positions, surface
brightnesses, and magnitudes of the objects discussed here.

21h 30m

20h 30m

20h 00m

β
HCG 88

ε
-10°
M73

Aquarius
Our sampler wraps up with HCG 88, about 15′ southeast of 4
and 5 Aquarii. It comprises a string of four galaxies that line
up brightest to faintest on a northeast-southwest line.
NGC 6978 stands at the northeast end of the string. It's
elongated northwest-southeast and has a bright core. About
2½′ to the southwest is NGC 6977, which is round with a
brighter core and a faint halo that, with averted vision, hints
at structure. The third galaxy in line, NGC 6976, is smaller
and fainter, but also round with a bright core. Last in line
and faintest is NGC 6975, which is an averted-vision streak
elongated nearly east-west.

21h 00m

ι
ε

M72

α
β

θ
CAPRICORNUS

Star magnitudes

me until I examined it in a scope with more aperture. The
group is located 3.5° east-southeast of Beta Andromedae and
just a little over 4° from M33.
The brightest member of the group, NGC 536, is elongated nearly east-west with a bright core. A faint star is
involved to the northwest of the nucleus. Almost 9′ west
is NGC 529, which is round with a bright core. NGC 531
is the third brightest component, but has a lower surface
brightness, so I found it to be the most difﬁcult of the group.
NGC 542, small and thin, angles almost northwest-southeast. It lies 2.5′ southeast of NGC 536.

0°

2
3
4
5
6

-20°

HCG 88

NGC 6978

NGC 6977

Galaxy Hunters, Away!

N O V E M B E R 2 0 17 * S K Y & T E L E S C O P E

NGC 6975

p DUCKS IN A ROW The galaxies of HCG 88 conveniently form a line
according to brightness. NGC 6978 and NGC 6977 hold the northwestern
end of the line, NGC 6976 takes up the central position, and NGC 6975
faintly pins down the southwest end. Even with 18 inches of aperture,
you're unlikely to detect any structure here. Look instead for circular fuzz
patches that brighten toward the center.

K EN T E. B IGGS

¢ Contributing Editor TED FORTE is the operations director of
Patterson Observatory in Sierra Vista, Arizona, and an officer of
the Huachuca Astronomy Club. He writes a monthly astronomy
column for his local newspaper, the Sierra Vista Herald.

NGC 6976

Many of the objects that comprise the Hickson Compact
Groups are also contained in Halton Arp's Atlas of Peculiar
Galaxies (https://is.gd/ArpCatalog) and exhibit the properties
that make those objects so very compelling in the eyepiece.
Evidence of interaction between the members, past or present, abounds. Gaseous streamers, warped disks and arms,
violent starbursts and disturbed-looking dust lanes provide an
endless array of interesting features that make these groups
tantalizing targets in scopes of all sizes.
Sometimes the ﬁrst reaction to a Hickson Compact Group
will be to dismiss it as invisible. Some observers playfully use
the words "lumpy darkness" to describe the vague impression
of that "something" at the edge of detection that faint galaxy
groups often display. It's here that experience and persistence
along with good dark adaption are critical. While many
Hickson groups will be beyond detection by most observers,
you should never reject a group out of hand. The only realistic
way to determine if a group can be seen is to actually look.
The patient and determined observer of these systems will be
well rewarded. Try them, and you're sure to conclude that the
less-traveled road can make for a remarkable journey.

https://www.is.gd/HicksonGroups https://www.is.gd/ArpCatalog

Sky and Telescope - November 2017

Table of Contents for the Digital Edition of Sky and Telescope - November 2017