Sky and Telescope - July 2016 - 59

OBSERVING

Steve Gottlieb

Going Deep

A Herculean Challenge
Test your skills with this ambitious array of galaxies.
1959 Catalogue of Interacting Galaxies (S&T: Sept. 2014,
p. 60). The warped tidal tail of NGC 6040A and knots of
young, massive star clusters point to this pair's ongoing
interaction.
Next in line is NGC 6041, another double galaxy
with the designation VV 213. Initially I noticed a single
elongated patch extending southwest to northeast. But
when the seeing settled, the patch resolved into NGC
6041A, a small hazy oval just 15″ × 10″ and NGC 6041B,
a barely non-stellar companion a mere 20″ southwest.
Look for two 10.7- and 13.5-magnitude stars 2′ due south
that align nicely with the pair. Despite its modest visual
appearance, NGC 6041A is a giant elliptical and one of
the most luminous members of the cluster. Its extensive
halo may include the cannibalized remains of several of
its former neighbors.
See if you can glimpse IC 1170, just 1′ west of this
twosome. With averted vision it popped into view as a
ghostly sliver. The last in this galactic string is NGC
6042, a compact 12″ knot with a brighter nucleus. It
sits 1.5′ southeast of NGC 6041B and 1.7′ northeast of a
prominent 10.7-magnitude star.
16h 10m

16h 05m

Abell 2151

16h 00m

15h 55m

+19°

+18°

HD 144149
5

SERPENS CAPUT

+17°

HERCULES
4
Star magnitudes

Rich galaxy clusters vary signiﬁcantly in the distribution of galaxy types, the degree of central concentration,
and whether there is a single dominant member. In some
clusters a central ultra-luminous elliptical (cD-type) holds
court over a contingent of lenticular (S0) and E-type galaxies. The Hercules Galaxy Cluster (Abell 2151), on the other
hand, is irregular in structure, has no leading member,
and abounds with star-forming spirals (roughly 50% of
the population). This galactic assemblage includes a wild
assortment of edge-on and face-on spirals, merging pairs,
and distorted interacting systems.
Abell 2151 is also one of the most distant clusters that
can be explored in depth with a 12-inch scope (a few
of the brightest members may be visible in an 8-inch),
though you'll need dark skies, a detailed chart, and
plenty of patience. Keep in mind the photons from these
faint fuzzies have been traveling some 500 million years
to reach your eyes!
The NASA/IPAC Extragalactic Database (NED, ned.
ipac.caltech.edu) lists 300 members of Abell 2151 within
a 1.5°° area. The central 18′ × 6′ region, called Abell
2151C, is the richest region, containing over 15 members
from 14th to 15th magnitude. I ﬁrst tackled the cluster in
1983 through a 13.1-inch reﬂector but recently used my
18-inch f/4.3 reﬂector to re-survey the area. Since these
galaxies are typically quite small, try using a magniﬁcation of 250× or higher if the seeing allows - the boost
will increase visibility and help to resolve several close
galactic pairs.
The heart of the cluster is a snap to locate. Start at
5th-magnitude 5 Herculis, just east of the distinctive
triangular asterism forming the head of Serpens Caput.
Then either nudge your scope 37′ due east or star drift
for 2½ minutes to 6.8-magnitude HD 144149. The interacting pair NGC 6040 is just 9′ east-southeast of this
star; it anchors a 4′ string of a half-dozen galaxies.
NGC 6040A, a highly inclined spiral, showed as a
diﬀuse oval, elongated 2:1 from southwest to northeast.
Occasionally, a star-like nucleus glimmered in the misty
patch. I found NGC 6040B, a round 15″ fuzz spot,
snuggled up against NGC 6040A's south limb. The pair
forms Arp 122, from Halton Arp's 1966 Atlas of Peculiar
Galaxies, and VV 212, from Boris Vorontsov-Vel'yaminov's

5
6
7
8
9
10

+16°

Sk yandTelescope.com July 2016

http://ned.ipac.caltech.edu http://ned.ipac.caltech.edu http://www.SkyandTelescope.com

Sky and Telescope - July 2016

Table of Contents for the Digital Edition of Sky and Telescope - July 2016