101 SKY OBJECTS
WWW.ASTRONOMY.COM 1316 The Hercules galaxy cluster
For observers who own or have access to large telescopes and a dark viewing
site, one of the most impressive sights in the sky is the Hercules galaxy cluster.
Also known as Abell 2151, this group of galaxies takes the term deep sky to a
whole new level. It lies at the astounding distance of 500 million light-years.
If your scope has a go-to drive, its database may not contain Abell galaxy clus-
ters. Instead, just target this cluster’s brightest member, elliptical galaxy NGC 6047.
Without a go-to, first find magnitude 5.0 Kappa (κ) Herculis. From that star, move
1° northwest and your field of view will land on hundreds of galaxies, most of
which glow too faintly to see. But that still leaves several dozen within the range
of moderate amateur instruments. Around half of all the members in the Hercules
Cluster are spirals. And several of those galaxies are in the process of merging.
Seven members of Abell 2151 are brighter than 14th magnitude, although three
just barely crest that level: NGC 6047 (magnitude 13.5), NGC 6061 (13.6), IC 1194A
(13.6), NGC 6055 (13.7), IC 1185 (13.9), NGC 6045 (13.9), and NGC 6056 (13.9). Of
these, NGC 6045 is the most interesting visually because of the magnitude 15.
lenticular galaxy PGC 84720 that lies at its eastern tip. The pair gives most the
impression of a hockey stick or the letter L.
To successfully observe this galaxy cluster, use at least a 12-inch telescope
and magnifications in excess of 250x. High magnifications increase the con-
trast between extended objects like galaxies and the background sky. Because
Abell 2151 spans more than 1°, you’ll need to move your scope around a bit to see
the maximum number of galaxies, especially when observing at high power. — M.B.15 NGC 4565
Observers love looking at face-on spiral
galaxies. There’s something captivat-
ing about seeing a disk of light with
a concentrated core and magnificent
sweeping arms containing millions of
stars suspended in your eyepiece. But
edge-on spirals offer an alternative view,
and some observers enjoy them just as
much. The finest example in the sky is the
Needle Galaxy (NGC 4565), discovered by
William Herschel in 1785.
NGC 4565 lies in the constellation
Coma Berenices, 3° southeast of magni-
tude 4.3 Gamma (γ) Comae Berenices.
It is the second brightest member of the
Coma I Group, which contains some two
dozen galaxies and may include up to
20 additional members. The center of
this group lies approximately 47 million
light-years away.
The Needle Galaxy glows at magnitude
9.6 and is easily visible through an 8-inch
or larger scope. It’s not precisely edge-on
because its plane inclines about 3.5° to
our line of sight, but it’s close enough.
Images show its length-to-width ratio isroughly 25 million Suns. American astronomer Albert George
Wilson discovered it in 1950 while examining a photographic
plate taken by the 48-inch Schmidt camera at Palomar
Observatory.
Astronomers think Leo I may be the youngest of the Milky
Way’s dwarf spheroidal galaxies. None of its stars seem older
than about 10 billion years, and most of them formed between
2 billion and 6 billion years ago. And no stars (or only an
extremely small number) have formed in the past billion years.
That may be because around a billion years ago, Leo I made
its closest approach to the Milky Way. Our larger galaxy’s
gravitational influence may have stripped away all the gas
available to create stars.
Although Leo I is easy to find, it’s not all that easy to observe.
To locate it, point your telescope 20' north of magnitude 1.
Regulus (Alpha [α] Leonis). Unfortunately, the glare from that
star (especially if the humidity is high) can mask the subtle
glow of Leo I, which has a magnitude of 11.2. That dim light
is further reduced, spread over an area measuring 10' by 7'.
The best strategy is to place Regulus just outside the field
of view to the south. Start with a low-power eyepiece and
increase the magnification to get the best view. — M.B.SERGEY TRUDOLYUBOVabout 8-to-1, but visual limitations mean
the object will appear five times longer
than it is wide.
The center of the galaxy appears to
be a small bulge. Recently, astronomers
used the Spitzer Space Telescope to
study the Needle at infrared wave-
lengths. They discovered that its central
bulge is box shaped, meaning that its not
actually a bulge, but a bar.
At high powers, you might detect the
dust lane running along NGC 4565’s
entire length. It’s easiest to pick out
against the core. If you’re using at least
a 14-inch scope, look ¼° west-southwest
for one of the Needle’s companion galax-
ies, NGC 4562. At 14th magnitude, it’s a
tough catch.
One final note of caution: If you’re
star-hopping to this galaxy rather than
using a go-to drive, don’t confuse the
Needle Galaxy with the Silver Needle
Galaxy (NGC 4244). That object lies 12.5°
to the north-northwest and is in the
constellation Canes Venatici the Hunting
Dogs. — M.B.DAN CROWSONTONY HALLAS