48 ASTRONOMY • JANUARY 2022
68 Stephan's
Quintet
Stephan’s Quintet sits just 0.5° southwest of NGC 7331,
making it one of the easiest tight groups to find. Halton
C. Arp listed this compact group as Arp 319 in his 1966
Atlas of Peculiar Galaxies. Later, he used NGC 7320’s
anomalous redshift — much lower than the others’ in
the group, indicating it is much closer — to question
the validity of redshift as a reliable measure of distance.
But Hubble Space Telescope observations dismissed
his concerns, as NGC 7320 resolves into stars, while
the Quintet’s four other galaxies do not. This proves
NGC 7320 is indeed an interloper not physically part
of the crowd: NGC 7320 is a small Sb galaxy 39 million
light-years away. NGC 7317, NGC 7318A, NGC 7318B, and
NGC 7319 are 7.5 times more distant.
NGC 7317 is a magnitude 13.6 E1 elliptical galaxy
roughly 1' across. Magnitude 14.4 NGC 7318A is listed
as either Sc or E2, and is a compact 0.9'. It sits behind
NGC 7318B. These two are interacting, surrounded by
disturbed gas and dust. NGC 7318B is a magnitude 13.9
barred spiral with dimensions of 1.9' by 1.2'. Hubble photo-
graphs show twisted arms rich in HII regions, blue stars,
and dark nebulae, particularly along one of its arms,
which has been stretched by gravitational interactions.
NGC 7319 is a 14th-magnitude SBbc galaxy whose
arms have also been highly disturbed by interactions.
It is a type 2 Seyfert galaxy with an energetic core.
NGC 7320 is a magnitude 13.6 galaxy spanning 2.2' by
1.1'. Despite its proximity to the Local Group, it shows
little detail beyond a brighter central hub.
Stephan’s Quintet is very small and easy to overlook
when scanning around NGC 7331. Telescopes 12 inches
and larger — and very dark skies — are ideal to resolve
its members and to look for nuclear brightening. — A.G.
69 The HerculesCluster
The Hercules Cluster (M13) is often cited as the grandest globu-
lar cluster north of the celestial equator. Located 25,000 light-
years away, it hosts more than 100,000 stars crammed into a
volume of space roughly 150 light-years in diameter.
Although it is visible to the naked eye on dark, transparent
nights, it seemingly went undocumented until Edmond Halley
first laid eyes on it in 1714. Charles Messier subsequently added
it to his famed catalog (as entry 13) a century later, but he only
described it as a starless nebula since his telescopes were
incapable of unlocking any stars within. That task was left to
William Herschel, who also coined the term globular cluster.
The easiest way to locate M13 using either binoculars or
a telescope is to draw an imaginary line between the bright
stars Vega (Alpha [α] Lyrae) in Lyra, to M13’s east, and Arcturus
(Alpha Boötis) in Boötes, to its west. M13 is about one-third of
the way from Vega to Arcturus, along the western side of the
Hercules Keystone, which is a trapezoidal asterism formed by
four bright stars.
M13 is a beautiful sight through any telescope. A 4-inch
scope will begin to show that there is much more here than just
a nebulous glow, as the cluster’s edges begin to dissolve into
myriad points. Doubling the aperture causes M13 to explode
into a huge globe of tiny stars. If you have an 8-inch or larger
scope, look carefully and see if you notice how the outer mem-
bers form chains, or lines, radiating outward from the cluster’s
core. Their appearance reminds many of the legs of a spider.
Those same larger scopes reveal an intriguing feature first
discovered around 1850 by Bindon Stoney, an astronomer
working for William Parsons, Earl of Rosse, at Birr Castle in
Parsontown, Ireland. Near the cluster’s core, he noticed three
subtle, comparatively star-poor lanes spaced about 120° apart
that seemingly intersect to roughly form the letter Y. Some
DAN CROWSONtoday refer to these as propellers. — P. H.