A (6)

http://www.skyandtelescope.com.au 53

to the sizable open star clusterIC 4756,averypretty
sightwhenviewedthroughbinocularsoratelescope
withawidefieldofview.My130-mmrefractor
displaysabout100moderatelybrighttofaintstars
loosely scattered across 45′of sky, with no tendency
to grow more crowded toward the centre. The group’s
leading light is a yellow, 6th-magnitude gem adorning
the southeastern reaches of the cluster; however,
it’snotatrueclustermember.Acolourful,8th-and
9th-magnitude stellar pair guards the cluster’s west-
southwestern border. The brighter star has a golden
sheen, while its neighbour 2′to the north-northwest
gleams yellow.
Thomas William Webb discovered IC 4756 and
included it in his first edition (1859) ofCelestial Objects
for Common Telescopesunder the listing for NGC 6633.
He wrote, “Between it andθ, nearer the former, is a
beautiful large cloud of stars, chiefly 8 and 9 mag., a
nearer part, apparently, of the Galaxy: visible to the
naked eye, and requiring a large field.” Indeed, IC 4756
abutsaprominentbranchofthesummerMilkyWay’s
misty river, which delineates the plane of the galaxy that
we call home.
Thebright,yellow-orangestardanglingsouthof
IC 4756 isΟΣ 360 (STT 360). Its designation tells us
that it’s one of the double stars discovered by Otto
Wilhelm Struve, who succeeded his father, Friedrich
Georg Wilhelm Struve, as director of Russia’s Pulkovo
Observatory in 1862.ΟΣ360 is a close, unequal
pair,thusitrequireshighmagnificationtosplit.
The components are a scant 1.7′′apartandgleamat
magnitudes 6.9 and 9.1. The companion makes its
appearance west and a bit north of its primary when
viewed through my 130-mm scope at 234×.
ΟΣ360formsaveryshallowcurvewithtwostars
of similar magnitude to its west. The more distant one
is a double discovered by the elder Struve,Σ 2342 (STF
23 42). It’s a very wide stellar pair when seen through
the 130-mm refractor at 37×, with a white primary
whose much fainter companion to the north looks
yellow-white.
Now let’s turn our attention to the region north
of Alya, where a few little-known deep sky wonders
reside.
In 2003, amateur astronomer Dave Riddle reported
a “trapezium-type asterism” to the Yahoo observer’s
group Amastro. He described it as a small ring of six
stars about 1′ across. Its four brightest suns make a
nearly perfect trapezoid. Trapezium-type systems were
defined in the 1950s by Armenian astronomer Viktor
Ambartsumian as multiple stars for which “the ratio
of the greatest to least space distances between the
components is less than three.” The prototype is the

multiple star system at the heart of the Orion Nebula.

Through my 25-cm reflector at 43×, Riddle’s

Trapezium (Riddle 2) is a small fuzzy spot at first

glance, but careful examination reveals three stars. The

star at the trapezoid’s western corner becomes faintly

visible at 68×, and all four are obvious at 166×. I couldn’t

discern the other two ring stars, which feebly shine

at about 15th and 17th magnitude. The brighter one

might be visible through a 25-cm scope when observing

conditions are good, but the seeing (atmospheric

steadiness) was poor on the occasions that I observed

this asterism. The trapezoid is also visible through

my 130-mm scope. Three stars appear at 63×, and the

fourth is reasonably easy to spot at 164×.

Just 17.6′ north-northeast of Riddle 2, we find the

Riddle 2

Archinal 1

YM 16

θ

IC 4756

ΟΣ 360

Σ 2342

SERPENS

CAUDA

18 h 55 m

+6°

18 h 50 m 18 h 45 m 18 h 40 m 18 h 35 m

+4°

Star magnitudes

6

5

7

8

9

10

Sights in Serpens Cauda

Object Type Mag(v) Size/Sep RA Dec.

θSer Double star 4.6, 4.9 22.3′′ 18 h56.2m +4° 12 ′

IC 4756 Open cluster 4.6 40 ′ 18 h38.9m +5° 26 ′

ΟΣ 360 Double star 6.9, 9.1 1.7′′ 18 h38.7m +4° 51 ′

Σ 2342 Double star 6.5, 9.6 34.1′′ 18 h35.6m +4° 56 ′

Riddle 2 Asterism 11.4 1.0′ 18 h54.5m +5° 16 ′

Archinal 1 Open cluster — 1.5′ 18 h54.8m +5° 33 ′

YM 16 Planetary nebula 13 6.1′×5.0′ 18 h55.0m +6° 03 ′

North is up in this diagram. In the table, angular sizes and separations are from recent

catalogues. Visually, an object’s size is often smaller than the catalogued value and varies

according to the aperture and magnification of the viewing instrument. Right ascension

and declination are for equinox 2000.0.