COTM: Serpens Cauda and Scutum

Peter Ogilvie

Serpens Cauda

Serpens, the serpent which Ophiuchus carries, is unique among the constellations in being divided into two separate parts - Serpens Caput (the head) and Serpens Cauda (the body) - with Ophiuchus in between. Lying mostly along the galactic plane, Cauda suffers from heavy obscuration by dust. The brightest star is Eta Ser, a yellow giant of magnitude 3.26.

Although none of the objects in S.Cauda could be described as spectacular there are some worthwhile objects for even small telescopes. All observations below were carried out with a 25cm (10 inch) Schmidt-Cassegrain.

Double stars

59 Ser. (18h 27.2, +00 12) A beautiful unequal pair, magnitudes 5.3 and 7.8 at 4" (arc seconds) separation. Described by Hartung in his Astronomical Objects for Southern Telescopes as deep yellow and white (spectral class gG0 +A2); I see them as pale yellow and bluish at 200X magnification. The colour contrast is quite pleasing. The giant primary itself has two spectroscopic companions (A2+A0) making this an impressive 4-star system (gG0+A2+A0 plus the visual A2 secondary).

Theta Ser. (18h 56.2, +04 12) A nice easy pair of white A5 stars, mag 4.5 and 5.4 at a wide 22".

Struve 2375. (18h 45.5, +05 30) Two white A0 stars mag 6.2 and 6.6 at 2.5", nicely split at 150X magnification.

Struve 2303. (18h 20.1, -07 59) Unequal pair. White F2 star, mag 6.6 with mag 9.1 companion at 2.4". Requires high magnification because of the difference in brightness.

AC 11. (18h 24.9, -01 35) Pair of white F2 stars mag 6.8 and 7.0, in an orbit calculated at 240 years. Some 30 years or more ago they were only 0.7" apart (see Hartung). They are now wider at 0.9" (1994), nicely split at 150X or more.

Clusters

M16 (NGC 6611). (18h 18.8, -13 47) A large scattered cluster immersed in a faint nebula, this object (made more famous recently by the Hubble telescope's close-up of the three gaseous pillars at its centre) is seen easily in 50mm binoculars or finderscope, including the nebulosity. Through the telescope however, the nebulosity is very faint and fairly low power is required. Adeep sky filter makes it easier, an OIII filter is better. Also in the same binocular view, just over the border in Sagittarius is M17, the 'Swan' or 'Omega' nebula, well worth a closer look.

IC4756. (18h 39.0, +05 27) Quite a large cluster more than a degree in diameter, this one also is faintly visible in the 8x50 finder, as is the nearby NGC 6633, just over the border in Ophiuchus. In the telescope they are best viewed with a low power. Curiously, Hartung overlooked both of these readily visible clusters in his listings, yet included more difficult objects.

Globular clusters

NGC 6535. (18h 03.8, -00 18) A small circular haze with only a few very faint stars visible. It is a fairly distant object.

NGC 6539. (18h 04.8, -07 35) Not an easy object, I passed right over it three times before I finally saw a very faint featureless haze. Paradoxically this is in fact the second closest globular, but interstellar absorption is particularly heavy in this region. Nearby NGC 6517 is somewhat easier, being more condensed but also unresolved.

Scutum

Scutum (originally Scutum Sobieskii, the shield of Polish hero Sobieski) adjoins Aquila and was formerly included in it. It was introduced by Hevelius in 1660. Unlike Serpens Cauda, much of Scutum lies in a very rich star field close to the Milky way plane.

Double stars

Struve 2325. (18h 31.4, -10 48) This pair is immersed in a large, very faint nebula IC1287. The primary is a very hot B3 star appearing white at mag 5.8, with a 9.1 companion at a wide 12 arc seconds.Very easy at 60X magnification. Struve 2373. (18h 45.9, -10 30) Fine pair of white F-type stars, mag 7.1 and 8.1 separated by 4.2". Easy at 100X.

Clusters

M11 (NGC 6705). (18h 51.1, -06 16) Seen easily (but not resolved) in binoculars or very small telescopes, M11 is one of the most beautiful clusters in scopes of 8 inches or more. It has the look of a rather loose globular - a myriad of faint stars, concentrated yet delicate in appearance, a very pleasing effect. It has a brighter orange star in the middle.

M26 (NGC 6694). (18h 45.2, -09 24) A small easily resolved group of 40-odd stars, not very conspicuous despite being in the Messier catalogue.

NGC 6625. (18h 23.2, -12 03) A very loose cluster of faint stars, with a number of slightly brighter ones forming a distinctive S-shape.

NGC 6664. (18h 36.7, -08 13) A loose scattering of dim stars about 1/3 degree east of the 4th mag. orange star Alpha Sct.

NGC 6649. (18h 33.5, -10 24) This is a very small cluster, appearing as simply a few stars on a fainter unresolved glow.

NGC 6704. (18h 50.9, -05 12) A small dim group, resolved but not very remarkable. Globular cluster

NGC 6712. (18h 53.1, -08 42) Described by Hartung as moderately bright and partly resolved with a 20cm (8 inch) scope, I see it as a soft but conspicuous glow, unresolved in the 25cm scope.

Also scattered throughout Scutum are a number of small, faint planetary nebulae - a challenge for all but large instruments. All in all, Serpens Cauda and Scutum can provide a pleasant, if not exactly warm couple of hours deep sky hunting.