A Starhop Through Scutum
August 2008 :
My submission last month may have prompted some of you to wonder why I did not include some of the more well-known Messier objects in a listing of seasonal deep sky objects. M22, M4, M8, M20, M16, M17, M24 and others were omitted--for a good reason. Most of you have either already seen, or will view in the future, these “standard” items. My choices were intended as alternatives to the summer “menu” so often followed at public star parties and/or by those who may not be familiar with observing beyond the Messier catalog. Several fine M-objects were included, though. Those items, to me, seemed not very well-known to many amateurs, so I made sure to list them.
I now want to concentrate on a rich, star-packed region of the sky in the constellation Scutum, which lies between Aquila to the north and east, Serpens Cauda to the north and west, and Sagittarius to the south and southeast. The wonderful Scutum Star Cloud stands out on dark, clear nights away from heavy light pollution as a conspicuous patch of luminosity within the Milky Way, the most concentrated such patch to be discerned by the naked eye from our latitude of approximately 42° north.
Actually, the central mass of the Great Sagittarius Star Cloud, centered about 2° north-northwest of 3rd magnitude Gamma Sgr (the Teapot’s Spout), is brighter but it is nearly 20° further south in declination than the Scutum Cloud. The combination of much more pronounced atmospheric extinction, greater effects of local skyglow from the low altitude, and the seasonal problem of summer-associated haze and murk in even seemingly clear skies when viewed through denser layers of atmosphere down low is usually enough to render the Sagittarius Star Cloud inferior in appearance to the Scutum Cloud. (This would not be the case if we could see the southern skies from a latitude a good deal south of our local latitude).
Back to Scutum: I’ll break my own tradition regarding well-known Messier objects by first mentioning the marvelous, very rich, deservedly famous M11, the so-called “Wild Duck Cluster” at RA 18h 51m, Dec -6° 18’. No wonder we all love this one as a telescopic target! It is about magnitude 6 with a magnitude 8 star that stands out from all other cluster members, of which there may be several hundred to possibly 1,500 or more. You can use M11 as a starting point for star-hopping to other key parts of Scutum. (Depending on sources, a recent value for this open cluster’s distance is around 6,000 light years.) A large region of dark nebulosity begins just ¼° due north of M11 and continues north to a point ¼° south of the border with Aquila. The western edge of this mass lies ¼° to the east of magnitude 4.2 Beta Scuti, a yellowish star which is the 2nd brightest in all of Scutum.
E. E. Barnard’s “B” catalog of over 340 dark nebulae is the most well-known such listing among several that feature these seemingly star-free “holes” in the sky along or adjacent to richer sections of the Milky Way. The region in Scutum that I just described actually is comprised of several individually-numbered “B” dark spots, too numerous to list here. With one huge exception, dark nebulae nearly always require moonless, dark, clear, transparent skies in order to be distinguished well. Famous examples include the Coalsack in Crux, the Pipe Nebula in Ophiuchus, and the difficult but worthwhile Horsehead in Orion. The exception to the demanding criteria regarding visibility is the very long “Great Rift” that appears to divide the Milky Way along the plane of the galactic equator from Cygnus at the Rift’s northern terminus down through several constellations southwestward, over many degrees of sky in length. A notable section of this Rift adjoins the Scutum Star Cloud to the northwest. Reasonably clear, dark nights in low light pollution areas will reveal the Great Rift to the naked eye!
Just ½° northwest of M11 lies one of four fairly bright stars that, together, comprise a striking asterism that somewhat resembles a greatly enlarged version of the Trapezium in the Orion Nebula, M42. (The side nearest M11 is much more sharply angled than any of the Trapezium’s sides, though. The other three sides of this pattern in Scutum are fairly square to one another.)
The star nearest M11 - the one marking the angled “point” of the group - is ADS 11719, also identified as HD 174208. It’s a wide, easy double of mags 6.1 and 8.6; spectral class K1 & K0, separation 114”. At the northwest corner of the group - about 1? from M11, of a similar golden hue to ADS 11719 - is a very notable variable star, R Scuti. This is one of the first dozen or so variables to have been discovered and is explained fully in Burnham’s, pages 1747-1750. R has a range of mag 4.5 at maximum to 8.2 at minimum over a period of 140.05 days. Distance is perhaps 1,400 light years. The two fainter stars at corners of the southern side of the asterism are both white and close in brightness. The star southwest of ADS 11719 is also a double, though not as dramatic. This is ADS 11695, also listed as Struve 2391. Magnitude 6.5 and 9.8 stars comprise the pair, at 38” separation. Altogether you have a four-point asterism with two doubles at one end and easy color-contrast between the brighter, deep golden stars and fainter, white ones. If you enjoy bright, pretty starfields, take a line beginning at R Scuti and extend it first through the magnitude 7 star at the southwest corner of my asterism, then continue southwest through a ragged line of four semi-bright stars spaced approximately equidistant from one another, set amid a sparkling little starfield. (This line is not a straight one, but it’ll serve the purpose - distance from R Scuti down through my row is about 1.8?) The average magnitude of these four is 6.6 and their spectra range from F7 to M0.
Try spotting a small, concentrated patch of stars just 1.5? to the west-southwest of R Scuti: NGC 6683 at RA 18h 42m, Dec -06? 17’, size 11’. Although bearing the NGC number, it’s probably not a true open cluster, just a small concentration of a starfield within the star cloud, and perhaps not too conspicuous. Just ¼? due west of this is another dark nebula, B103, one of the better ones among many in Scutum. The blankness of the Great Rift begins, in turn, just west of B103. A much larger starfield concentration which is not a cluster - and is many times bigger than NGC 6683, in angular size - is NGC 6682, centered at RA 18h 39.5m Dec -04? 46’. This spot is due west of Beta Scuti by 1.9?, just west of the Galactic Equator. Notice, if you can, how abruptly the starry region “drops off” to the seeming void of the Great Rift, which begins just west of NGC 6682!
Open cluster M26 is a worthwhile, but small, condensed object at RA 18h 45.2m, dec -09? 24’. This is a magnitude 8 cluster about 15’ in size. Use magnitude 4.7 Delta Scuti, which marks nearly the center of Scutum, as a reference for star-hopping: M26 lies 0.8? southeast of Delta. Another fine object to hunt down is the globular cluster NGC 6712 at RA 18h 53.1m, dec -08? 42’. There are no bright stars nearby from which to star hop. Still, at magnitude 8.2 and 7’ in size, you won’t want to miss it - I speak from experience, having stumbled upon NGC 6712 by lucky accident years ago.
Do you like red stars? In southwest Aquila near the northeast border of Scutum, at RA 19h 04.4m, Dec -05? 41’, is a classic bright, easy carbon star: the variable V Aquilae, magnitude 6.6 to 8.4, period 353 days, semi-regular. This star is a real “honey,” having fine reddish color and positioned luckily roughly mid-way between two bright naked-eye stars, magnitude 3.4 lambda Aquilae at 1? to the northeast and magnitude 4.0 12 Aquilae about ¾? to the west. An easy and beautiful find.
I’ll finish with a personal favorite: a triangular asterism that resembles an arrowhead, with a great orange carbon star at its western tip, the variable S Scuti, magnitude 6.7 to 9.0, at RA 18h 50.3m, dec -07? 55’. Five other reasonably bright stars make out the arrowhead, it’ll stand right out to you. Four other stars stream off to the southwest, forming-perhaps- the arrow’s rather short, curved shaft. Except for the clusters mentioned, low-power views will definitely be the most pleasing in Scutum. Rich-field scopes and large binoculars will work best. Ideal magnification range for sweeping around Scutum is 15x-25x.
No mention of star clouds would be complete without including M24, the fantastic and glittering Small Sagittarius Star Cloud in northern Sagittarius! Low power is the rule here and 10x to 15x binoculars are superb for such an object. No wonder Messier actually included such a detached, bright Milky Way region in his catalog as an independent object! You’ll easily spot this at RA 18h 17m, Dec -18? 30’. Magnitude is 4.2 total and size is 2? by 0.9?.
