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Small Sagittarius star cloud: the Sagittarius Milky Way is host to dark nebulae and open clusters
Sky & Telescope. 130.1 (July 2015): p56+.
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On moonless nights away from the glow of outdoor lighting, the misty fall of the Milky Way tumbles down to the horizon through Sagittarius. Its gossamer glow is fashioned from remote swarms of innumerable stars, and the silvery splendor of their intermingled light shows us the plane of the disk-shaped, spiral galaxy we live in. The Sagittarius Milky Way is interlaced with dark rifts. For the most part, the stars that lie along this section of the Milky Way, as well as the dark clouds that decorate it, lie within the Sagittarius-Carina Arm of our galaxy. This is the next spiral arm inward from ours, and it blocks the view beyond. Within the dark rift, however, a gap allows us to peer deeper into the galaxy. The stars that shine through this hole make up Messier 24, the Small Sagittarius Star Cloud.

Messier 24 is sometimes called Delle Caustiche, a name attributed to the 19th-century Italian astronomer Angelo Secchi. However, Secchi made it clear in his 1877 book Le Stelle that he was only describing a small part of M24. He writes of a little cloud, less than half the Moon's apparent diameter, made up of a multitude of separate groups of tiny stars. Two of these groups are charted as seen through a 9.6-inch telescope. The first is labeled "Gruppo delle Caustiche" (Group of Caustics), because its diminutive stars are arrayed in arcs that resemble caustic curves. Secchi describes the second group, close south of the first, as a circular collection of beautiful starlets arranged in several rays diverging from its brightest star. Its chart is labeled "Gruppo a raggera" (Sunburst Group). He refers to yet another section, next to the Sunburst, as a magnificent system of crossed arcs, the middle strewn with faint stars too numerous to count.

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Indeed, one can't help but point a telescope anywhere within the 2[degrees] x 1[degrees] oblong of M24 without being struck by the richness and variety of the star fields. Through my 130-mm refractor with a wide-angle eyepiece at 23x, M24 spans most of the field of view. Its best-known features are the dark nebulae Barnard 92 and Barnard 93, seen in projection against the cloud like dusky eyes in a fuzzy face. B92 is a nearly north-south ink spot covering about 13 1/2' x 8'. B93 is an 8' x 3' band with a less pronounced extension bending southward from its southwestern end. This eye seems to be winking. Collinder 469 is a little knot of stars just a few arcminutes off the extension's end. A very long and distinctive line of faint stars sweeps east-northeast to west-southwest across M24. The star chain skims north of B92 and B93, and it has a northward bump between them.

The open cluster NCC 6603 is a nicely obvious patch of haze flecked with a few superimposed stars. It's perched near a red-orange star, which is the middle star in the northern arm of a 20' V of 7th- and 8th-magnitude stars. The middle star in the southern arm of the V is the double SHJ 264 (S,h 264). Its whitish components are well separated, with the 7.6-magnitude companion 17" northeast of its 6.9-magnitude primary. The pair's designation tells us that it's the 264th entry in James South's and John Herschel's multiple star catalog of 1824.

Although I can't fit all of M24 in the field of view at 63x, it's amazing how much more obvious and intricate the dark nebulae are at this magnification. A fairly conspicuous thread runs east-northeast from B93, leading to a large area of patchy darkness that contains Barnard 307. Much dark nebulosity spreads west from B92, and a long, forked patch (Barnard 304) reaches southwest. Collinder 469 and NGC 6603 share a field of view. Cr 469 shows six stars that form a capital A pointing northeast, while pretty NGC 6603 is a granular patch of mist. At 117x, Cr 469 displays 11 stars in a group whose longest dimension is about 3V4'. A bit larger but much more crowded, NGC 6603 is sprinkled with many faint to very faint stars over haze. It sports a prominent southeast-northwest band of stars that cuts across the cluster's center.

NGC 6603 is wonderfully transformed by the 10-inch scope. At 213x, it's a beautiful cluster of myriad diamond-dust stars, with little unresolved haze remaining.

You might think that M24 would be a terrible place to look for a petite planetary nebula, but I was surprised to find NCC 6567 reasonably easy to spot through my 130-mm scope. At 37x it appears bluish and minuscule, but most definitely not stellar. A magnification of 117x reveals a tiny blue-grey disk that's fairly bright. A dim star sits just off the nebula's eastern side. At 205x the nebula seems to have a brighter center.

Through my 10-inch reflector at 115x, NGC 6567 presents a strikingly blue-green disk that I judge to be about 9" across.

A few other star groups from Charles Messier's famous 18th-century catalog keep M24 company. Its nearest neighbor on the sky is the open cluster Messier 18, which hovers over (north of) the star cloud's northeastern end. My 130-mm refractor at 48x shows 15 moderately bright to faint stars, most arranged in a cute shape resembling a slightly mangled hairpin. At 91 x I count 24 stars loosely scattered across approximately 7V2'.

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Two richer star clusters bracket M24. To the west we find Messier 23, which blossoms into a populous collection of 9th- to 12th-magnitude stars when seen through my 130-mm scope at 37x. The cluster spans about V20 and boasts about 90 stars strung in glittering chains meandering through the group. Pushing the scope east from M24 brings me to Messier 25, a lovely cluster of 70 mixed bright and faint stars prettily displayed in sprays and gently curving lines. The four brightest stars shine gold, and there's an adorable little D of seven faint stars at the group's heart. The nice double star 3' south of the D is Burnham 966 CD ([3966 or BU 966), with a yellow primary whose companion rests 11" to its south. Altogether, the stars form a vaguely rectangular gathering that's about 40' long, tipped a bit east of north.

The Small Sagittarius Star Cloud plumbs our galaxy to considerable depth. Its stars spread from about 10,000 to 15,000 light-years away from us. This would include stars from the Scutum-Centaurus Arm, the major spiral arm between us and the galactic center. M24 may also include stars within a minor spiral arm farther inward, the Norma Arm. Thus, M24 is not a true physical object, but merely a pile-up of stars along our line of sight.

At a distance of 12,000 light-years, NGC 6603 is actually within the bounds of the Star Cloud. It's difficult to tell where NGC 6567 fits into the scene. As is often the case with planetary nebula, even recent distance estimates are widely varied--in this case from 5,500 to 12,000 light-years. The other objects are in the foreground, and we simply see them projected against M24. Collinder 469 is about 4,800 light-years distant, while the dark nebulae are roughly 700 light-years away from us.

Most Messier objects are also listed in the 1888 New General Catalogue of Nebulae and Clusters of Stars (NGC), but neither M24 nor M25 is among them. They were added to the 1908 supplement to the NGC, known as the Second Index Catalogue of Nebulae and Clusters of Stars (IC). In the catalog, IC 4715 (M24) is described as a most extremely large cloud of stars and nebulae and IC 4725 (M25) as a cluster, pretty compressed. We can just call them beautiful.

Select Objects from the Small Sagittarius Star Cloud

Object             Type         Mag(v)          Size/Sep

Messier 24   Star cloud          2.5     2[degrees] x 1[degrees]
NGC 6567     Planetary Nebula    11.0              12"
Messier 18   Open cluster        6.9              7.2'
Messier 23   Open cluster        5.5               29'
Messier 25   Open cluster        4.6               32'

Object          RA             Dec.

Messier 24   18h 16.8m   -18[degrees] 33'
NGC 6567     18h 13.8m   -19[degrees] 05'
Messier 18   18h 20.0m    17[degrees] 06'
Messier 23   17h 57m     -18[degrees] 59'
Messier 25   18h 31.8m   -19[degrees] 07'

Angular sizes and separations are from recent catalogs. Visually,
an object's size is often smaller than the cataloged value and
varies according to the aperture and magnification of the viewing
instrument. Right ascension and declination are for equinox 2000.0.

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Sue French welcomes your comments at scfrench@nycap.rr.com.

Source Citation   (MLA 8th Edition)
French, Sue. "Small Sagittarius star cloud: the Sagittarius Milky Way is host to dark nebulae and open clusters." Sky & Telescope, July 2015, p. 56+. Academic OneFile, http%3A%2F%2Flink.galegroup.com%2Fapps%2Fdoc%2FA417569982%2FAONE%3Fu%3Dmcc_pv%26sid%3DAONE%26xid%3D7472cfc9. Accessed 20 Nov. 2018.

Gale Document Number: GALE|A417569982