Archive for July 2017

August 2017 Observer’s Challenge, Globular Cluster, M24 and the Many Other Wonders and Treasures Hidden in the Depths of the Sagittarius Milky Way by Sue French

July 18, 2017

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 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 calls the second group, close south of the first, 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.

Indeed, one can’t help but point a telescope anywhere within the 2° × 1° oblong of Messier 24 without being struck by the richness and variety of the star fields. Through my 130-mm refractor with a wide-angle eyepiece at 23×, 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 13½′ × 8′. B93 is an 8′ ×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 NGC 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 V’s southern arm 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 63×, 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 117×, Cr 469 displays 11 stars in a group whose longest dimension is about 3¼′. 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.

You might think that M24 would be a terrible place to look for a petite planetary nebula, but I was surprised to find NGC 6567 reasonably easy to spot through my 130-mm scope. At 37× it appears bluish and minuscule, but most definitely not stellar. A magnification of 117× reveals a tiny blue-grey disk that’s fairly bright. A dim star sits just off the nebula’s eastern side. At 205× it seems to have a brighter center. Through my 10-inch reflector at 115×, NGC 6567 presents a strikingly blue-green disk that I judge to be about 9″ across.

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

Sue French