M24 Star Cloud, Open Cluster NGC 6603, Dark Nebula Barnard 92 and 93

Observer’s Challenge Report:  AUGUST 2017 OBSERVERS CHALLENGE – M-024

Image of the M24 complex by James Dire from Hawaii

M24

 

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

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 

 

 

David J. Eicher

The starcloud M24, also known as the Small Sagittarius Starcloud. It is a dense patch of Milky Way, detached from its surroundings by lanes of dark nebulae. The cloud shines at magnitude 4.5, and measures 120′ x 40′ across. Its entire area fits into a binocular field, making for a spectacular sight. Telescopes don’t show the whole cloud, but several telescopic objects lie within and around the piece of Milky Way Galaxy.

The open cluster NGC 6603, which appears as a condensation in the rich background of starcloud M24, measures 4′ across and contains 50 stars of 14th magnitude and fainter, giving it a total magnitude of 11.4. Telescopes operating at high power show this misty spot as being slightly nebulous, giving the impression of an unresolved globular. The object looks similar to NGC 2158 in Gemini, the little cluster sitting beside M35. Also within the cloud is the bright, tiny planetary nebula NGC 6567, which glows at magnitude 11.5 and measures 11″ x 7″ in diameter. It is rather difficult to locate among the richness of the stellar background, but medium powers reveal the nebula’s fuzziness. Seeing 6567’s 15th magnitude central star is a difficult task even for large telescope owners: it is easily overpowered by the nebulosity. Another object immersed in M24 is the dark nebula Barnard 92, which measures 15′ across and lies on the starcloud’s northwest edge. On good dark nights it is visible as an obvious “hole” in the glittery backdrop of stars.

David J. Eicher, The Universe from Your Backyard – A guide to Deep-Sky Objects from Astronomy Magazine

 

Roger Ivester

Messier 24 is a rich detached section of the Sagittarius Milky Way, best observed with binoculars. M24 is also known as the the little star cloud with a size of 2º x 1º which makes it a bit large for most telescopes, and is best observed with binoculars.

It was my plan this year to use a small 3-inch rich-field telescope with a 4º FOV to finally attempt that pencil sketch which I’ve wanted for the longest time. Unfortunately the weather in North Carolina has been rainy and cloudy for most of the year to-date. I’ve had very limited time outside this year, so that wide-field pencil sketch of M24 and all of the integrated sights and features will have to wait for another year.

In the northeast section of the star cloud lies a faint and small open cluster, NGC 6603. Using a 3.5-inch Maksutov-Cassegrain telescope at 52x, I found it difficult to locate, but this was due in-part to the light glow in my southern sky. I could not resolve this cluster, which appeared only as a faint mostly round glow.

Over the years, many amateurs have confused NGC 6603 as being M24.

Roger Ivester

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