Great Globular Grab

GREAT GLOBULAR GRAB

Whether the Skies grown old here shrink their frame,

And through the chinks admit an upper Flame,

Or whether here the Heaven's two Halves are joyn'd,

But odly clos'd, still leave a Seam behind.

Or here the parts in Wedges closely prest,

To fix the Frame, are thicker than the Rest.

Like clouds condens'd appear, and bound the Sight,

The Azure being thickened into white.

~ Manilius

To the ancients, the Milky Way often represented a heavenly path for the souls of the dead to travel. It certainly was seen this way by the American Indians, the Milky Way known as the ‘Path to the Hereafter’ which reflected many ancient legends. The Greek philosopher Democritus may have been the first to suspect the Milky Way as made up of innumerable faint stars unresolved to the naked eye. Of course, with the invention of the telescope, much of it was resolved into stars interspersed with dust and gas.

Galileo was the first to use his telescope in a serious way, mapping the heavens including resolving M44 ‘Praesepe’ or the ‘Beehive Cluster’ into a family of stars.

"I have observed the nature and material of the Milky Way. With the aid of the telescope this has been scrutinized so directly and with such ocular certainty that all the disputes which have vexed philosophers through so many ages have been resolved, and we are at last freed from wordy debates about it. The Galaxy is, in fact, nothing but a congeries of innumerable stars grouped together in clusters."

The first globular cluster resolved into stars was M22 by Sir William Herschel, though it was known before then. Globular clusters contain the oldest stars in the Galaxy, situated in what is called the Galactic Halo. Most of the 150 or so globulars lie approximately at the same distance from the Galaxy's nucleus as our Sun. Therefore, from our vantage point, most seem to lie toward the Galactic Centre in Sagittarius and nearby Scorpius and Ophiuchus. Globular clusters have been very important for studying stellar evolution as they contain a pot-pourri of stars of many sizes. These stars all lie at the same distance from us which allows a Hertzsprung Russell diagram to be drawn up and their absolute magnitude and spectral type to be plotted.

For the deep sky observer, they remain a delight to observe, ranging from the giant omega Centauri (which may be a small spheroidal galaxy) and 47 Tucanae to the faint Palomar and Terzian clusters which challenge the most serious observer. I have decided to pick a piece of the heavens and unravel some of the celestial treasures that know no man made boundaries or constraints. On this note, I give you the ‘Great Globular Grab’. The area I have chosen is in Sagittarius, where else, and it would be wise to have a starting point and systematically tramp the stellar path. All these balls of suns lie within or very near the asterism known as the ‘Teapot’. Bright stars serve as signposts and direct the observer on their hallowed journey.

Seat belts on, we are under way. The charts below show the objects in question and each object has been highlighted by a light orange circle to make detection (hopefully) easy.

First of all locate the bright, orange giant star lambda or Kaus Borealis. It represents the top of the lid in the ‘Teapot’. This magnitude 2.8 orange-coloured jewel lies a lot closer than the subjects of this article do, around 77 light years. Before we move to our first globular cluster, I can't resist moving just over 1° directly south of lambda and centering on a superb asterism. This delightful grouping of bright white stars forms a beautiful triangle. Burnham 133 (mag. 6.9, 7.0) lies on the western edge of a 20'.0 field of view. It is a close binary of 1".3 separation in PA (position angle) 251° but needs high power to resolve. Another pair (mag. 6.3, 8.8) lies 8'.9 south and slightly east, separated by 54".5 in PA 136° but are not listed as a binary. Double star WNO 6 (mag. 6.7, 8.1) of our asterism lie 15'.5 to the northeast and is listed as a pair, separated by 41".9 in PA 182°. Measurements since 1890 have shown no change in WNO 6's separation or PA.

After this distraction and it happens all the time in this star-spangled region, move back to lambda. Around 1° to the northwest lives NGC 6626 or M28. Discovered by Messier in July 1764, he described it as a:

“nebula containing no star... round, seen with difficulty in 3.5-foot telescope, diam 2'." Sir William Herschel captured it pretty well; “1799, August 1, 20 feet telescope, It may be called insulated though situated in a part of the heavens that is very rich in stars. It may have a nucleus, for it is much compressed towards the centre, and the situations is too low for seeing it well. The stars of the cluster are pretty numerous.”

M28 is one of the more compact clusters but a 30cm aperture at high power can resolve the stars. It has a bright central region and there are many outlying stars radiating outwards in lines and sprays.

Around 40' to the southeast of lambda lies our next object NGC 6638. Sir William Herschel discovered NGC 6638 in 1784. This is a small and faint cluster in a 20cm aperture from city skies. A bright stellar core is easily seen though no stars are resolved. Large telescopes should make better use of it and no doubt resolve many stars.

Once again starting at lambda, move 2° to the northeast to discover a pretty pattern of six bright stars including the orange giant 24 Sagittarri. Nearby, east and slightly north lies one of the gems of the southern skies. NGC 6656 (M22) is a large and bright cluster of infinite beauty. The brightest stars glow at 11th magnitude and are visible to a 20cm telescope, even from city skies. One of the best views I have had of M22 was on a night of very good seeing around midnight. Using a 20cm f6.7 reflector and a power of 48x, this beauty appeared as a large, very bright, irregular cluster with chains of bright stars radiating outwards to the southwest. Employing a higher power of 90x, it took on a 3D effect, the chains of bright stars covering a fainter milky-white background of unresolved stars. Truly a magnificent object that should be returned to again and again and again. Due to M22 lying within 1° of the path of the planets (the Ecliptic), the moon and planets can come very close to M22, sending astrophotographers scurrying for their cameras.

The small and faint globular NGC 6642 lies just over 1° to the northwest. Once again, using a 20cm aperture, the light polluted Auckland skies interfered, rendering 6642 as a smudge of weak light, found after a bit of searching and muttering. Still, four down and plenty to go.

Our next sign post to the stars is M8, the ‘Lagoon Nebula’. I'm afraid that I am going to do a desperate thing here and dismiss M8 with great unwillingness. Moving a little southeast brings forth NGC 6544, a delightful cheery cluster lying in a lovely field of unrelated stars. The stars are packed well within its boundaries but a larger telescope should have no trouble in resolving most.

NGC 6553 lies a little farther south and slightly east, not as attractive as it's neighbour as heavy obscuration by dust and gas interferes. It appears elliptical in shape and there is little central condensation. It may not be the "belle of the ball" but is worth admiring anyway.

Gamma Sagittarri represents the tip of the ‘spout’ of our celestial ‘teapot’. Here we have an orange K-type giant, twin to lambda mentioned previously. Less than 1° north lies the cepheid variable star W Sagittarii. Between these two bright stars lie a charming dainty couple, NGC 6522 and NGC 6528. This area is overwhelmed by Milky Way stars and in photographs, both small globulars almost disappear from view. We are now looking toward the Galactic Centre, a thriving metropolis of yellow elderly stars and obscuring dust and gas.

NGC 6522 is the larger and brighter of the two, both discovered by none other than Sir William Herschel, on the same night, in 1784. A 25cm aperture resolves a few stars of 6522 but makes heavy work of 6528. A larger telescope is needed for some resolution. Though these two apparently lie in the same region of the sky, they are in fact unrelated, NGC 6522 being farther away but appearing brighter and larger because of less obscuring dust. Sir John Herschel was clearly overwhelmed by this region of space when writing:

"This and I. 49 (NGC 6522) occur on a ground so astonishingly rich and stipped over with stars individually discernable, as hardly to admit a pin's point between the stars, and this fills more than the whole field or many fields"

NGC 6569 lies just over 2° southeast of gamma. Once again, in photographs the field contains much obscuring dust and thousands of overlaid stars. In a 20cm telescope, it appears as a disappointing small and faint glow with a stellar looking bright core region. Larger apertures are needed to begin resolution.

Around 43' to the west lies another globular but the obscuring ‘veil’ does its trick here as well. NGC 6558 appears as a very small and faint smudge, easily overlooked. Sir John Herschel discovered it in 1834 and resolved it into faint stars.

Delta, another K-type orange giant of magnitude 2.7 marks our next signpost. Around 47' to the southeast lies NGC 6624, a small but bright globular cluster with a brighter stellar core. A 30cm aperture will resolve it into a myriad of faint stars.

Move on south to find epsilon Sagittarii, an early B9 white giant star of magnitude 1.8. There is a line of three 6th and 7th magnitude stars running toward the northeast culminating in a star of magnitude 5.4. About 38' to the north lies our next target, M69. Discovered by Lacaille in 1752, this beautiful elliptical ball of suns shows some resolution around the periphery with a 20cm aperture at 90x magnification. Low magnification of around 50x showed the cluster to advantage including the pretty surrounding starfield and there is a bright 8th magnitude star close to the northwest.

Moving back to the magnitude 5.4 star on our star trail, move slightly east to a magnitude 5.3 and 7.0 pair of unrelated stars. Less than 0.5° to the east lies NGC 6652, a fainter and smaller cluster than M69. High power showed an irregular form and a brighter stellar core.

Next we move just under 2° northeast to M70, a small but easy globular with a very bright central core. There was no sign of resolution at 90x magnification.

Our final object on this visit is M54 farther to the northeast. It would be best if we centre on magnitude 2.6 zeta Sagittarii, a close double of spectral type-A giant and dwarf stars. Around 1.7° to the southwest brings us to M54, a smallish but very bright cluster with a brighter core. There was no hint of resolution with a 20cm aperture at 90x magnification. No doubt, a larger telescope and higher power would work the trick.

Well, that ends our brief tour of only one small part of the treasure trove of Sagittarius. Clear skies and good hunting.

~CosMos

Note: Selected terms have been linked to Wikipedia to give an understanding of their meaning and Messier objects have been linked here as well. NGC objects have been linked to a photograph on the NGC/IC Project website. Charts above are edited from Uranometria 2000.