The Globulars of Sagittarius

Table

As the warm summer evening twilight fades and the night skies are revealed, my gaze is always drawn to the magnificence of the summer Milky Way. Here the awesome star fields are peppered with clusters and nebulae as we gaze along the spine of our galaxy. I always find myself, though, returning to the glowing globulars-symmetrical spheres of one hundred thousand to one million stars which formed in the earliest stages of our galaxy ten to fifteen billion years ago. As Harlow Shapley deduced, the globular clusters form a halo around our galactic nucleus and as the center of the milky is located in the direction of Sagittarius, we naturally find the highest concentration in this constellation. Of the 150 or so known globulars, 29 can be found in this constellation, 24 more in Ophiuchus while 18 reside in Scorpius. Between these three constellations, this amounts to nearly half of the total numbers of globulars.

Globulars tend to be relatively bright and compact and so often they have a very high surface brightness. This is despite the fact that they often lay in the direction of heavy obscuration by dust in the plane of the milky way which can diminish starlight by several magnitudes. The brightest globulars like Omega Centauri, and even M13 are naked-eye objects while the smallest optical aid including your finder scope will reveal scores of globulars.

Although they are similar as a class, globulars vary considerably in appearance visually. Except for highly obscured or distant globulars, they are easy to locate as a class, so the principal challenge for the visual observer is the degree of resolvability into stars. This depends greatly on 3 factors. First, is the degree of concentration of the stellar members in the cluster (rated on a 1 to 12 scale with a smaller number indicating a concentrated cluster and a larger number indicating a loosely packed cluster). Tightly packed clusters tend to resist resolution but will have a high surface brightness. This aids some extremely distant clusters such as NGC 2419 in Lynx to be easily visible in an 8" scope. Secondly, the distance to the globular plays a factor with the closest globulars like M4 and Omega Centauri generally appearing larger and brighter. Medium or loosely concentrated clusters may allow a high degree of resolution in an 8" if they are relatively nearby. A third factor which can come into play is the extent of dust in the plane of the milky way along our line of sight to the object. Some globulars discovered photographically in the past 35 years are visually unobservable due to a combination of a sparse population, extreme distance, and high obscuration of dust in the galactic plane.

The easiest place to start is Lambda, which forms the top of the "teapot" asterism. Just 40' east, you'll find NGC 6638, a compact object with a high surface brightness. In my 17.5" it appears fairly bright and round, perhaps 2 arc minutes in diameter. The halo has a fairly even surface brightness but increases in to a small bright core. At 220X, a few extremely faint stars, magnitude 15-16 are resolved at the mottled edges of the halo.

Head back to Lambda and move 1° to the northwest and you'll run across M28 , discovered by Messier in 1764. Although his description, reads in part "nebula containing no star", my C-8 at 200X just resolved the lively halo into many faint stars. With a 13", M-28 appears very bright, and fairly large with a highly mottled core. and the outer halo is resolved into many stragglers arranged in long spidery chains. A few dozen stars are resolved at 290X including a prominent star chain leading to the north.

If you scoot your scope 2.5° northeast of Lambda and glance in your finder, you should easily find M22, the crown jewel globular in Sagittarius. This globular was known before Messier's time, and its discovery is generally attributed to an obscure amateur, Abraham Ihle. In a dark sky, this globular is a faint naked-eye object and is a strong rival to M13 in beauty. My C-8 revealed a few 100 stars mag 11-13 blanketing the core and halo. In my 17.5" operating in a dark sky, the extreme richness is shocking and takes your breath away!

For a change of pace, a little over 4° east-northeast of M22, you find a pair of faint naked-eye stars, Nu1 and Nu2, also known as 32 and 35 Sagittarii. Look 2' south of 35 Sag and you should find a very small and faint patch catalogued as NGC 6417 . In my 13", this hazy globular is fairly faint with a low surface brightness. Five stars are resolved including two pairs though these are possibly superimposed field stars.

Forming the north vertex of a large equilateral triangle with M22 and NGC 6417 is a globular discovered photographically in the 1950's from the POSS, Palomar 8 . I'm very surprised this globular was not discovered visually at an earlier date as it is an easy object with direct vision in my 13". It is slightly elongated north-south and a faint star appears at the south end. A threshold star can be glimpsed at the west end but there is no other resolution over the disc.

A little over a degree northwest, you can find another fine specimen, NGC 6642, floating in a rich milky way star field. Although my C-8 could not resolve this globular, the 17.5" showed many details. Within the 2' diameter round disc was a bright core which seemed slightly offset while the mottled halo revealed approximately six 15th magnitude stars. The globular is collinear with two 11th and 13th magnitude stars located 2' and 2.5' northwest.

A pair of globulars, NGC 6522 and NGC 6528, can be easily found just northwest of Gamma, which forms the spout of the "teapot" which is embedded in the rich milky way. In my 13", NGC 6522 appears moderately bright and mottled, with one brighter star visible on the east side but no other resolution. You'll find NGC 6528 in the same field 15' east. This globular appears as a fainter twin of NGC 6522, though prominently visible in my 13", I found no resolution into stars.

Just over 2° southeast of Gamma, you can find NGC 6569 , located 9' north of a 7th magnitude field star. Although faint with my C-8, it is a fairly bright, compact object in my 17.5" just 2.5' in diameter. There is an irregular surface brightness to the globular, appearing mottled with darker areas on the east side. Still there is no clear resolution into stars. A similar object, NGC 6558, is situated just 45' west and you may be able to capture both in the same field.

If you head east to third magnitude Delta, you should find NGC 6624 in your finder scope just 50' to the southeast. This globular has a very high surface brightness and appears very symmetrical, about 3' in diameter. The center contains a sharp bright core and a stellar nucleus. With my 17.5" there are hints of resolution in the outer halo particularly on the north edge with six stars magnitude 15 glimpsed. A close double star with components of 12th and 14th magnitude is located 2' WSW and the star field in general is rich in faint stars.

About 2.5° southeast of NGC 6624 you’ll run across another Messier entry, M69, discovered by Lacaille at the Cape of Good Hope in 1752. With a high surface brightness, you should have no problems with this globular, and in my C-8 it appears fairly bright, with a bright core and a lively halo. A few stars are resolved at the edges and my 17.5" resolves the halo into a mass of faint stars.

Continue another 1° southeast from M69, and you'll find NGC 6652, 7' southeast of a 7th magnitude field star. This high surface brightness globular has a compact bright nucleus in my C-8 and a 13th magnitude star is visible at the southwest edge but no other resolution was seen. With a 17.5" scope, NGC 6652 is fairly bright, and slightly elongated east-west, 2.0'x1.5' in dimensions. A sharply defined small bright core is prominent with a substellar nucleus embedded. The outer halo is mottled but unresolved except for the 13th magnitude star 1' WSW of the nucleus and a 14th magnitude star at the east edge of the halo.

From here a 1°40' jaunt to the northeast will bring you to M70, discovered by Messier in August 31, 1780. With my C-8 operating at 220x, a few faint stars are resolved in the halo but the bright core remains unresolved. The halo is unsymmetrical, being slightly flattened on the east side. A very small bright core is prominent with my 13", and many faint stars glitter in the outer halo while a bright string of stars trail to the north-northeast direction.

Although there are no bright naked-eye stars in this region, a 3° shift to the northeast should reveal M54 in your finderscope. Messier made this discovery on July 24th of 1778. This globular has an unusually high surface brightness yet is not easy to resolve. With my 17.5" it is moderately large, 3' in diameter and very bright, increasing to a sharp bright core. The globular has a grainy, mottled appearance and five brighter stars (possibly field stars) are visible around the edges.

From here, let's head south just over 6° towards the Corona Australis border to a remarkable region containing the globular cluster NGC 6723 and a complex nebulous region in the same field including NGC 6726/27/29. NGC 6723 is a showpiece globular and would have been picked up by Messier if it was further north. An 8" begins to resolve the cluster around the edges of the mottled disc while my 13" and 17.5" reveal dozens of stars across the disc and at the edges of the halo.

Let's continue our journey at the Lagoon Nebula. You can get hung up in this area for quite awhile but tear yourself away and head just 50' southeast to NGC 6544. With my C-8 a brighter core is visible with 2 brighter stars resolved in the core and a few stars at the edges of the halo. Although grainy, the cluster resists further resolution. At 220X, this cluster appears 4'x3' in my 17.5", elongated northwest-southeast. The outline is noticeably irregular and several faint stars and other field stars are resolved at the edges. Several superimposed stars are visible over the mottled disc including 2 or 3 13th magnitude stars in tight knot near the center.

At this point, we can slide 1° southeast and run into NGC 6553. My C-8 reveals a north-south elongation to the globular and a star at the north edge but no other resolution is evident. With a 17.5", the disc is grainy and mottled and besides the bright 12th magnitude star at the north edge, 4 or 5 faint stars are visible at the edges of the halo. A rich milky way star field adds to the visual enjoyment of this globular.

To end our tour of the Sagittarius globulars, we need to head southeast out of the rich Milky Way fields. Scan about 8° east southeast of 2.6 magnitude Zeta Sagittarii and you should have no problem picking out M55 in your finder. This globular was another Lacaille find in his journey to the Cape in 1752. With a concentration class of 11, this object is very loosely compressed and resolves easily in small scopes. In my C-8, scores of faint stars are scattered across the entire 10' diameter. Due to its low concentration, there is no compact core, just an unresolved background haze. M55 appears nearly fully resolved in the 13" and 17.5" with a couple of hundred stars of varying brightness glowing over a background haze.