Sagittarius is the 9th constellation of the zodiac. Located just east of Scorpius, it lies in one of the richest regions of the Milky Way. This constellation holds a multitude of objects to fill many nights of observation. There are more Messier objects in this constellation (15) than any other including numerous NGC objects. There are over 30 globular clusters and more than 30 open clusters. Many planetary nebulae and diffuse nebulae are also present, however most planetary objects are too faint for amateur observation.
I refer to the outline of Sagittarius as a teapot as this is easier to picture than the archer aiming its arrow at the heart of Scorpius. I guess a tea pot is not as heroic as a mythological figure.
Locate Sagittarius high in the Eastern sky. The teapot handle towards the east, the spout to the west and the lid pointing north.
All observations in this review were made with a 128 mm (5 inch) refractor, 8 mm Radian (130 x), 12 mm (86 x), 17 mm (61x), and 27 mm (38 x) eyepieces, 2 x Barlow. 7 x 50 binoculars were also used. The objects were all located using star maps and a planet-sphere guide without any automated Go-To aids. If I can do it, anyone can find these objects using the brightest stars of the constellation as a guide.
In a dark moonless sky I was able to locate all objects listed below with binoculars. Although there is not much detail seen and some of the smaller globular clusters only a pinpoint haze, it is worthwhile seeing how many you can see with your binoculars.
Near γ Gamma Sag. (at the tip of the teapots spout) are a faint pair of globular clusters NGC6522 and 6528 magnitude 8.6 (5.6’ size) and mag. 9.5 (3.70’). At 130 x magnification, small unresolved nebulous spots are seen. NGC6528 is 15 arc minutes east of NGC6522 and the smaller and fainter of the pair. They can both be seen in the same field of view through your eyepiece with a small triangle made up of 4 stars located almost directly between the two. NGC6522 and NGC6528
If Sagittarius the Archer was to raise his Bow (located at gamma) a few degrees, the arrow would pass through the Milky Way to the Galactic centre with NGC 6522 lying close to this point. The 15’ field around this globular is known as Baade’s Window, after German-American astronomer Walter Baade who used it to estimate the distance of the centre of the Galaxy.
There is a nice line of small globular clusters along the base of the teapot between Epsilon and Zeta Sag. NGC6715 (M54), 6681 (M70), 6652, 6637 (M69). And 6624 near delta Sag. See Eugene O’Connor’s expanded challenge below. These are all easily located and appear as small unresolved nebulosity in my 5 inch.
NGC6715 / Messier 54 (left) – NGC 6681 / Messier 70 (right)
M28 mag. 6.9 (11.20’), another easily located small globular lying 1° north west of Lambda (top of teapot lid) appears small with a condensed unresolved core at 130x and a sprinkling of faint outlying stars.
1 ° SE of Lambda a fainter 9th magnitude globular NGC6638 can fit in the same field of view with low magnification using a 40 mm eyepiece.
NGC6809 (M55) is a nice magnitude 7 globular located 8° east-southeast of Zeta Sagittarii covering 19.00’. It seems quite a large cluster after observing all the faint fuzzies I have been observing. The stars are more open in this one with a nice large scattering of outliers.
We cannot pass NGC6656 (M22) the “Great Sagittarius Cluster”, 2.3° NE of Lambda, third brightest cluster in the sky with over 100,000 stars. Magnitude 5.1 covering 24.00 arc minutes. Visible to the naked eye it is an easy object for binoculars. The cluster does not appear round with a bright rich condensed centre and a broad scattering of outliers easily resolved with the 5 inch, a 3 inch scope will also resolve the brighter stars. Stephen O’Meara calls this the “Crackerjack Cluster” after the boxes of sweet popcorn with a surprise treat inside ready to be discovered. I must say I consumed many a box of crackerjacks in my childhood, having to search for the surprise, just as you have to search for surprises within each object you observe in the sky.
M25 is a nice bright open cluster of about 50 stars. At magnitude 4.6 it is visible with the naked eye. Easily located by following a line along the teapot lid from Delta to Lambda the distance to M25 (6°) is slightly larger than the distance between these two stars. In the centre of the cluster I found groups of curving stars. From a line of 3 deep yellow stars there runs a widening curve of equal stars amongst these is the Cepheid type variable star U Sagittari which waxes and wanes between mag. 6.3 and 7.1 every 6 days 18 hours. In this region of brilliant pinpoint I could see the form of a Giraffe (thought I would have name this one since there are no Giraffes represented in the heavens). The line of stars above form the rump and back leg, the nice curve of stars the tail. To the left of this in my refractor telescope’s field of view, there is another line of three stars the front legs and chest, a small box now forms the body. Two more stars off the top of the box form the neck and crown of the head, with another three stars finishing with the nose pointing down.
M8 The Lagoon Nebula or Hour Glass Nebula, 4° SW of Mu Sag. is spectacular as usual with its rolling nebulosity and easily defined dark channel through the centre. This beautiful cosmic cloud is a popular stop on telescopic tours of Sagittarius. Eighteenth century cosmic tourist Charles Messier cataloged the bright nebula as M8. Modern day astronomers recognize the Lagoon Nebula as an active stellar nursery about 5,000 light-years distant, in the direction of the center of our Milky Way Galaxy. Hot stars in the embedded open star cluster NGC 6530 power the nebular glow. Remarkable features can be traced through this sharp picture, showing off the Lagoon’s filaments of glowing gas and dark dust clouds. Twisting near the center of the Lagoon, the small, bright hourglass shape is the turbulent result of extreme stellar winds and intense starlight. At the nebula’s estimated distance, the picture below spans over 60 light-years. With an OIII filter the nebulosity and dark lane were much more pronounced. The 5 inch gathered just enough light to cope with the filter. Then using a UHC-Filter the image was even more spectacular, providing more definition. Another prominent feature within the eastern half of the nebula is the open star cluster NGC6530. Start with lower magnification on this one, the complete nebula and open cluster are best viewed at 38 x magnification but it could be magnified further to get more detail if your scope is capable.
Just south of the Lagoon are two more faint 8 magnitude globular clusters NGC6544 and 1° south NGC6533. These are very small, unresolved, with a condensed centre and granular in appearance, with 6533 a bit more elliptical.
M 20 The Trifid Nebula lies 5,000 lights years away 1 ½ ° north of the Lagoon Nebula. The Trifid illustrates three different types of astronomical nebulae; red emission nebulae dominated by light emitted by hydrogen atoms, blue reflection nebulae produced by dust reflecting starlight, and dark nebulae where dense dust clouds appear in silhouette. The bright red emission region, roughly separated into three parts by obscuring dust lanes, lends the Trifid its popular name. But in this sharp, colorful scene, the red emission is also surrounded by the the telltale blue haze of reflection nebulae. Pillars and jets sculpted by newborn stars, below and left of the emission nebula’s center, appear in Hubble Space Telescope close-up images of the region. The Trifid Nebula is about 40 light-years across. The central double star HN40 at magnitudes 8 and 9 is easily located. HN40 is actually a multiple star and a 3rd fainter star was visible at 122 x magnification. Using a 12” (30 cm) aperture telescope will be required to see a fourth star. Eugene can see the fourth in his 16 inch. How many can you see in your telescope?
M17 (NGC6618) the Swan Nebula (also known as the Omega or Horseshoe Nebula) is located near the border of Serpens Cauda. This is a beautiful emission nebula with the distinct shape of a swan floating in the heavens at 86x mag. Stellar winds and energetic light from hot, massive stars formed from M17’s stock of cosmic gas and dust have slowly carved away at the remaining interstellar material producing the cavernous appearance and undulating shapes. M17 is also known as the Omega Nebula or the Swan Nebula.Take your time time to observe the region surrounding this nebula for fine wisps of nebulosity.
Easy Deep Sky Challenges
I always enjoy revisiting many Deep Sky objects in Sagittarius. Apart from the more obvious highlights, I would like to draw your attention to an easy- to- find circlet of globular clusters which link Zeta and Delta Sagittarius. I had no difficulty in star-hopping these in the space of a half hour with the 8” on a blustery night in late July.
They are – moving west from Zeta – M54, M70, NGC6652, M69 and NGC6624. Apart from NGC6652, they are all brighter than magnitude 8. What makes this an interesting trip is the contrast in both the appearance and fields where these objects lie. M54 is small and circular with a bright core and a large halo. M70 is less bright but in a rich setting with two wide sets of double stars in the field. NGC6652 is quite a small, dim object in a rich field below a double star. M69 is a broad, dense globular with a bright star to the north of the field. NGC6624: Another fairly dim object but set in an almost straight line of stars.
It will be an interesting exercise, when you have studied these five to blow your mind by hopping across to the nearby Crackerjack Cluster, M22. You will now see why it is called the third most impressive Globular Cluster in the sky.
Finally, if you want to reach for a final challenge, try the Tom Thumb Cluster, further west in this part of the sky. It is like a triangle of stars saturated with scintillating star dust in the 8”. These nights may be cool and windy but the Deep Sky never looks better. Wrap up and get out there.
Sources used: The Messier Objects – Stephen O’Meara 1998;
Hartung’s Astronomical Objects for Southern Telescopes;
Burnham’s Celestial Handbook;
Collins Pocket Guide, Stars & Planets 1993;