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Sagittarius

by | Jul 28, 2021 | 0 comments

Sagittarius, the Archer, lies straight above the southern horizon on August evenings at mid-northern latitudes. The band of the Milky Way runs through the western part of Sagittarius; in fact, the core of the Milky Way is located in the direction of this constellation. Because of its position along the Milky Way, Sagittarius contains many star clusters and nebulas.

Trifid Nebula
Trifid Nebula. Photo © Shaphan Shank.


M22 is a bright globular star cluster located 2.5° northeast of Kaus Borealis, the northern star in the bow of Sagittarius. M22 is my personal favorite globular cluster, although there are a couple of larger and brighter ones that are too far south to easily observe from Virginia. Under a dark sky, this cluster is faintly visible to the unaided eye as a fuzzy spot. Binoculars show M22 easily, but a telescope is needed to resolve its stars. M22 is, however, one of the easiest globular clusters to resolve. This cluster’s core is quite large and slightly elongated.


M8, the Lagoon Nebula, lies about 6° west of Kaus Borealis. M8 is as large and nearly as bright as the famous Orion Nebula, making it one of the easiest nebulas to observe. This nebula is fairly obvious to the unaided eye as an elongated bright spot in the Milky Way. Telescopes at low magnification give the best views of M8. Even a small telescope can reveal the bright knots and dark lanes in this nebula, but, as usual, visible detail increases with telescope aperture.


Just over a degree north of the Lagoon Nebula is M20, the Trifid Nebula. This nebula is fainter and much smaller than the Lagoon, but it is still fairly bright as nebulas go. The Trifid is oblong, with the northern half composed of reflection nebulosity and the southern half composed of emission nebulosity. Lanes of dark nebulosity divide the Trifid’s southern half into three sections. The Trifid is visible as a small fuzzy spot, but a telescope at moderate magnification is needed to really view the nebula well. A mid-sized telescope under dark skies should show the Trifid’s dark lanes. Using averted vision and an OIII or UHC filter can really help reveal the detail in this nebula, although a filter will block the reflection nebulosity in the northern half of the Trifid.


The Lagoon and Trifid aren’t the only bright nebulas in Sagittarius. If you follow the Milky Way northeast about 5° from the Lagoon Nebula, you will come upon a much larger bright area of the Milky Way. This brighter patch is cataloged as M24, and is simply a “star cloud” in the Milky Way. M24 is an interesting sight with a binocular, but it is also a useful starting point for finding two emission nebulas, M17 and M16.


M17, the Swan or Omega Nebula, lies about 2° north-northeast of M24. This nebula is relatively bright and easy to detect with binoculars, although you’ll need a telescope at moderate magnification to really be able to observe it well. The brightest part of M17 really does resemble a swan, with a long body and a curved neck. Careful observation under dark skies should also reveal fainter nebulosity around the swan shape.


M16, the Eagle Nebula, sits another 2.5° north of M17, or 4.5° north of M24. This nebula actually lies in the constellation of Serpens Cauda, but it is easier to find by starting from Sagittarius. M16 is similar in size to M17, but it is somewhat fainter. It takes quite a bit of imagination to make an eagle out of M16, but careful observation should show some of the nebula’s shape and structure. An OIII filter enhances the contrast of this nebula and makes it considerably easier to observe.


Sagittarius holds so many bright deep sky objects that some of its less-famous objects often get overlooked. Such is the case with NGC 6520 and Barnard 86. This open cluster/dark nebula pair lies 2.5° north of Alnasl in the western part of Sagittarius. The cluster and nebula lie side by side in a rich Milky Way field. The two objects are quite similar in size, giving the impression that the star cluster was scooped out of the Milky Way, leaving a murky hole where the dark nebula is. The best tool for viewing the pair is a mid-sized or larger telescope at perhaps 100–150×, since both objects are rather small. The pair is striking under a dark sky, but light pollution greatly reduces the contrast of the dark nebula.


Although each individual object deserves careful telescopic observation, it is also rewarding to scan the Sagittarius area of the Milky Way with a binocular. Besides the objects mentioned here, Sagittarius holds a number of bright open star clusters that are easy to observe with binoculars. Few regions of the sky hold as many bright nebulas and clusters as the Milky Way around Sagittarius.

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