There are 24 Messier objects between RA 18 and RA 21, the second-richest octile of the Messier list after the one that includes the Virgo Cluster. The abundance of Messier objects is due to the fact that this part of the sky contains the widest, richest, and brightest part of the Milky Way, with some of the finest star clusters and nebulae in the sky.
These late-summer Messier objects are both a blessing and a curse for urban and suburban observers north of latitude 30N or so. The blessing is that unlike the faint galaxies of spring, many of these are bright showpiece objects that are spectacular even through heavy light pollution. The curse is that they are heavily concentrated at southerly declinations, where they never really climb out of the soup of light pollution along the horizon.
In addition, they are well-placed only during the summer, when the nights are at their shortest and the logistics of observing are most problematic. But they are not quite as bad in this regard as the objects between RA 15 and RA 18, at least not for people who do most of their observing in the evening rather than in the pre-dawn. These objects become well-placed in the evening sky just as the nights start to become significantly longer, so although they are slowly slipping out of grasp, the ever-earlier evenings give you a chance to admire these showpiece objects well into autumn, if you start as soon as the sky gets dark.
For instance, M8, the quintessential late-summer object, is at its highest at 10PM daylight time around August 1, which is around when the sky becomes fully dark at my latitude of 42N. One month later, the sky is fully dark at 9PM, and M8 is only an hour past its prime. On October 1, the sky is fully dark at 8PM, and although M8 is then 2 hours past its prime, it is still moderately well placed, just 5 degrees lower than optimal.
For convenience, I have included M23 in this section, despite the fact that its RA is three minutes shy of 18:00, because it clusters naturally with the other objects in Sagittarius. I have tranferred the three eastermost objects (M29, M72 and M73) to the early-fall section to augment the measly five objects that actually fall properly in that section, between RA 21 and RA 23:59. Here, then, are the late-summer Messier objects:
For a key to this table, see
Key to the Tables.
In the unlikely event that you tire of the objects in this section, M13 and M92 remain reasonably well-placed in the evening sky throughout the summer for observers in the North Temperate Zone, and several of the objects in the early- autumn section lie quite far north, and rise reasonably high fairly early in the summer. As for galaxies, the king of them all, M31, begins to rise in the eastern sky just as M51 is disappearing in the western sky.
M23 is a delightful open cluster in its own right, but its primary significance for me is as the forerunner of the magnificent Sagittarius Messier objects, and of the summer Milky Way in general. I love observing the distant galaxies of spring, but I am always glad when they yield place to our own galaxy. Lying on my back on a warm summer night, I can see far more detail in the Milky Way with my unaided eyes than I can see in any other galaxy with the largest telescope I have ever used.
To locate the Sagittarius Messier objects, you must first locate Sagittarius itself, which is not as easy as it should be under bright skies at my latitude of 42N, due to its southerly declination and the fact that many of its signature stars are only third magnitude. Although Sagittarius means “archer”, most modern astronomers prefer to picture it as a teapot, with the handle on the east and the Milky Way issuing out of the open spout on the northwest. The only genuinely bright stars in the constellation are mag 2.1 Sigma in the handle and mag 1.8 Epsilon at the SW edge of the base. The key stars for locating the Messier objects are Lambda, Delta, and Gamma on the top of the kettle, all mag 2.7 – 3.0.
M23 lies a little under 2/3 of the way between Lambda Sgr and Xi Serpentis, but that is a long enough stretch to make it hard to locate M23 by the point-and-shoot method. It helps a lot if you can see Mu Sgr (mag 3.8) partway between Lambda and M23. Failing that, it is a long 6-degree starhop from Xi Ser or a very long 10-degree starhop from Theta Oph or Lambda Sgr.
In my 7×35 binoculars, M23 is visible as a faint, subtle cloud of light in under a suburban sky, but invisible from the city. In my 70mm scope, it is barely visible at low power but quite attractive at 60X both in the city and in the suburbs, showing a few stars with direct vision in the city and about a dozen in the suburbs. In both cases, the directly visible stars are supported by a much richer background of stars that can be seen only with averted vision.
In my 178mm scope, M23 is fairly well resolved at all powers both in the city and in the suburbs, showing several dozen stars in a 25′ circle. Because extra magnification is not needed to bring out the faint stars, as it is in the smaller scope, M23 shows best at modest power, around 40X, where enough of the background is visible to set the cluster off well.
M8, the Lagoon Nebula, is a close rival of M42 and M45 as my very favorite Messier object to view with small instruments under dark skies. Unfortunately, the nebulosity that makes M8 so magnificent is hurt badly by light pollution, especially at my latitude of 42N, where M8 never rises higher than 24 degrees off the horizon.
M8 is the same kind of object as M42, a dense cloud of gas which is actively giving rise to new stars. M8 appears somewhat fainter than M42, but that is due entirely to the fact that it is much farther from us, some 5000 light years distant as opposed to 1500 for M42. In fact, M8 is considerably larger and more active than M42.
M8 is a very large object, listed as 90′ by 40′ by many sources, but the portion that is visually observable is much smaller; the bright part of the nebulosity is no more than 30′ in the E-W direction and 20′ N-S. Somewhat E of center is a cluster of about two dozen stars, mostly ninth and tenth magnitude, arranged in a crude rectangle about 10′ from NE to SW and 7′ in the narrower axis. This cluster has the separate designation NGC 6530.
Encircling NGC 6530 is a ring of stars considerably brighter than any in the cluster proper, the brightest of which is the mag 6.0 star Sagittarii directly W of the cluster. This star, and its companion Herschel 36 to the WSW, are the primary illuminators of M8, just as star C in the Trapezium is the primary illuminator of M42. Like the Trapezium stars, 9 Sgr and Herschel 36 are O-type stars emitting the great majority of their energy in the ultraviolet spectrum, and causing the hydrogen and oxygen in the gas cloud to fluoresce in a few specific wavelengths of visible light.
Even brighter than 9 Sgr is mag 5.4 7 Sgr 15′ to the WNW. 7 Sgr is flanked by a few other brightish stars which appear to form a sparse mini-cluster in their own right.
The nebulosity is brightest around 9 Sgr and Herschel 36, especially in very small and intense patch about 1′ in diameter around the latter star. The star cluster NGC 6530 is also suffused in a somewhat fainter nebulosity, and those two patches of nebulosity are connected by an arch to the N. In the middle of all of that is the famous dark patch, like a patch of water surrounded by a coral reef, which gives the Lagoon Nebula its name. There is also a very faint nebulosity surrounding the whole object, but it is quite invisible under urban or suburban skies, even with a nebula filter. A few isolated bright patches do show through under good conditions, notably near the mag 7 star E of NGC 6530 and around 9 Sgr.
M8 is very obvious to the naked eye under dark skies, looking like a small disjoint piece of the Milky Way some distance W of the main body, and somewhat brighter. Even under suburban skies, I can see M8 fairly easily with my unaided eyes, although not in the city. I have never been sure whether I am actually seeing the nebulosity or the cluster of stars contained within.
If you cannot see M8 directly, which is likely, it is a moderate 4-degree starhop from mag 3.8 Mu Sagittarii, or you can locate it by the fact that Mu Sgr and M8 are equidistant from Lambda Sgr. If you cannot see Mu, you might try extending the line from Phi Sgr through Lambda and bending slightly S, or you can starhop 5.5 degrees from Lambda.
Under urban skies at 42N without a nebula filter, the view of M8 is dominated by the stars rather than the nebulosity. Even my 7×35 binoculars show the brighter stars easily, such as 7 Sgr and 9 Sgr, and they show NGC 6530 as a fairly bright cloud with a few stars peeking through. In my 70mm scope at 60X, NGC 6530 is resolved into about a dozen stars, and the nebulosity near 9 Sgr and surrounding the cluster is faintly visible. NGC 6530 is well resolved at all powers in my 178mm scope, with higher powers showing more detail and lower powers giving a better overview. 60X may be the best compromise. Again, the embedded nebulosity is visible, still faint but much bolder than in the 70mm scope.
Adding a nebula filter under urban skies changes the view completely. The stars become much less evident, and the star cluster NGC 6530 nearly disappears in my 70mm scope. The nebulosity, however, becomes very much bolder and brighter, and the Lagoon shape becomes quite obvious, which is not true without the filter.
M8 does much better under suburban skies; the nebulosity is quite obvious even in my 7×35 binoculars, and is prominent in both my scopes even without a filter. Using the 178mm scope with the filter, the nebulosity is overpowering in the brighter sections, and shows a wealth of fine detail throughout.
M20 and M21
Moving north along the axis of the Sagittarius Milky Way, the next Messier objects after M8 are M20, another bright nebula with embedded stars, and M21, a rather unremarkable open cluster. M20 is a magnificent object under dark skies, especially in large telescopes, but its nebulosity is much more subdued than M8’s, and it suffers very badly from even modest light pollution.
M20 and M21 are mostly easily reached by starhopping from the much more prominent M8, or simply by moving the telescope north two degrees. All three objects (M8, M20, and M21) are visible simultaneously in a rich-field telescope at low power, forming what many people consider to be the single finest wide telescopic field in the whole sky.
M20 and M21 lie at opposite ends of Webb’ Cross, a wonderful asterism of stars from mag 6 to mag 8 arranged like a cross with the long arm faintly curved and the short arm, consisting of the three brightest stars, an arc severely concave to the S. The whole formation is about 40′ long, and M21 surrounds the northernmost star, while M20 surrounds the whole S section. The cross is quite obvious in my 7×35 binoculars, but M21 and M20 show only as vague glows in that instrument under suburban skies, and not at all under urban skies.
M21 is rather small and sparse, and the stars have a wide range of brightness, from the central mag 7 star down to mag 12 or 13. It is also set against a fairly rich background, which makes it look even less prominent. Many of the stars are arranged in a nearly perfect circle about 3′ in diameter with the mag 7 star on the S and a complete void in the middle, which gives the cluster a rather unusual appearance once it has been detected. The remaining stars are scattered loosely in a circle some 10′ to 15′, and it is quite impossible to say where the cluster ends.
M21 is reasonably well resolved in my 178mm scope at high power, even under urban skies, and enough stars show in the 70mm scope under suburban skies to make the flesh the cluster out. Only half a dozen stars are visible in the 70mm scope under urban skies, and they do not add up to a convincing cluster.
Under urban skies, M20 is quite invisible in my 70mm scope, and I am not 100% sure that I have seen it in my 178mm scope either. The best place to look is around the lovely double star Herschel 40, which forms the S end of Webb’s Cross. If I look hard, I see just a hint of nebulosity, maybe 5′ across, surrounding the star. The nebulosity — real or imagined — remains but is not improved by my nebula filter.
M20 is much more definite under suburban skies, but is still only a pale shadow of its true self as seen under dark skies. Again, it appears as a 5′ circle around Herschel 40, faintly present in my 70mm scope and fairly obvious in my 178mm scope. In both scopes, a nebula filter makes the nebulosity much more definite, but it does not make it seem any larger or show any more detail.
I have never been able to see under suburban skies any of the wonderful detail that makes this nebula famous, notably the 15′ circle around Herschel 40 split into three or four lobes by striking dark lanes, which gives this object its popular name the Trifid Nebula, famous from science-fiction horror novel and movie. I have also never had a hint in city or suburbs of the secondary nebulosity some 15′ to the N, separated by a wide dark lane from the primary nebulosity.
Moving 5 degrees NNE along the galactic axis from M20 and M21, we encounter M24, one of the most unusual objects in the Messier list. Indeed, this is technically not an object at all. Messier called it a star cluster, but that implies a collection of stars which are physically related and bound to each other by gravity, which is not true for M24. Instead, M24 is a star cloud, a rich portion of the inner Milky Way which happens to be visible to us because of a gap in the dust clouds that normally prevent us from seeing far along the galactic plane, particularly towards the center, where stars, gas, and dust are all most dense. If not for the dust clouds, this whole quadrant of the sky would be ablaze with light, as M24 is.
M24 is sometimes called the Small Sagittarius Star Cloud. The Great Sagittarius Star Cloud is the “steam” that issues from the spout of the teapot, N of Gamma and Delta Sagittarii, some ten degrees S of M24. The Great Sagittarius Star Cloud is even more remarkable than the lesser cloud, being a portion of our galaxy’s central bulge, again showing through a gap in the celestial clouds, whereas M24 is part of one of the inner spiral arms. The actual center of the Milky Way lies some distance WSW of the Great Sagittarius Star Cloud, but it is completely blocked from us in the visual spectrum by dense clouds of dust. It does shine through in X-ray and radio frequencies, where it is called Sagittarius A.
Like M8, M24 is very obvious to the naked eye under dark skies, and visible but somewhat subtle in the suburbs at 42N. Again like M8, it appears like a piece of the Milky Way, not as clearly separated from the main body as M8, but much larger than M8, and much brighter than most of the Milky Way.
If M24 is invisible to the naked eye, it can be found some 2.5 degrees N of Mu Sagitarii, assuming that that mag 3.8 star is visible. Failing that, it can be found 2/5 of the way from Lambda Sagittarii to Nu Ophiuchi
M24 is huge by telescopic standards, some 2 degrees along the long axis, from NE to SW, and 30′ to 45′ across. It is best seen at very low power under dark skies in an instrument large enough to fit the whole thing lengthwise. But it can still be appreciated well in a scope whose maximum FOV is in the 1 – 2 degree range by scanning the object lengthwise. A 1-degree FOV fits just enough of the relatively sparse background on either side of M24 so that one can appreciate M24’s richness and separate identity.
Unfortunately, under urban skies, the background is filled with light pollution rather than darkness, and enough of M24’s stars are blotted out to detract greatly from the overall impression. M24 does appear as a markedly rich star field in my 7×35 binoculars, in my 70mm scope at 16X, and in my 178mm scope at 28X, but the true grandness of this star cloud is lost.
M24 fares much better under suburban skies, where even my 7×35 binoculars are big enough to resolve a fair number of stars, and where the contrast between M24 and the background is much more prominent. Again, it looks best at the lowest possible power in both of my scopes.
M16, M17 and M18
Just N of M24 lie three closely spaced Messier objects: M18, M17, and M16, moving S to N. M16 happens to be just across the constellation boundary into Serpens, but I think of all three objects as part of the Sagittarius Milky Way The three objects all fit together with M24 in the field of most hand-held binoculars, and my 70mm F/6.9 scope at lowest power is just able to fit the three objects without M24.
M18 is by far the least interesting of the three objects. It is a perfectly respectable open cluster, and might even seem rather attractive in other parts of the sky, but it is totally overwhelmed by its neighbors.
M17, the Swan or Omega Nebula, is an emission nebula like M8 or M42, and every bit a match for those two. Its surface brightness is not as high as the Hughenian area of M42 or the intensely bright patch in M8 near 9 Sgr, but the bright part of M17 is much more extensive than the bright part of either of those other nebulae. Like M8 and M42, M17 is giving birth to a population of brand new stars, but these are entirely shrouded by gas and dust, and cannot be seen in the visible spectrum. Infrared photographs, however, reveal a budding star cluster within the nebula.
M16, sometimes called the Eagle Nebula, is yet another star cluster embedded in nebulosity, but it is at the opposite end of the spectrum from M17. In M17, the nebulosity is bright and bold, but the cluster is invisible. In M16, the cluster is prominent and attractive, but the nebulosity is subtle even under dark skies, let alone in city or the suburbs.
Once any member of this triplet has been found, the other two are easy to locate by moving the scope slightly S or N. They are also all easy to find off the N end of M24. I generally find M16 the easiest of the three to pick up in binoculars or a finderscope, especially under heavy light pollution, but M17 is also quite easy to see with even the most modest optical aid under all but the worst skies. Both objects show at low power as unresolved patches of light.
Like the other objects of the Sagittarius Milky Way, these are all surprisingly far from any truly bright star despite being located in the one of the richest parts of the sky as measured by the density of stars mag 6 and fainter. When looking for them under skies to faint to see M24 directly, your best bet may be to starhop from Nu Ophiuchi, but the hop is long and arduous and the anchor star is not terribly bright, at mag 3.3.
M18, the southernmost of the three, has two different aspects under suburban skies or darker. At very low power, as in 7X binoculars or at low power in my 70mm scope, it shows as a small, faintish cloud of light, with only the central mag 8.7 star possibly visible. At 60X in my 70mm scope, I can see half a dozen stars around that central star in a rough 7′ circle, and high power in my 178mm scope brings in perhaps another dozen fainter stars.
Under urban skies, the cloud-of-light view is lost entirely, making M18 invisible in my 7×35 binoculars or in my 70mm scope at low power. At 60X in my 70mm scope, I can make out the brighter stars with averted vision, and M18 is adequate although unimpressive at 120X in my 178mm scope.
Under urban skies, M17 is readily visible in both my scopes without a filter, but far more impressive and detailed with the aid of a nebula filter. The same is true under suburban skies, but of course both the filtered and unfiltered views are improved by the darker skies.
The most prominent feature of M17 is a broad, bright bar of light running some 10′ from WNW to ESE, and about 3′ wide. This is what shows in the smallest instruments and in the worst conditions. A second, fainter bar of nebulosity running roughly N-S joins the first at the WNW end, making a striking checkmark shape. Under darker skies, you can see that the vertical bar bends to the W, forming the neck of the Swan as seen in an inverting telescope, and you can also see some much fainter nebulosity surrounding the whole, but I have never seen these in city or suburbs even with the aid of a nebula filter.
In the city, without a nebula filter, M16 shows as a star cluster — a very attractive cluster, but utterly without nebulosity. This fact is actually more apparent in my 178mm scope than in my 70mm scope, because the former scope resolves M16 well, whereas the latter shows enough stars near the edge of resolution to add up to some false sense of nebulosity. The cluster consists of five mag 8 and mag 9 stars, two very close to each other on the W edge, with some twenty fainter stars filling the intervening space, all in a 6′ circle.
Under suburban skies, the star clusters shows much as in the city, but a faint nebulosity is visible in my 70mm scope without a nebula filter, and more prominent, but only slightly so, in the 178mm scope. The nebulosity is only really visible around the mag 9.5 star in the NW corner and its mag 11 companion.
Adding a nebula filter changes M16 dramatically. The cluster becomes much more subdued, and pretty much disappears in my 70mm scope under urban skies. In the city, the filter gives much the same view of the nebulosity that one gets without the filter in the suburbs, a faintish cloud in the NW corner. In the suburbs with the filter, the nebulosity in the NW corner becomes quite bright and prominent, and fainter tendrils stretch out from there to suffuse the entire cluster.
Even under the darkest skies, the nebulosity in M16 is a difficult subject for the visual observer. But for astrophotographers and imagers, this is one of the most spectacular subjects in the sky. Most famous is the famous Pillars of Creation image from the Hubble Space Telescope.
M25 frames the Sagittarius Milky Way on the E as M23 frames it on the W. M25 is another fine open cluster, but whereas M23 is rich in moderate and faint stars, M25 is a fairly coarse agglomeration of bright stars.
M25 is a 4.5-degree star-hop from Mu Sagittarii, or if you are already pointing at M24, you may be able to find it by moving due E. It may also be visible to the naked eye under good suburban skies.
M25 shows easily in my 7×35 binoculars as a bright patch, with several stars resolved, especially under suburban skies. The telescopic view is best in my 178mm scope, but the 70mm scope does nearly as well, and the cluster is relatively impervious to light pollution, because most of the stars are quite bright (mag 7 – 9). In all cases, the view is best at roughly 60X. Higher powers constrict the field of view too much to set this fairly large and coarse cluster off from the rich background, and lower powers fail to do justice to the tight clump of faintish stars near the center of the cluster.
M25 contains a little knot of ten stars near the center in an area about 5′ x 2′ elongated E-W with a pretty arc of five or six stars just S of that, and a looser arc of stars near the S border of the cluster.
M22 and M28
Just as Ophiuchus contains a wealth of globular clusters W of the main axis of the Milky Way, so Sagittarius contains a fine collection of globular clusters trailing the Milky Way to the E, and the finest of these by far is M22. Nearby M28 is another fine globular cluster, but very much eclipsed by its magnificent neighbor.
Let us start with M28, the westernmost and lesser of these clusters. M28 is very easy to find, just one degree NW of mag 2.8 Lambda Sagittarii.
Under urban skies at latitude 42N, M28 is not exactly hard to see in my 70mm scope, but it is not at all prominent, possibly requiring averted vision. In my 178mm scope under urban skies, it is obvious at all powers, showing at 120X as a small circle of light about 1.5′ to 2′ in diameter, strongly concentrated towards the center.
Under suburban skies, M28 is bright and concentrated, almost starlike in my 70mm scope at 40X or 60X. In the 178mm scope, in shows a bright 2′ core inside a halo perhaps 4′ across, fading out gently at the edges.
M22 is a little harder to locate but much brighter, bigger, and easier to see. If you take a line from Gamma Sagittarii through Lambda Sgr and continue it another 2.5 degrees ENE of Lambda, you reach M22. M22 is fairly easy to see in my 7×3 binoculars, although it is somewhat washed out but light pollution at my latitude of 42N, especially under urban skies.
In my 70mm scope, M22 is obvious at all powers under all skies, showing about 4′ – 5′ across under urban skies and more than 6′ across under suburban skies at 60X. The brightness is fairly uniform across the face of the cluster, but moderately concentrated towards the center. It is distinctly grainy under suburban skies, and resolves a few stars with averted vision under dark skies.
In my 178mm scope, M22 is magnificent at all powers under all skies. Several stars are resolved at 120X under urban skies and many more peek through intermittently. Under suburban skies at 120X, M22 shows at least 7′ across, but fades out vaguely to hint at a much larger disk. The central area is very bright, glowing as if on fire. I can pin down the location of two dozen stars, and far more are visible intermittently.
M54, M69 and M70
Three Messier globular clusters lie along the S edge of the Teapot asterism. Moving W to E they are M69, M70, and M54. None of them can be resolved easily in medium-sized scopes even under dark skies, let alone under urban or suburban skies, but they are all interesting and attractive objects. M54 also has the distinction of being the only globular cluster in the Messier list which arguably belongs to a galaxy other than the Milky Way.
Epsilon Sagittarii, the star at the SW corner of the Teapot, is the brightest star in the constellation at mag 1.8, making it quite prominent even under urban skies at latitude 42N, where it never rises more than 14 degrees off the horizon. M69 lies 2.5 degrees to the NE, in the direction of Phi Sagittarii (mag 3.1), and M70 lies 4.5 degrees ENE, nearly halfway to Zeta Sagittarii (mag 2.6). M54 lies 1.5 degrees WSW of Zeta, just N of the line connecting that star to Epsilon.
Nominally, M70 is slightly fainter and very slightly larger than M69, but according to my visual impressions, M70 is quite a bit larger, with significantly lower surface brightness, and quite a bit harder to see under heavy light pollution, especially at low magnifications.
In my 70mm scope at 60X, M69 is fairly easy to see, but M70 is elusive, especially under urban skies, where it requires concentrated effort with averted vision. M69 is 3′ or under, with a concentrated core, while M70 is a vague, about 4′, uniformly faint across the disk.
Both clusters are much easier to see in my 178mm scope at 80X to 120X, even fairly bold and attractive, but they do not show much more detail than in the smaller scope.
M54 is much more remarkable in every way. This globular cluster is inherently very bright, but it is by far the most distant of all the Messier globulars, some 70,000 light years distant, on the far side of the galactic core. Recent analysis indicates that M54’s motion, and the motion of many nearby stars, is radically different from the prevailing motion of that portion of the Milky Way. This is attributed to the fact that those stars, and M54, are actually part of a small galaxy, the Sagittarius Dwarf, which is currently colliding with the Milky Way. In all probability, the Sagittarius Dwarf will not survive the collision; its stars will be swallowed up in the Milky Way’s disk, while M54 will be added to the halo of globular clusters circling the Milky Way’s core.
M54’s great distance makes even its brightest stars appear quite faint, so M54 is quite hard to resolve even in fairly large amateur telescopes under good conditions, and quite impossible with medium-sized scopes under urban or suburban skies. However, M54 is quite bright and concentrated, giving it a high central surface brightness which makes it quite easy to see even under heavy light pollution.
In both my 70mm scope and my 178mm scope, M54 is readily visible at all powers under all skies, but it is quite small, nearly starlike. The best views are at fairly high powers, 60X in the 70mm scope or 80X to 120X in the 178mm scope. It shows a vague disk about 1′ across under urban skies, and maybe 2′ across under suburban skies.
Trailing far E of the Milky Way and the Teapot, in a part of the sky otherwise devoid of interesting deep-sky objects, lies the remarkable globular cluster M55. M55 appears almost as large as M4, and has even lower surface brightmess, making it quite prominent under dark skies but difficult to see under heavy light pollution, especially considering its low maximum altitude above the horizon for northern observers.
Finding M55 is a major nuisance even under dark skies; it is nowhere near any plausibly bright star. I find the best strategy to be the arduous 8-degree starhop from Zeta Sagittarii.
I can resolve M55 fairly easily in my 178mm scope under dark skies, but I have never seen any hint of resolution under urban or suburban skies. Due to its immense size and low surface brightness, M55 under light pollution shows best at fairly modest magnifications, around 30X in my 70mm scope and 60X in my 178mm scope. As with other large, diffuse objects, the view varies more due to sky brightness than to aperture.
In the city, M55 is difficult to see even with averted vision in both of my scopes, showing up best when I pan around the area so that M55’s apparent motion catches my eye. It appears enormous (over 7′) and vague.
M55 is much easier to see under suburban skies, but it still appears both vague and faint. Again, it is enormous, around 10′ across, nearly as big as the distance between the prominent pair of mag 8 stars 45′ to the NNW.
M11 and M26
Returning to the main axis of the Milky Way, we move north in the Scutum star cloud, an area full of magnificent deep-sky objects including two open clusters catalogued by Messier: M11 and M26. M26 is rather attractive under dark skies, but it is the faintest open cluster in the Messier list, which makes it disappointing under bright skies and/or in small instruments. M11, by contrast, is extremely bright and attractive in all instruments and under all skies. Indeed, many people consider it to be the most beautiful open cluster in the sky.
For all of its wonderful deep-sky objects, the constellation of Scutum is a sorry affair, composed of four fourth-magnitude stars in a nondescript pattern. It is unconvincing under dark skies and may be entirely invisible under urban skies. In general, I find it easier to locate objects within the Scutum star cloud off of Auqila to the NE. Aquila can be spotted immediately by brilliant first-magnitude Altair with its attendants Tarazed (Gamma Aquilae, mag 2.7) and Alshain (Beta Aquilae, mag 3.7) to the NNW and SSE respectively. The four third-magnitude stars making up the rest of the outline of Aquila (the Eagle) are less prominent, but they make a distinctive and attractive pattern.
The tail of Aquila is composed of three stars arcing SSW: Lamdba Aquilae (mag 3.4), 12 Aquilae (mag 4.0) and Eta Scuti (mag 4.8) just over the border into Scutum. One or more of them may be invisible to the naked eye under urban skies or poor suburban skies, but the arc should be striking in binoculars or a finderscope. If you continue from Lambda Aql through Eta Sct and then on for another 1.5 degrees, you reach M11. Continuing from Lambda through 12 Aql and on for another 5.5 degrees, you reach M26. Alternatively, M26 is an easy starhop from Eta Sct via the line of 5th and 6th magnitude stars S and SW of Beta Sct, and then back to M26 via bright Epsilon and Delta Scuti.
M11 in some ways resembles a globular cluster more than an open cluster. It contains hundreds of stars packed very tightly, but all of the stars are fairly faint, and the range of brightness among the stars is fairly narrow. Therefore, like globular clusters, M11 appears as an intense but unresolved patch of light in very small instruments and at low magnifications. Indeed, M11 is visible as a patch of light under virtually any skies with almost any optical aid. It is quite obvious in my 7×35 binoculars under urban skies.
Only three stars are clearly resolvable in my 70mm scope at 60X: a mag 8.6 star near the center and a pair of 9th magnitude stars near the SE edge. Under suburban skies, a few other stars occasionally pop out with averted vision, particularly on the S side. All of this is set against a lovely bright glow of unresolved stars about 8′ square.
M11 is breath-taking in my 178mm scope. At low power, it shows as a fan of light stretching W of the bright central star, scattered with faintish stars, and with a few other stars set around. At 120X, the fan of light resolves into a huge number of stars, too many to count. Despite the superficial similarity to a globular cluster, M11 is much easier to resolve, and lacks the classic circular symmetry.
M26 is like a radically scaled-down version of M11. Like M11, it has a bright central star, but it is a magnitude fainter than M11’s. Like M11, M26 contains a scattering of medium-bright stars and a dense swarm of faint stars, but the latter are genuinely faint in M26 — near the limit of my 178mm scope under dark skies — and nowhere near as dense as in M11.
With a total magnitude of only 8.0, M26 is quite invisible both in my 7×35 binoculars and in my 70mm refractor under urban skies. The central star is indeed visible in the small scope, and perhaps one or two of the surrounding stars, but that does not add up to a cluster. The bright sky background camouflages the haze of light that would otherwise result from the unresolved stars.
Much to my surprise, M26 is visible in my 7×35 binoculars under suburban skies, showing as a tiny but fairly bright patch of light forming a nearly perfect isoceles right triangle with Delta and Epsilon Scuti. The view is quite similar at 16X in my 70mm scope, but using averted vision at 60X, I can resolve several stars in addition to the central star.
M26 does far better in my 178mm scope at 120X, which is enough aperture to resolve about a dozen stars under urban skies, and somewhat more under suburban skies. Nonetheless, the stars are a little too scattered to form a completely convincing cluster, especially under urban skies.
M27 and M71
Following the galactic axis far to the north of the objects we have discussed before, we come to two more Messier objects: the faint and unusually sparse globular cluster M71 in Sagittae, and big, bright planetary nebula M27 in Vulpecula. Although these objects are far E of Sagittarius, they are also far enough N so that they reach a reasonable altitude earlier for most observers in the North Temperate Zone, and they linger well into autumn.
These objects are placed in a rather inscrutable part of the sky. Vulpecula, in particular, is one of the sorriest constellations in the sky, composed entirely of faint stars arranged in no recognizable pattern. Sagittae is compact, and very shapely if you can see it, but two of its four essential stars are only mag 4.5 and the brightest is only mag 3.5, making it a challenge for urban and suburban observers. Fortunately, it is small enough to fit completely into the field of 7X or 10X binoculars, and it is well worth a look.
It is easy to locate the general area of the sky between brilliant first-magnitude Altair on the S and Albireo (Beta Cygni, mag 3.1) on the N. Albireo, by the way, is generally considered to be the most spectacular double star in the sky, readily split at 15X or higher, with wonderful contrast between the golden primary star and the blue secondary star. Having done that, look hard for Sagittae halfway between Altair and Albireo, and slightly E. Once this is done, M71 is very easy to locate halfway from Delta Sagittae to Gammma Sagittae, and just 20′ from (telescopically) bright 9 Sagittae. Delta Sge, Epsilon Sge, and M27 form a very obtuse, nearly isoceles triangle with Gamma at its apex.
M71 is unusually sparse for a globular cluster; in fact, it was usually considered to be an open cluster until the last few decades. The small number of stars gives M71 one of the lowest total brightnesses of any Messier globular cluster, but the individual stars are fairly easy to resolve.
In my 70mm scope, M71 shows best at 40X, where it appears as a faint cloud of light, about 3′ – 4′ across, best with averted vision but still perceptible with direct vision, even under urban skies. It is slightly extended NE – SW, with a faint extension to the N.
In my 178mm scope, M71 shows much as in the 70mm scope, but there are hints of resolution at 120X under urban skies, and at least a dozen stars are clearly perceptible with averted vision under suburban skies.
M27 is a completely different animal, the brightest planetary nebula in the sky by a fair margin. M27 also has high surface brightness, allowing it to show fairly well under heavy light pollution. M27 is readily visible, although surprisingly small, in my 7×35 binoculars under suburban skies but quite difficult under urban skies.
Like most planetary nebulae, M27 responds well to a narrowband nebula filter, but even if you own such a filter, M27 should be observed both with it and without it. The filter brings out certain aspects of the nebula, but others are best observed without it.
In my 70mm scope, M27 shows best at 60X, where it appears as a rectangle with rounded corners, extended NNE – SSW, with a hint of scalloping along the long sides.
The 178mm scope brings out the scalloped shape much more strongly, making it obvious why this is sometimes called the Dumbell Nebula. To me, it is more reminiscent of an apple with large bites taken out of the sides. The view is best at 80X to 120X. Using my narrowband filter, the bites on the sides cease to be hollow, and instead become filled with a faint nebulosity, especially under suburban skies. In fact, the faint nebulosity extends farther from the center than the bright apple part, so whereas without the filter M27 is about 5×3 extended NNE – SSW, with the filter it is about 8×5 extended WNW – ESE.
M56 and M57
The small constellation of Lyra contains two Messier objects: M57, the Ring Nebula, which may be the most famous of all planetary nebulae, and the modest globular cluster M56. These are the northernmost of the late-summer Messier objects, and also placed fairly far W, so that they reach reasonable altitude quite early in the summer, or even in the late spring. Few people go out of their way to observe M56, but M57 is one of the great showpieces of the heavens, and with the highest surface brightness of any Messier object, it is a particularly attractive target for urban and suburban observers.
The constellation of Lyra is one of the most prominent and attractive star patterns in the sky, although some of the stars may be difficult or invisible under urban skies. The eye is immediately drawn to Lyra by dazzling blue-white Vega, fifth brightest star in the sky, at magnitude 0.03. Native American legend has it that Vega and Altair are lovers from warring tribes, immortalized in the sky but perpetually separated from each other by the band of the Milky Way.
Vega forms a tight equilateral triangle with two fourth- magnitude stars, which attaches to a nearly perfect, elongated parallelogram to the S. Under very poor skies, only Vega and the two 3rd-magnitude stars at the far end of the parallelogram are likely to be visible, and the latter may be hard. Each of the six signature stars of Lyra is remarkable in some way; refer to a more general guidebook like Burnham’s for more information. For now, I will only comment that Epsilon Lyrae, Vega’s partner at the far N tip of Lyra, is the famous Double Double, resolvable into two tight pairs of stars at 100X or higher.
M57 is extremely easy to locate 40% of the way from Beta Lyrae at the SW end of the parallelogram to Gamma Lyrae at the SE end. But for a deep-sky object whose position is so tightly defined by bright stars, M57 proves surprisingly hard for many novices to find. The reason is that M57 is very small, the smallest of all the Messier objects. At low or even medium power, it may be hard to distinguish M57 from a star. Look for a star that appears slightly hairy.
In my 70mm scope under urban skies, M57 appears merely as a tiny but bright patch of light, slightly elliptical ENE – WSW. Under suburban skies at 60X, it begins to become apparent that the center of the disk is hollow, especially using averted vision.
In my 178mm scope at 120X, it is immediately obvious why M57 is called the Ring Nebula; the doughnut shape is evident even under poor urban skies.
M56 lies almost directly between Alberio (Beta Cygni) and Gamma Lyrae, 45% of the way from the former to the latter. But this is a fairly large stretch of sky, and M56 is fairly faint, so it may be hard to locate M56 just using the point-and-shoot method. If that fails, M56 is a somewhat easier starhop from Gamma Lyrae than from Alberio, due to a convenient chain of reasonably bright connecting stars.
M56 has fairly low surface brightness, making it a little hard to see under urban skies, especially in my 70mm scope, where it is barely perceptible with averted vision. It shows a little better in my 178mm scope at 120X, as a small cloud about 3′ across, almost evenly bright, but fading slightly at the edges.
M56 is considerably better under suburban skies, where it is reasonably easy to see even in my 70mm scope. In my 178mm scope at 120X, it shows as a bright core around 1.5′ – 2′, surrounded by a very faint halo about 4′ across. The whole thing seems slightly elliptical ENE – WSW, and the core seems to be subtly flattened on the S side. I can resolve a few stars in M56 using the 178mm scope under dark skies, but not under suburban skies.
Like M55, M75 floats all on its own, far from any other notable deep-sky object or reasonably bright star. But where M55 is enormous and vague, M75 is tiny and highly concentrated.
I have a habitual strategy for locating M75 which seems rather round-about but turns out to be quite efficient in practice. It relies on the fact that the handsome pair of mag 5 stars Rho and Pi Capricorni, close enough to fit easily in a wide telescopic field, point almost directly at M75. Take a line from Rho to Pi, bending slightly N, and you pass through mag 5 Sigma Cap. Half a degree S of Sigma, a line of three mag 8 stars starts a sweeping arc of other mag 8 stars leading W and S to M75.
Unfortunately, Rho and Pi Cap are usually too faint to be seen directly under urban or suburban skies. So I find them (very easily) by extending a line from the lovely star pair of Alpha and Beta Capricorni which dominates this part of the sky. Alpha is actually a pair of stars, mag 3.6 and 4.2, separated by 6.5′, which I can sometimes but not always separate naked-eye.
M75 shows best at highish power in both my scopes, around 60X in the 70mm scope and 120X in the 178mm. In all cases, it shows readily as a tiny, nearly starlike core surrounded by a small halo. Both the core and the halo appear to grow with darker skies, with larger apertures, and with higher magnifications.