Just as springtime is galaxy time, so early summer is time for observing globular clusters. Of the 15 Messier objects between RA 15 and RA 18, 12 are globular clusters. These include clusters like M5 and M13 which rank high among the showpieces of the northern sky and also modest clusters like M107 which show well only in large telescopes, especially under light pollution.
The concentration of globular clusters is no accident. Throughout the early summer, the Milky Way can be seen slowly rising almost parallel to the eastern horizon. Globular clusters are associated with the Milky Way, but they are not tightly bound to the galactic plane, like most nebulae and open clusters. The globular clusters of early summer are harbingers of middle and late summer, when the most magnificent section of the Milky Way lies directly overhead, and stretching down to the southern horizon.
A similar collection of globular clusters tags along behind the Milky Way in autumn. Globular clusters are heavily concentrated towards the galactic core, which lies in Sagittarius in the summer Milky Way. That is why so few globular clusters are visible in the winter, when we look out towards the edge of the galaxy.
For observers in the southern hemisphere, spring and early summer are also fine times for viewing nebulae and open clusters in the Milky Way proper, but this part of the Milky Way, through Centaurus, Crux, and Carina, is forever hidden from observers north of the Tropic of Cancer. Only around RA 17:30 in Scorpius does the galactic plane climb far enough north to be visible from Messier’s latitude. That accounts for the appearance of the two magnficent open clusters M6 and M7 at the very end of this section. M7 is the most southerly of all the Messier objects, but it is so bright that it shines even through very heavy light pollution low in the sky.
Here are the Messier objects from RA 15 through RA 18, excluding M23, at RA 17:56.8, which fits naturally with the late-summer objects.
For a key to this table, see
Key to the Tables.
With the exception of the two wonderful globular clusters in Hercules, the Messier objects from RA 15 to RA 18 are concentrated fairly far south, and so are best observed for the brief time when they are at their highest in the south. This is particularly true for the objects in Scorpius, a wonderful constellation which is never really well placed for observers much north of latitude 30N. That is a pity; Scorpius rivals Orion both in its naked-eye appearance and in the deep-sky objects that it contains.
Many of the late-spring galaxies, notably the ones in Ursa Major and Canes Venatici, remain well-placed well into the summer for observers in the North Temperate Zone. I often view them in the summer for a change of pace, but mostly I am drawn to the finest galaxy of all — our own Milky Way. You can see more detail in the Milky Way with your unaided eyes than you can see in any other galaxy through the finest telescope. Unfortunately, the finest objects of the summer Milky Way are quite far south, and are tantalizingly low in early summer evenings as seen from my latitude.
M5 is my personal favorite among all the globular clusters in the Messier list. M13 is better known, and undeniably magnificent, but I find M5 more shapely. M22 and M4 are easier to resolve than either M5 or M13, but they are too far south for optimal viewing from my latitude of 42N, especially under serious light pollution.
M5 is extremely easy to locate under dark skies, lying as it does just 22′ NW of 5 Serpentis. But at mag 5.1, 5 Serpentis is likely to be visible only in the best suburban skies. It can also be hard to distinguish 5 Serpentis from the numerous other stars in the vicinity, notably mag 5.2 10 Serpentis 2 degrees to the E and mag 5.4 6 Serpentis 1 degree to the SSE.
Under good urban skies and most suburban skies, with limiting magnitude of 4.2 or better, I like to locate 5 Ser and M5 starting with the head of Serpens, a striking nearly equilateral triangle of stars. Proceed from Kappa Ser, the northmost and faintest star of the triangle, through Delta Ser, and continue on slightly farther, and bending slightly south, and there is M5.
If the head of Serpens is not readily visible, then M5 is an 8-degree hop from mag 2.6 Alpha Serpentis.
M5 is readily visible under urban skies in my 7×35 binoculars as a small, bright, fuzz ball. It looks quite similar in my 70mm scope, with a small, very bright core around 2′ inside a fairly bright 5′ halo.
M5 really comes into its own in larger scopes. Even under urban skies, my 178mm scope shows several stars clearly with averted vision, while many more appear intermittently. M5 is magnificent in my 178mm scope under suburban skies, again showing a very bright 2′ core inside a 5′ halo, but also showing a much fainter halo extending nearly to a 15′ diameter, studded with a huge number of stars that appear intermittently with averted vision and about two dozen stars that appear consistently with direct or averted vision.
M5 appears better still in even larger scopes and/or under darker skies. The stars in the halo appear to lie upon well-defined arms, slightly curved, with the brightest and longest arm stretching nearly N.
M13 and M92
M13, the Great Cluster in Hercules, is the most famous globular cluster north of the Celestial Equator. As I have said, I do not consider it as attractive as M5, but it is indisputably magnificent. M92, the other cluster in Hercules, is overshadowed by its giant companion, but it is a wonderful cluster in its own right, almost as easy to resolve as M13, although much smaller. Both clusters have the virtue (for observers in the North Temperate Zone) of lying quite far north in the sky, so that they remain well-placed in the evening sky from early spring throughout the summer.
M13 and M92 are both located off the Keystone of Hercules. This is a lovely asterism, but a little hard to recognize even under dark skies, and much harder under significant light pollution. Epsilon Herculis, the faintest of the four stars of the Keystone at mag 3.9, may well be invisible under poor skies, and none of the stars is genuinely bright. The Keystone is also large and loose, making it even harder to identify.
One easy way to find the Keystone is to start with the unmistakable blue-white star Vega; the Keystone lies about 2/5 of the way from there to Arcturus. Somewhat farther along the same line is the much tighter and finer constellation of Corona Borealis. When looking for the Keystone, my eye is often caught by the head of Draco, an asterism with a very similar shape, but much brighter and tighter. Vega, the head of Draco, and the Keystone form a nearly equilateral triangle.
Once the Keystone is found, M13 is very easy to locate, 1/3 of the way from Eta to Zeta Herculis, on the W edge of the Keystone. M92 is a good deal harder to locate. It forms a slightly lopsided triangle with Pi and Eta Herculis, being about 1.5 degrees too far E to form an equilateral triangle. It also lies about 2/5 of the way from Iota Herculis to Eta. And it is a modest 6-degree star-hop from Pi.
Like M5, M13 is readily visible through even the smallest instrument even under very poor skies. In fact, M13 is visible to the naked eye as a small, faintish but fairly concentrated fuzzy patch under reasonably dark skies. M13 lies halfway between a mag 6.9 star and a mag 7.3 star, but slightly NW of the line connecting those stars.
In my 70mm scope, M13 shows a very bright 2′ core surrounded by a halo which ranges in size from about 3.5′ under urban skies to about 7′ under suburban skies. The halo appears distinctly grainy at high power under good suburban skies, but it does not really resolve into individual stars.
M13 shows the same 2′ core and 7′ halo in my 178mm scope, much brighter of course, and it also resolves numerous individual stars with at high power, especially with averted vision, and especially in the halo. I can pin down about 8 stars under urban skies and about 16 stars under suburban skies, and several times that number appear intermittently with averted vision in both cases. Several of the faint stars appear well outside the halo, to a diameter of 10′ – 15′, and although it is hard to tell which ones properly belong to the cluster, the borderline faint stars are clearly richer in the vicinity of the cluster than in the background.
M92 is also readily visible in my 7×35 binoculars under urban skies, but it is so small that it is a little hard to tell that it is non-stellar. A chain of 3 stars lies quite closeby to the NNE, and M92 is distinctly brighter than the northmost and brightest of those 3 stars, which is mag 8.6.
In my 70mm scope, M92 shows a very compact and very bright core about 1′ across, inside a faint halo about 2′ across under urban skies, and slightly larger under suburban skies.
The core appears even brighter and smaller in my 178mm scope, and a very faint outer halo about 4′ – 5′ in diameter appears around the brighter inner halo, especially under good suburban skies or darker. Numerous stars appear intermittently with averted vision in the inner halo. I can pin down about 5 under urban skies and maybe 10 under suburban skies.
M4 and M80
M4 is one of the closest globular clusters to Earth, at a distance of roughly 7,000 light years; therefore, it appears unusually large, and its stars appear unusually bright to us. It is also rather sparse as globular clusters go, giving it a fairly modest surface brightness. The brightness of the individual stars, combined with the fact that they are fairly widely separated, makes this probably the easiest of all the Messier globular clusters to resolve into individual stars in small instruments — at latitudes where it rises reasonably high in the sky.
At my own latitude of 42N, M4 never gets very far above the horizon, and in the city and the suburbs, it is always buried in the low-lying haze of light pollution, which greatly detracts from its appearance. Even so, it is a wonderful sight in my 178mm scope.
Just as M92 will forever be known as “the other globular cluster in Hercules”, so M80 is the other globular cluster in Scorpius. Although M4 and M80 appear close together in the sky, M80 is actually about four times farther from Earth, at around 27,000 light years. Morphologically, M80 is at the opposite end of the spectrum from M4; where M4 is large and diffuse, M80 is highly concentrated.
M4 is extremely easy to locate, being just 1.3 degrees W of dazzling, deep-red Antares, one of the most prominent stars in the sky. Antares means “rival of Ares”, Ares being another name for Mars, the Red Planet. Antares lies very close to the ecliptic, the path that the Sun and the planets follow through the sky, so Mars and Antares can sometimes be seen right next to each other, rivalling each other in redness and in brilliance.
M4 would be readily visible under dark skies if not for its proximity to Antares, which tends to overpower it. In my 7×35 binoculars, it is an easy target under suburban skies or darker, showing as a large but faint patch of light. Under urban skies, at my latitude of 42N, M4’s low surface brightness makes it invisible in small binoculars.
In my 70mm scope, at latitude 42N, M4 is barely visible as an extended patch barely brighter than the background, showing best at 40X. The view is far better in the suburbs, where M4 shows as a faint but definite 6′ circle, with two stars faintly visible on the W edge. Under dark skies, my 70mm scope resolves perhaps a dozen stars in and around a 10′ halo.
My first view of M4 through my 178mm scope in the city was unforgettable. At low power, the cluster was barely visible. At around 40X, a large, ragged, ethereal cloud began to take shape. When I raised the power to 120X, numerous stars suddenly swam into view out of a barely-visible cloud, with dozens more appearing at the edge of my field, and disappearing when I tried to catch them with direct vision. The overall effect was utterly magical.
In the 178mm scope under suburban skies, I can pin down about two dozen stars in M4, with numerous others at the edge of averted vision. A striking feature of M4 both visually and in photographs is the string of bright stars running in a straight north-south line, slightly W of the cluster’s center. Even when the individual stars cannot be resolved, the bright central line is quite prominent.
M80 can be a little tricky to locate, especially because its small size and high surface brightness make it appear nearly stellar at low power. One way to locate it is by aiming halfway between Antares and Beta Scorpii (Graffias), the topmost star of the prominent arc lying W of Antares. M80 also forms a nearly perfect parallelogram with Antares, Sigma Scorpii, and Rho Scorpii, but at mag 5.1, Rho is unlikely to be visible even in good suburban skies in the North Temperate Zone. Perhaps you can imagine where it lies. M80 is also a modest 3-degree starhop from Sigma.
In the eyepiece, M80 appears like a fuzzy star in the middle of a zigzag line of three other stars of similar brightness about 20′ long. Its high surface brightness makes it readily visile under urban skies both in my 70mm scope and in my 178mm scope, but nothing can be seen except for the tiny, nearly stellar core.
Under suburban skies, at high power, I can see a subtle halo around the miniscule core, about 1′ across in my 70mm scope and 1.5′ in my 178mm scope. There is no hint of resolution even in the larger scope.
M19 and M62
M19 and M62 are a pair of globular clusters, rather similar in size and brightness, that lie in the SW corner of Ohpiuchus, just E of the body of Scorpius. They are both fine and interesting objects in a large scope under a dark sky, but they are too far south to show well through light pollution at my latitude of 42N.
To find these objects, start with Antares, then locate Theta Ophiuchi 12 degrees to the E, and Epsilon Scorpii 9 degrees SSE of Antares. Both of those other stars may pose problems. Theta Oph is only mag 3.3, which would be quite bright high in the sky but may be hard to see if it lies close to the horizon. Epsilon Sco is amply bright at mag 2.3, but at my latitude, it is only visible from sites with an unusually low southern horizon.
M19 lies just S of a point 2/5 of the way from Theta Oph to Antares, and it is a modest hop from either star if that method fails. M62 lies just NW of a point 2/5 of the way from Epsilon Sco to Theta Oph, and is a modest hop from Epsilon. Once either of the two clusters is found, it is a modest star-hop to the other.
M62 is the brighter of the two clusters, and that is even truer in appearance than the raw magnitudes would suggest, probably because M62 is much more concentrated towards the center. M62 is easy to see under urban and suburban skies both in my 70mm scope and in my 178mm scope, and it appears strikingly similar regardless of aperture, magnification, and skyglow, as a concentrated 1.5′ circle.
M19 is rather hard to see in my 70mm scope under urban skies, showing as a 3′ circle with averted vision at 40X to 60X. It is much easier in my 178mm scope, but still is only borderline with direct vision at 120X.
Under suburban skies, M19 appears reasonably bright both in my 70mm scope and in my 178mm scope. Again, it appears about 3′ across, but in my 178mm scope, it is unmistakeably elliptical — a very exotic feature in a globular cluster — being slightly elongated north-south.
M9 is easy to locate, but it is one of the faintest and least interesting of Messier’s globular clusters.
To locate M9, start at Eta Ophiuchi, which at mag 2.5 should be readily visible in all but the worst skies. M9 lies 3.5 degrees to the SSE, more or less continuing the line of stars marking the W edge of Ophiuchus.
M9 is barely perceptible with averted vision in my 70mm scope under urban skies, showing as a 1′ smudge with a pointlike center. It is much easier in that scope under suburban skies, showing with direct vision as a 2′ circle with a brighter core.
M107 is even easier to locate than M9, and even fainter.
M107 lies 3 degrees SSW of mag 2.6 Zeta Ophiuchi, in the direction of Graffias (Beta Scorpii, the northmost star in the arc W of Antares).
Under urban skies, M107 is invisible in my 70mm scope and very difficult in my 178mm scope, showing as a 1.5′ circle with averted vision.
M107 is much easier to see under suburban skies, showing fairly easily with averted vision in my 70mm scope and with direct vision in my 178mm scope. In the smaller scope, it shows as a featureless 2′ blob, rather hard to pin down. In the larger scope, it is a vague circle about 3′ across with a brighter 1′ core.
M10 and M12
M10 and M12 are a pleasant change of pace from the faint, nearly featureless globular clusters described above. They are among the finest of all globular clusters — bright, bold, and easy to resolve. M10 and M12 are nearly twins, but M10 is slightly brighter, closer, and easier to resolve.
The clusters are separated by only 3 degrees, making it quite easy to star-hop from one to the other, but the pair is a bit of a nuisance to locate unless the sky is dark enough to see the mag 4.8 star 30 Ophiuchi, which lies just 1 degree E of M10.
The key to locating the general region of the sky is the wonderful star pair of Yed Prior and Yed Posterior, Delta and Epsilon Ophiuchi, respectively. They are not terribly bright, at mag 2.7 and 3.2, but their proximity makes them very striking; the only other place in the sky where two stars so bright appear so close together is in Orion’s belt. Both stars are also visibly reddish, especially Yed Prior.
Once the Yeds are located, Zeta Ophiuchi (the jumping- off star for M107) should be trivial. If you can also see Lambda Ophiuchi to the N, at mag 4.1, then you may be able to locate M10 by the fact that it forms a parallelogram with Lambda, Zeta, and Delta (Yed Prior). Alternatively, M12 is a moderate 6-degree hop from Lamda, or you can hop to either cluster from any of the other stars if Lambda is invisible or obscure.
Both clusters are fairly easy to see in my 7×35 binoculars under urban or suburban skies, but they are small enough so that it is possible to confuse them with stars.
Neither cluster shows much detail in my 70mm scope. Somewhat surprisingly, M12 is significantly less prominent than M10 in that scope both under urban and suburban skies, but neither cluster could possibly be called difficult to see. M10 appears more concentrated, roughly 4′ across with a 2′ core in the suburbs, and a fairly featureless 3.5′ circle in the city. M12 is a featureless faintish smudge in the 70mm scope from the city and a large, slightly dull blob in the suburbs.
Both clusters show far better in the 178mm scope, which is big enough to resolve quite a few stars in each cluster even in the city, and many more in the suburbs. Both clusters do best at high power — at least 120X — to bring out the individual stars. I can pin down 7 stars in M12 using averted vision in the city and 10 in the suburbs, with many more popping out intermittently. M10 is even better, showing 8 stars with averted vision in the city, and giving an overall impression of stardust in the background. I can make out perhaps two dozen stars in M10 in the suburbs, but some are hard to pin down.
M14, the easternmost globular cluster in Ophiuchus, is a let-down after M10 and M12, but it is not a bad cluster in its own right. It is, however, rather hard to locate within the huge and amorphous body of Ophiuchus.
Technically, M14 is smaller than M10 or M12, but it appears much bigger, because it lacks a concentrated core. The large apparent size and low surface brightness make M14 rather attractive under suburban skies or darker, but M14 suffers badly under heavy light pollution because of its low surface brightness.
The easiest way to find M14 is to start at Beta Ophiuchi, a reddish mag 2.7 star, but it is an arduous 8-degree starhop from there to M14. One could try aiming 2/5 of the way from Beta Oph to Xi Serpentis, but this is a bit of a long shot considering that M14 is not very prominent at low power. Also, Xi Ser may be hard to see given its southerly declination and its modest magnitude of 3.5.
Under urban skies, M14 is rather elusive both in my 70mm scope and in my 178mm scope, showing best with averted vision in both cases, although it is perceptible with direct vision in the larger scope. It appears quite similar in both scopes, as a large diffuse circle about 4′ to 5′ in diameter.
Under suburban skies, M14 is actually easier to see in my 7×35 binoculars than M10 or M12, because of its large size. It is fairly easy to see in my 70mm scope under suburban skies and quite prominent in my 178mm scope. In both cases, it is quite large and diffuse, about 5′ in diameter. The 178mm scope also shows a vague 2.5′ core. The appearance is not altered much by magnification, so M14 does best at medium power, especially in the larger scope, which allows one to see its setting in a rather attractive field of bright background stars.
M6 and M7
M6 and M7 are two of the brightest open clusters in the sky, a welcome change from the globular clusters of Ophiuchus. Both objects lie quite far south; in fact, M7 is the southernmost of all the Messier objects, and must have been a logistical challenge for Messier to observe from Paris, where it rises barely 6 degrees above the horizon.
Despite their southerly declination, both clusters are quite striking even for northern observers; in fact, Messier reported seeing both of them naked-eye, a feat that I can replicate easily at my latitude under dark skies. Unfortunately, light pollution makes a naked-eye sighting ulikely in urban or suburban skies at latitude 42N.
M6 and M7 lie a little under 4 degrees from each other, just off the end of Scorpius’ tail. The final three stars of the tail are quite bright, especially Lambda Scorpii, the last star, at mag 1.7. Unfortunately, at my latitude of 42N, it requires a low southern horizon to get a good view of the tail. M7 lies halfway from Kappa Scorpii, the next-to-last star in the tail, to mag 3.0 Gamma Sagittarii, and if that fails, there are ample stars in the area to support star-hopping. When in doubt, M6 is most easily located by star-hopping from M7.
M7 is the second brightest of all the Messier objects, behind M45, and also the second easiest cluster to resolve. M44 rivals it closely in both regards, but I find M7 tightr and more shapely than M44. Like M44 and M45, M7 has been known since Classical times, being recorded in Ptolemy’s Almagest. In fact, M7 is sometimes called Ptolemy’s Cluster.
M7 is immediately recognizable and faily well resolved with even the slightest optical aid, even under urban skies at my latitude of 42N. It shows about 10 stars using my 7×35 binoculars in the suburbs, and shows at least twice that number at 40X in my 70mm scope. Most of the stars are packed into a 30′ circle, but there are a few outliers in a much looser formation that are probably also part of the cluster. The 178mm scope is somewhat over-powered for M7, which is not especially rich in fainter stars, and which requires a fairly large field of view for proper appreciation.
M6 is fourth-brightest among the Messier clusters, but it is much fainter and smaller than M7. Morphologically, it is quite different from M7, with the stars more numerous and denser, but also much fainter on average. Under surburban skies, I can resolve three stars in my 7×35 binoculars, and the remainder form into an unmistakable haze. I have not positively identified M6 in my 7×35 binoculars under urban skies, where the haze is swallowed by the bright sky glow along the horizon and even the brightest stars are hard to resolve.
M6 appears well resolved in my 70mm scope at 60X both under urban and suburban skies, but a look through the 178mm scope reveals the fact that there are many stars too faint to see in the smaller scope. Regardless, M6 appears quite rich through both scopes. The brighter stars famously trace the outline of a butterfly, giving this its popular name of the Butterfly Cluster. The bright red variable star BM Scorpii lies at the tip of the E wing