Messier Guide: Early Autumn

In my part of the world (the U.S. Northeast), and in many other places, early autumn is the most pleasant time of year for astronomy. The nights are reasonably long, the biting insects begin to die off, the temperature is delightful, and the probability of clear skies is at its highest. Unfortunately, early autumn is also the season when the fewest Messier objects transit the meridian in the evening sky. There are a number of first-rate deep-sky objects between RA 21:00 and RA 0:00, but for whatever reason, Messier failed to catalog most of them.

Because late summer is so rich in Messier objects and early autumn so poor, I have included M29, M72, and M73 in this section although they do not quite qualify according to RA. I have also included M103 because it forms a natural pair with M52, the other Messier object in Cassiopeia. Here, then, are the objects described in this section:

Obj S178 S70 U178 U70 Type Con RA Dec Mag PBrt SBrt Size
M29 2B 2B 2B 3C OCL Cyg 20:23.9 +38:32 7.0 —- 19.9 7.0
M72 3C 4C 3C 4C GCL Aqr 20:53.5 -12:32 9.3 19.3 21.8 5.9
M73 3C 4C 4C OTH Aqr 20:59.1 -12:38 9.6 —- 18.2 1.0
M15 2A 2B 2A 2B GCL Peg 21:30.0 +12:10 6.2 16.3 20.2 12
M39 2B 2B 2B 2B OCL Cyg 21:32.2 +48:26 4.6 —- 20.8 32
M2 2B 2B 2B 3C GCL Aqr 21:33.5 -00:49 6.5 16.6 20.7 13
M30 2B 2C 2B 2C GCL Cap 21:40.4 -23:11 7.2 17.5 21.0 11
M52 1A 2B 2B 3C OCL Cas 23:24.2 +61:35 6.9 —- 21.1 13
M103 2B 3C 3B 3C OCL Cas 01:33.2 +60:42 7.4 —- 19.9 6.0

For a key to this table, see
Key to the Tables.

If you exhaust the objects in this section, feel free to jump to the next; all of the Messier objects between RA 0:00 and RA 3:00 except for M77 are well placed for nothern observers through the autumn. In particular, M31, the great Andromeda Galaxy, twin to our own Milky Way, is the premier astronomical target throughout autumn and well into winter. Thanks to the increasingly early sunsets, it is also possible to observe many of the late-summer Messier objects early well into October.

M72 and M73

M72 and M73 are two of the peskier objects in the Messier catalog, especially for urban observers. M72 is the faintest of the Messier globular clusters both in total brightness and in surface brightness. M73 is almost certainly just an asterism, a chance alignment of stars, and not a deep-sky object at all.

M72 and M73 lie in the far southeast corner of Aquarius, just over the border from Capricornus. They are just 1.5 degrees from each other, visible simultaneously in a wide-angle field, but you may have trouble recognizing them at such low power under heavy light pollution.

It’s easy to find the right general section of the sky starting at Alpha and Beta Capricorni, the brightest stars for a long way around, and a very striking pair. From there, it may (or may not) be possible to locate Epsilon Aquarii (mag 3.8) and Theta Capricorni (mag 4.1). If you can see both stars naked-eye, you may well be able to locate M72 and M73 2/5 of the way from Epsilon Aqr to Theta Cap. If not, you must starhop from Epsilon if you can see it, or from Aplha Cap if you cannot. A line from nearby Xi Cap through Alpha conveniently points almost directly at M72 and M73.

M72 shows as a faint, subtle smudge about 2′ across. I see it as slightly elongated from ENE to WSW, a rather exotic feature for a globular cluster, and not evident in the photographs. It ranges from difficult with averted vision using my 70mm scope under urban skies to fairly easy with direct vision using my 178mm scope under suburban skies. In the latter situation, I can almost see or imagine a vague circular 4′ halo around the brighter central section. M72 shows best at fairly high power, around 60X in my 70mm scope and 120X in my 178mm scope.

M73 consists of four stars in a tight Y pattern about 1 minute across. The brightest star is mag 10.4 and the faintest mag 11.9. In my 70mm scope, it is somewhat challenging even under the best skies, and quite invisible under urban skies, where only one of the stars is above the limit of visibility, and that not by much. Viewing M73 through the 70mm scope at 60X under suburban skies, it is apparent why Messier catalogued this as a nebulous object. The four stars merge together into a vaguely nebulous mass, out of which one or two stars poke intermittently.

Viewing M73 through my 178mm scope at 120X, the illusion of nebulosity disappears, but it is still rather hard to resolve the two faintest stars, the upright of the Y, which also happen to be the closest stars to each other. Many people scorn M73, considering it to be the most inconsequential of all the Messier objects, but I find the Y pattern to be rather attractive.

There is an ongoing controversy about whether M73 is a true, albeit very poor star cluster, or an asterism — an accidental alignment of stars. Recent observations strongly suggest that the stars are at significantly different distances, so that M73 must be an asterism rather than a cluster. Statistically, there should be about one group of four stars in the whole sky this bright and also this close together, and apparently, there is indeed just one such group, namely M73.

It should be mentioned that the Saturn Nebula, NGC 7009, lies just 2 degrees NE of M73. This is one of the brightest planetary nebulae in the sky, a much finer deep-sky object than either M73 or M72, and also one that shows particularly well under bright skies. Even the smallest aperture is sufficient to show this object, but medium to high power are required to show its bright blue-green disk clearly. That, no doubt, is why Messier failed to notice it. Very high power, good seeing, and a fairly large scope are required to see the extraordinary faint lines protruding on either side of the disk which give this nebula its name.


M30 is fairly bright and has fairly high surface brightness, making it relatively easy to see from northerly cities despite its southerly declination.

M30 lies in the SE corner of Capricornus, 3.3 degrees ESE of the mag 3.8 star Zeta Capricorni. Zeta is obvious in the suburbs, but quite difficult to see from the city at my latitude of 42N due to its low maximum altitude. If Zeta is invisible, you must star-hop from Delta Capricorni 7 degrees N, the brightest star in the constellation at mag 2.8. M30 can also be located at the right angle of a triangle with Delta and Zeta as the corners.

M30 is faintly visible from the suburbs in my 7×35 binoculars. In my 70mm scope, it is moderately obvious from the city and very obvious from the suburbs at all powers, showing as a brightish disk 2′ – 3′ across concentrated gradually to a very bright, small center. The view in my 178mm scope is much the same, but brighter, and there is a hint of graininess at 120X, especially in the suburbs.

M29 and M39

The constellation of Cygnus contains many fascinating clusters and nebulae, but for whatever reason, Messier cataloged only two of them: the open clusters M29 and M39.

Cygnus is one of the most magnificent of constellations to the naked eye, especially under a dark sky, where one of the brightest sections of the Milky Way can be seen to run neatly down the long axis of the constellation. The visibility of this part of the Milky Way when near the zenith can be taken as an indicator of fair-to-good suburban skies.

The five brightest stars of Cygnus outline the Northern Cross. All of them should be readily visible even in the worst skies, and the two brightest are among the brightest stars in the sky: Deneb (Alpha Cygni, mag 1.2) at the NE end of the long arm, and Sadr (Gamma Cygni, mag 2.2) at the center of the cross. The faintest of the five is Albireo, Beta Cygni, at the SW end of the long arm, but this is also a star well worth knowing, as it is often considered to be the most beautiful double star in the sky, resolvable with difficulty in hand-held binoculars and with ease in any telescope.

M29 lies 2 degrees due S of Sadr (Gamma Cyg), an easy starhop and also quite easy to find by point-and-shoot. Paradoxically, I find that despite its small size, it can be hard to recognize at high power, but shows up well at low power. In my 7×35 binoculars under suburban skies, it stands out immediately as a tiny, unresolved bright patch. M29 is a moderately poor but also fairly tight group, containing half a dozen mag 9 stars and one or two dozen fainter stars in a 6′ – 7′ circle. As M29 lies in a fairly rich section of the sky, only the brightest stars really stand out from the background, and in my 70mm scope, only the brightest stars are visible at all.

M39 is more or less opposite from M29 in every way. It lies quite far from the main body of Cygnus, closer to the main body of Cepheus than to the Northern Cross. It is very large, very bright, very coarse, and all of its stars are nearly equal in brightness.

Under dark skies and good suburban skies, M39 is faintly visible to the naked eye, but it is rather hard to locate under bright skies, as the nearest reasonably bright star is Deneb, over 9 degrees distant. One possible strategy is to locate it first using binoculars, where it stands out well, and should be at least partially resolved into its individual stars. Under good urban skies or better, M39 can be located with respect to Rho Cygni (mag 4.0) 2.9 degrees to the S and Pi2 Cygni (mag 4.2) 2.6 degrees to the ENE. Failing that, you are stuck with the long starhop from Deneb.

Most of the significant stars of M39 are mag 7 through mag 9, and so readily visible even in my 70mm scope under urban skies. In general, I find that this cluster shows best at the lowest possible magnification, although it contains a number of star pairs, or wide doubles, which may profit from slightly higher powers. The cluster is half a degree across, and I find that it is framed poorly at any power much above 40X.

M2 and M15

M2 and M15 are a fine pair of globular clusters. M15 lies about 12 degrees due N of M2, not really all that close, but these clusters tend to be closely associated because they are very similar in size and brightness, and because there are no other bright deep-sky objects anywhere nearby. To my taste, M15 is the more attractive of the two clusters, slightly brighter, more concentrated, and definitely easier to resolve into individual stars.

Both clusters show easily in 7×35 binoculars or a decent finderscope, and grab the eye as soon as they enter the field of a telescope’s eyepiece, and both form easily recognizable patterns with bright stars, so it should be possible to locate them with point-and-shoot or point-and-sweep methods.

I have two standard ways to find M2. One is to note that it lies at the right angle of a triangle with Alpha and Beta Aquarii as the other corners, and the sides runnning almost due E-W and N-S. The other is to use the prominent W-shaped asterism composed of Eta, Zeta, Gamma, and Alpha Aquarii. A line from Eta through Alpha, extended an equal distance to the W, lands nearly on top of M2.

M15 is even easier to locate, off the SW crook of the stick figure of Pegasus formed by Theta and Epsilon Pegasi. A line from Theta through Epsilon, extended 50% farther, lands almost on top of M15.

In my 70mm scope, M2 shows as a 3′ halo around a very bright 1.5′ core, both in the city and in the suburbs. The view is similar in my 178mm scope, but it is also possible to see that the core grows gradually brighter towards an extremely bright, small center. The halo is grainy, but not really resolved except for a few stars that pop out intermittently, especially in the suburbs. The area immediately around M2 seems to be over-rich in faint stars, which may or may not be associated with the cluster.

M15 appears quite similar to M2 in both scopes, but the core is somewhat smaller and more concentrated, and the halo extends somewhat farther, roughly to 5′ using my 178mm scope in the suburbs. The core and halo both seem to be subtly truncated on the E, as if obscured by a cloud of dust. In the 178mm scope, M15 resolves distinctly better than M2, the halo being studded with stars when scanned with averted vision in the suburbs.

M52 and M103

M52 and M103, the two Messier objects in Cassiopeia, are wildly different from each other. M52 is one of the finest open clusters in the sky: big, bright, and rich. M103 is one of the smallest and faintest clusters in the Messier list. What is particularly puzzling about this is that Cassiopeia is crammed with magnificent open clusters; in fact, there are half a dozen clusters within two degrees of M103, of which at least one (NGC 663) is far more prominent and attractive than M103, especially in small instruments and under poor skies.

Lying as they do within 30 degrees of the celestial north pole, M52 and M103 can be viewed at any time of year from my latitude of 42N, and are reasonably well placed any time from mid summer through late winter. They are also quite easy to locate, as Cassiopeia is one of the most compact and recognizable of all the constellations. Because Cassiopeia is so nearly symmetric, I sometimes have trouble remembering which end is which. Remember that the W side is the one containing the star Eta, which at mag 3.5 is nearly as bright as the five stars that outline the constellation’s basic shape. The E side is more extended, and ends with the faintest of the five basic stars (Epsilon, mag 3.3). By the way, Eta is a lovely double star, splitting easily at 50X.

Under decent suburban skies or better, M52 is easy to find by extending a line from Alpha Cassiopeiae through Beta Cassiopeiae and continuing in the same direction for the same distance, and then 20% farther. This is a rather long stretch, but it should work because under good skies, M52 is immediately obvious even in very small apertures and at very low powers as a small but well-defined cloud of light. Under very bright skies, the cloud of light barely stands out from the background, making it quite easy to sweep right over M52 without noticing it. In that situation, I find it advisable to starhop from Beta Cas.

In my 70mm scope at low power, M52 appears as a cloud of light some 8′ in diameter, with a brightish (mag 8.3) star near the edge. This star, by the way, is an impostor, not a true cluster member, but it dominates the visual impression of the cluster. At 16X, the cloud is distinctly grainy under suburban skies, but not really resolved. At 60X, I can make out about 6 stars with averted vision under urban skies, and somewhat more under suburban skies.

In my 178mm scope, M52 is fairly well resolved even at low powers and even under urban skies. At 120X, it shows two dozen stars under urban skies, and three dozen under suburban skies, with a sense of many more near the edge of visibility. The overall impression is of a very rich and dense cluster, not unlike M37 or M11.

M103 lies just S of the line from Delta Cassiopeiae to Epsilon Cassiopeiae, just one degree from Delta. It can be hard to locate without a detailed chart, because it is a rather inobtrusive cluster lying in a very rich star field, with other equally prominent clusters nearby.

In very small instruments, I find that M103 stands out best at very low power, where the cluster’s stars merge into a tiny cloud of light about 5′ across. In fact, in some ways I find M103 more prominent in my 7×35 binoculars than in my 70mm scope, where the cluster is partially resolved even at the lowest possible power.

The view in my 70mm scope at 60X is dominated by the four brightest stars, ranging from mag 7.3 to mag 9.1, which roughly outline the shape of the Greek letter Gamma. A few more stars are visible using averted vision at 60X, especially under darker skies, but the overall impression is of a very sparse cluster, perhaps just a chance alignment of stars.

In my 178mm scope at 120X, at least a dozen additional stars are immediately obvious along the outline created by the four bright stars, even under urban skies. That is enough stars in a small enough area of the sky to make it immediately obvious that M103 is a true cluster. Occasionally, the mag 10.7 star at the SE edge of the cluster catches my eye, forming a five-star dipper together with the four bright stars already mentioned. The star at the center is distinctly reddish, and the northmost of the bright stars is the double star Struve 131, but the secondary is far enough from the primary, and faint enough, that it may just look like another cluster member.

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