From 1946 to 1994, Sky & Telescope magazine featured a column called Deep-Sky Wonders, written by amateur astronomer Walter Scott Houston. In it he explored the dim denizens of the deep sky asterisms, double and variable stars, open and globular clusters, nebulae, and galaxies.
Deep-Sky Wonders, a month-by-month selection of those columns, was published in 1999 and is available from Sky Publishing. The following seasonal guide to a variety of open clusters consists of excerpts from the book.
The Jewel of Autumn
The Double Cluster, NGC 869 and 884, also known as h and Chi (χ) Persei, is a jewel. To the naked eye it shines with a steady glow, while telescopically its majesty is so compelling that it causes many observers to neglect fainter offerings in the area. These rich conglomerations of stars were known to Hipparchus in the second century B.C. But we can wonder how many Mongol shepherd youths before him knew about these fuzzy twin lights in the autumn and winter sky.
In the telescope this tremendous blaze of scintillating suns makes a commanding entrance into the eyepiece field. One can look for a long time at the many doubles, the colors, the winding patterns, as the dense cores of the cluster thin out slowly to merge finally in the star-rich background of the galaxy itself. Gazing at these clusters produces a succession of feelings too subtle and too complex to be captured by words along. Each of these two open clusters would stand well on its own, but they are even more spectacular because, less than a degree apart, they're visible in the same low-power field.
M34 in Perseus is an open cluster that receives relatively little amateur attention because it's overshadowed by the more spectacular Double Cluster. While not as rich, M34 is one of the finest sights in wide-field telescopes that can be found. It lies about half way between the stars Gamma (γ) Andromedae and Algol, with a total brightness equal to that of a 5.2-magnitude star.
Thomas W. Webb justly called attention to M34 as a grand, low-power, rich-field object. I feel that 15 x 65 binoculars give the best impression; the low power allows plenty of dark sky surrounding the cluster to enhance contrast. The sky background is sprinkled with faint stars, so it may be easier to decide where the cluster edge lies by using a small rather than a large telescope. More magnification merely spreads out the few bright stars that the binoculars show perfectly well.
Many observers see its stars arranged into distinct curved lanes that diverge from the cluster's center. I see three noteworthy curved rays of stars running out from the center which are very evident in my 4-inch Clark refractor at 40x; indeed, they even show in binoculars. Many of the stars also form pairs.
More Autumn Clusters
Just southwest of Beta (β) Cassiopeiae are two 5th magnitude stars, Sigma (σ) and Rho (ρ) Cassiopeiae, which are separated by a little less than 2°. Halfway between them is NGC 7789, an open cluster with about the same apparent diameter as that of the Moon.
NGC 7789 is one of those rare objects that is impressive in any size instrument. With a 4-inch rich-field telescope the cluster appears as a soft glow nearly ½° across and speckled with tiny, often elusive, individual stars. The 12-inch f/17 Porter turret telescope at Stellafane picks up more than 100 stars. Through a 16-inch aperture the view is spectacular, and the whole field is scattered with diamond dust. And a 22-inch Dobsonian reflector in the clear skies of California gave a most impressive view with countless sparkling points filling an entire 60x field.
Cassiopeia also contains the bright cluster M52, which lies between Cassiopeia and Cepheus on the edge of the dark lane in the Milky Way that divides these two constellations. M52 has a total magnitude of 6.9 and is visible in any finder. Recent studies indicate that this object is one of the richer and more compressed open clusters known. It's also relatively young, being some 20 million years old and comparable in age to the Pleiades.
Unlike some open clusters, M52 shows increasing richness with larger-aperture telescopes. Not large as galactic clusters go, it's 13' in diameter, but packed into that area are more than 150 stars of 11th magnitude and fainter. If it were not for its irregular outlines, M52 might well be mistaken for a globular cluster.
Marvels in Taurus
The constellation Taurus is well placed in the evening sky. Situated along the western edge of the Milky Way, Taurus might be expected to contain swarms of open clusters that pepper the constellations of Cassiopeia, Perseus, and Auriga. But this is not the case. Yet Taurus is still an observer's paradise; objects within its borders include the magnificent naked-eye Pleiades and Hyades star clusters.
The Hyades star cluster is rather close to us, and its stars appear spread over quite a large area. Unlike the Pleiades, no nebulosity is associated with the group. In fact, whenever I think I've sighted the wispy glow between the Pleiades, I quickly turn to the Hyades to check for a glow there, too. If I see any, then I know to blame a slight dewing of the optics, even if their surfaces look clear.
Winter brings many cold but often clear nights. On such evenings, when the stars sparkle like diamonds, nothing is more spectacular than M45, the Pleiades. It's delightful in any instrument, from the naked eye to the largest amateur telescope, although I find large binoculars give the most impressive view. Almost every culture, past and present, mentions in its folklore the dazzling stars of this nearby cluster.
Have you ever tried to count the Pleiades with the naked eye? Do not consult a chart while you are trying to count them. Instead, make a careful drawing of what you see and compare it with a chart later. Depending on light pollution and sky conditions, most persons can see between four and six naked-eye Pleiads. Traditionally, the average eye can see six stars here, the exceptional eye seven. Ten bear names or Flamsteed numbers. Perhaps because it has been repeated so many times that the number of naked-eye Pleiads is six or seven, too many observers quit counting before really reaching their limit. Many observers can reach magnitude 7.5 with the naked eye. But in the sooty skies of our populated areas, it's now not uncommon that no stars can be distinguished; the eye sees just a shimmering patch. So the number of Pleiades stars visible is really an index of the transparency of the atmosphere, and the cluster does not make a valid eyesight test. If you look at this cluster only infrequently, one glance will not tell you much about the sky conditions. But with practice you'll know by the cluster's appearance whether the night is a particularly good one.
Three Clusters in Auriga
Galactic clusters are legion in the winter Milky Way, and overhead Capella shepherds a profusion of them in Auriga. The prettiest of these is M37, dimly visible to the naked eye on extremely clear nights, a little southeast of the midpoint of a line from Beta (β) Tauri to Theta (θ) Aurigae. In binoculars or small finders, M37 appears as a milky patch, while a 3-inch shows a fine swarm of stars. In his 5.9-inch refractor, W. H. Smyth saw "the whole field being strewed as it were with sparkling gold-dust." M37 is also the most striking of the three Messier clusters in the constellation, the brightest (visual magnitude 6.2), and has the most stars 150, according to the Atlas Catalogue. The cluster is rather open, that is, the stars are not impressively concentrated toward the center. Whether your telescope is a small instrument or a large reflector, M37 is a treat. The view is further enhanced by the background of the Milky Way.
Nearby is M36, a rich cluster of fainter stars, somewhat smaller than M37, but also impressive. The cluster measures 12' in diameter and contains 60 stars. It's sparse compared to M37, and visually the stars tend to form a blunt cross.
Moving "down" the Milky Way we run into such variegated star fields and clusters that it's almost impossible to know where to halt, but this might very well be at M38. Although this cluster is well within the star-strewn Milky Way, it's usually visible to the naked eye without much effort. Evenly compressed into a glowing ball two-thirds the diameter of the full Moon are over 100 softly blazing stars. M38 is magnificent in any sized instrument. Photographs usually show a departure from circularity, a feature quite evident to visual observers. (For more on open clusters visible during the winter, see "Winter Clusters Galore.")
M35 My Favorite Cluster
M35 is my personal favorite open cluster. Located about 2½° northwest of Eta (&eta) Geminorum, it is an impressive frame of bright stars with a softly flaming background of fainter ones, seemingly containing hundreds of members. Charles Messier, however, was not the first to call attention to M35. That honor appears to go to the Swiss astronomer Phillipe de Cheséaux, who in 1746 called it "a star cluster above the northern feet of Gemini."
Amateurs often test themselves by trying to view the cluster with the naked eye. This puts it in the same class as the galaxy M33 and the zodiacal light. Brian Skiff in Arizona concluded from a study of 434 stars in M35 that its total magnitude was 5.1, but the bright background glow of the Milky Way may be a handicap. Nevertheless, one clear morning in September 1984, Sky & Telescope's Dennis di Cicco chanced upon the cluster with his naked eye.
To me, M35 seems most lovely in a 6-inch at 40x though I must admit that, through a 36-inch telescope and a wide-field eyepiece, this blaze of interwoven stars is an awe-inspiring sight. With a home-made wide-field eyepiece on my 10-inch reflector I could get all of M35 into a single field. The view was too beautiful to describe with mere words. Bright stars were scattered with cosmic recklessness across the field, and it was difficult to establish where the cluster's edges dissolved into the stellar background.
The Rosette Nebula in Monoceros (see "Observing Nebulae Season by Season") is an object with far better name recollection among today's amateurs than NGC 2244, the open cluster it surrounds, even though the latter was the only object mentioned in observing guides until recently. NGC 2244 was itself once a test for the naked eye. The brightest star here is 6th-magnitude 12 Monocerotis, but it's quite likely a foreground star and not an actual member of the cluster. Sky Catalogue 2000.0 lists NGC 2244 as having about a hundred stars and a total brightness equal to that of a 4.8-magnitude star. With an estimated age of 3 million years, the cluster is very young; in fact, the Rosette Nebula is the cloud of gas and dust that gave the cluster its birth.
Two Pretty Spring Clusters
There are quite a few deep-sky objects near the limit of normal naked-eye vision that serve as guides to atmospheric conditions overhead. One of the oldest and best known involves the Beehive, M44, in Cancer. With some experience it’s possible to estimate the limiting magnitude of a telescope from a naked-eye view of M44. As early as several centuries BC, the invisibility of M44 was considered an omen of coming rain. Today amateurs can use the same observation to judge the night sky’s transparency.
Its true nature was first discerned by Galileo, who described it in his 1610 astronomy pamphlet Sidereus Nuncius as “the nebulae called Praesepe, which is not only star only, but a mass of more than 40 small ones.” Indeed, only the slightest optical aid is needed to resolve the cluster. In low-power fields, finders, and binoculars, M44 is a brilliant show object. It has no sharp boundary. No one can say for sure where the cluster’s faint glow merges into the placid sky background. And the center is hardly brighter than the edge. The cluster appears as a ghostly sheen on cobwebs at least a degree in diameter, sometimes maybe two. Through a large telescope the view is not particularly impressive, but the cluster is an exciting object for binoculars and rich-field telescopes.
The open star cluster M48 (in Hydra) was long believed to be a "missing" object until Harvard astronomer Owen Gingerich linked it with NGC 2548, which Caroline Herschel discovered in 1783. If Gingerich is correct, the original published position for M48 was about 5° in error. Seemingly, Messier made a mistake of 5° in declination, but his right ascension is correct. But the identification seems pretty certain since there is no other nearby candidate matching Messier's visual description of M48.
This sparse sprinkling of stars has roughly the angular size of the Moon. Because it's a very loose group, it's best viewed with low magnification or the finder. W. H. Smyth with his 6-inch refractor saw this cluster as "a splendid group, in a rich splashy region of stragglers, which fills the field of view, and has several small pairs, chiefly of the 9th magnitude." In my 5-inch binoculars, its shape appears distinctly triangular. This cluster contains about 60 members brighter than 13th magnitude. The total magnitude is about 5.8, and the English author and observer Kenneth Glyn Jones notes that many people can see the cluster's glow with the naked eye. Being so bright and large, it would seem that M48 would be easy to find. Experience suggests otherwise. The cluster is sparse and the background rich. I've never been convinced that it's visible to the naked eye, but it does show nicely in small telescopes.
There is never a shortage of deep-sky objects. Whatever the season, the sky holds more than enough of these delights to keep you busy all night, every night if you take the time to search them out with good charts and referemce books. In the summer evening sky, almost every direction we look offers something interesting.
The area of Ophiuchus and Serpens is a turmoil of stars mixed with clouds of bright and dark nebulosity. Surprises in this part of the sky are common. One example is the open cluster IC 4756 in the eastern half of Serpens (Cauda). It's one of the largest such objects in the heavens, appearing more than 1° across and just a little smaller than the Beehive cluster, M44, in Cancer. Some 80 stars between 7th and 12th magnitude are evenly scattered across IC 4756's diameter. It appears as a patch of the Milky Way to the unaided eye. Binoculars or a finder will easily reveal its individual stars.
The most northern galactic cluster in the sky, NGC 188, is also one of the oldest. It's located just 4° south of Polaris. NGC 188 is 15' across, so use low power. It contains 150 stars, most of which are fainter than 13th magnitude. On fine nights I see it as a ghostly glow in the 4-inch Clark refractor.
Inside the Cepheus pentagon is a more challenging object, NGC 7142. John Herschel described it accurately as "a large, rich, loose cluster of stars of magnitude 10 or 11." To me it seemed an evenly spread layer of small stars. NGC 7142 doesn't show in a 2-inch finder, and I generally search with the main telescope after plotting the cluster on a detailed chart. High magnification helps after this cluster has been found.
Open cluster NGC 7510 in Cepheus is about 9th magnitude and 3' across. Of it, a Canadian amateur Dunstan Pasterfield writes, "Very easy to find, an attractive object framed by surrounding stars. It has an unusual shape, like a very thin arrowhead that is slightly bent at the tip. About 710 stars delightful thing."
Finally, what open cluster is visible to the naked eye, has no Messier or NGC number, and was called "gossamers spangled with dewdrops" by the 19th-century astronomy popularizer Garrett P. Serviss? Almost every skygazer has seen this group at one time or another, and its official designations are Melotte 111 and Collinder 256. The answer will surprise many, for the cluster is the shimmering haze of 5th- and 6th-magnitude stars we call Coma Bernices. It's a real cluster and not just a change alignment of stars. There are about 80 members scattered across 5° of sky. Coma Bernices is only 260 light-years away and is one of the nearest open clusters. Therefore its stars appear well separated. If they are a bit too faint for your naked eye, a simple 2x or 3x opera glass gives a wonderful view.