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The M31 challenge: amateur telescopes operating at high powers can reveal stunning details new to backyard observers.

Autumn evenings bring into view the most sought-after galaxy in the heavens - M31, the Great Spiral in Andromeda. With an apparent magnitude of 3.6 and a distance of 2.9 million light-years, M31 is an easy naked-eye object, appearing as an elongated 4 [degrees] xl [degree] cloud about 7 1/2 [degrees] northwest of Beta ([Beta]) Andromedae. In photographs the galaxy is one of the most spectacularly detailed objects in the heavens and has long fascinated amateur astronomers with its mysterious beauty. But its proximity also makes M31 an ideal laboratory in which amateurs can study objects internal to another galaxy and structures that would otherwise go unnoticed in more distant systems.

Telescopically, 1' of M3 l's surface equals 845 light-years, a convenient scale for visual studies. Over the last few years I have used a 24-inch f/5 telescope under moderately dark skies (at an elevation of only 250 feet) to log, among other features, a total of 50 deep-sky objects within that galaxy. I've since compiled a list of 20 of the most visible stellar associations, open clusters, and globular clusters that should be within reach of amateur instruments. Observers using small telescopes can hunt down some of the brighter objects, while larger telescopes might reveal stunning and rarely seen details.

The key to observing many of these objects is the use of high power. I had made earlier visual studies of some of these features with relatively low powers, around 150x, but that was not adequate to show them as anything but stellarlike. But powers higher than 300x will show some of them as extended with other subtle details. Don't worry about image quality. Even if the stars appear bloated at high power, the stellar groupings in M31 will fuzz out and be noticeable as truly deep-sky objects.

OB Associations

The large, loose groupings of young, hot stars known as OB associations are found in the dust- and gas-rich regions of a galaxy's spiral arms. They are the largest stellar groupings in M31, and about 200 have been cataloged. There are no associations closer than 15[minutes] to the galaxy's center. And beyond 75[minutes] the density falls rapidly, with no association located more than about 130[minutes] from the nucleus.

Observations. NGC 206 or A78. Nestled between two dust lanes in the galaxy's southwest sector is the largest and most conspicuous OB association in M31. NGC 206 is a giant grouping that measures 2[minutes]x4[minutes]. Distinct and easily seen, the cluster is elongated north-south and separated into two groupings. The northern lobe is brighter and shines with a nebulous glow. The southern lobe is larger and a bit fainter, and a star lies between the two clusterings. Through large apertures, faint stars pop in and out of view.

A54 is a very nice stellar association involved with a cloud of glowing hydrogen known as an H II region. At 277x, it looks like a large, faintly glowing rectangle of light (elongated east-west) with a noticeable star in the center of the haze.

A61 is another large H II region/stellar association, lying north of M31's nucleus. At 277x, the object appears as a very dim, uniform glow.

A67. A small, ill-defined patch, A67 looks like a faint galaxy about 1.5[minutes] in diameter. Fairly well pronounced at 435x, it appears to have a slight brightening to the middle with a faint star just to the north. I can make out the entire stellar association, not just the core.

Globular Clusters

Globular clusters are the easiest stellartype objects to observe within the Andromeda Galaxy. With a visual-magnitude range of 13.7 to 18.1 and an apparent size of 4[inches] to 10[inches], some are accessible with small to moderate-size telescopes - if one knows where to look. In 1985 a total of 509 Andromeda objects were listed as globulars, but many of these have been reclassified as stellar objects, open clusters, or distant galaxies. M31 has about 300 true globulars compared to approximately 150 known in our galaxy.

Observations. G76/V12 (magnitude 14.3). Located 11[minutes] to the southwest of NGC 206, this globular is bright and easily seen at all powers. The globular lies between a star of similar magnitude 30[seconds] to the southeast and a brighter star 1[minutes] to the northwest. At 333x, it looks like a "soft" star, but at 624x the object is spectacular, showing a fuzzy halo with a brighter center.

G78/V42 (magnitude 14.3) is one of the easiest globulars to observe and is a remarkable object at high power, appearing fuzzy and extended with a slightly brighter middle. G78 looks more diffuse than the pair of stars that precede it to the west.

G213/V64 (magnitude 14.7). Southeast of M31's nuclear region is another bright and easy target. At low powers G213 shines as a soft, fuzzy star, while 624x shows it as an extended object with a very diffuse halo and a bright core.

G233/V73 (magnitude 15.4) is a fairly easily seen globular at all powers. At 428x, G233 has a faint stellar core surrounded by a diffuse halo.

G244/V116 (magnitude 15.4). The globular forms the base of a T asterism with three stars to its northwest. At 428x it is easily seen as a diffuse extended object with a very slight central brightening.

G251 (magnitude 17.7). Despite its published magnitude, this object is bright and readily seen. At 428x it appears stellar and is the brightest in a chain of three objects oriented southwest-northeast. At 624x it appears slightly softer than a nearby star 1.3' to the southwest.

G257/V106 (magnitude 15. I) is obvious at 624x as an extended, diffuse spot with a brighter middle and surrounding halo.

G272/V101(magnitude 14.7). Located 1.5[minutes] to the southeast of a prominent star, G272 is a tenuous glow at moderate magnifications, while 624x shows it as a faint, diffuse patch with a brighter center.

G280/V282 (magnitude 14.2) is among the brightest globulars in the galaxy and can be seen at almost any power, displaying a bright center and a faint surrounding halo. A star of similar magnitude trails 1[seconds] to the east-southeast.

Open Clusters

M31's open star clusters are very difficult to observe. A total of 403 have been listed with measured diameters ranging from 1.8[seconds] to 22[seconds]. The open clusters of M31 are young objects similar to the Milky Way's Pleiades in Taurus and Double Cluster in Perseus. The youngest clusters - only one million years old - are more plentiful in the inner areas, while older clusters around 100 million years of age are farther out and somewhat fainter in luminosity. The M31 open clusters are highly clumped in their distribution and generally show no concentration toward the center. They are limited exclusively to the disk region and are good tracers of the spiral-arm pattern.

Observations. C179. Located west of M32, C179 is also an H II region and appears as a dimly glowing patch slightly elongated north-south; the southern part is slightly brighter. The halo fans out to the north, making it look like a very small, low-surface-brightness galaxy. It responded slightly to a narrowband filter, proving the existence of some emission nebulosity.

C202 and C203. Both are easily seen and are among the brightest clusters in the galaxy. Located to the northwest of M31's companion galaxy, M32, the two clusters are oriented north-south within a common halo. The northern member, C203, is the brightest and definitely extended at 339x. C202 appears as a stellar object 30[seconds] to the south. This pair may be similar to the Double Cluster.

C263 is within the OB association A62. At 425x the cluster appears as a faint double star (separated by 20[seconds]) inside a faintly glowing halo. A prominent asterism of four stars points directly to the cluster.

C275. At 277x this cluster and H II region is rather easily seen as a dim glow, slightly elongated east-west. To its west is a pair of stars oriented north-south but not involved with the cluster.

C306. At 277x this cluster looks almost stellar. Three stars arranged in a crescent shape lie to its west.

C311, C312, and C313. The group is easily seen at 277x as a dim and diffuse object elongated northeast-southwest. There is a hint of very faint stars popping in and out of view. I could not resolve the group into three separate objects.

C410 is a star cluster/H II region fairly easily seen at 277x as a small, slightly elongated object with a north-south orientation and a brighter central region. Very faint stars seem to pop in and out of view. This cluster is a part of the larger OB stellar association A33. C410 does not respond to a nebula filter.

A Good Club Project

Many astronomy clubs today have access to large-aperture telescopes. Searching for the objects listed here could be a good club project, especially at star parties. The objects I selected for this article are within the grasp of determined amateurs using a wide variety of apertures. Don't let the magnitudes of some of the objects dissuade you from looking for them. Amateurs using telescopes as small as 4 inches in diameter have seen objects listed as 14th magnitude. Also, I've found that catalog magnitudes do not always match visual impressions, which should spur you to attempt some of the fainter objects. I'd be interested in hearing about any successes. You can send the reports to me at the address below or e-mail me at larrymit@netcom.com.

In 1994 LARRY MITCHELL visually discovered Supernova 1994S. His address is 5923 Black Maple Lane, Houston, TX 77088.
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Author:Mitchell, Larry
Publication:Sky & Telescope
Date:Nov 1, 1997
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