Exploring messier 31: backyard scopes can reveal dozens of star clusters in the Andromeda Galaxy.
At magnitude 13.7, G1 is by far the brightest globular cluster in M31--and indeed, it's the most luminous globular in the entire Local Group. Like the Milky Way globular Omega Centauri (and others), G1's stars vary considerably in age, so many astronomers suspect that it's actually the core of a captured dwarf galaxy.
Use the star chart and image at right to star-hop to G1, which lies well outside M31's visible disk. The giant globular appears obviously non-stellar through my 8-inch Newtonian at 61x, and at 348x I can occasionally glimpse the two foreground stars on G1's western edge. Mackie's 12.5-inch at 264x shows a "condensed core with soft halo," and the halo looks as large as Saturn's disk in my 16-inch at 261x. While you're in the area, seek out G1's junior partner G2, which is 2 magnitudes fainter.
Now let's search M31's visible disk, starting near its southwestern end with the huge stellar association NGC 206, which is visible in 4-inch refractors and prominent through an 8-inch scope under dark skies.
With a 10-inch telescope, 14.2-magnitude G76 is the easiest globular to find because it lies within a distinctive Cassiopeia-like asterism near NGC 206. Observers are always delighted when I show them G76 in their own telescopes. I once saw G76 with my 8-inch at 348x on a night with excellent transparency and very good seeing, but that was about my 20th try with that aperture! G76 is only 3.6" in diameter, but that's larger than most. Using my 16-inch on a superb night I wrote that "G76 has a huge halo at 522x with a stellar core." I wrote this after a night of viewing many fainter globulars, most of them mere sparks, so "huge" is a relative term.
With my 16-inch at 366x, the open cluster C107 appears stellar with direct vision, but it swells into a tiny fuzzball with averted vision. From there, let's star-hop northeastward along the M32 side of the Andromeda Galaxy. The open cluster and H II region C179 is a small smudge at 229x; two faint and starlike globular clusters, G96 and G87, are neighbors.
Open clusters C202 and C203 lie only 16" apart. The northern one, C203, is bright and stellar, whereas C202 is fainter and quite extended. Adjacent G156 can be seen despite its proximity to a 9.3-magnitude star. At magnitude 15.6, this was the faintest globular that Mackie found with his 12.5-inch. At 316x he logged it as an "averted vision fleck of light."
Skipping 35' northeast, I reach my favorite high-power field in the galaxy. It features the large H II region and open cluster C410 bracketed by two of the easiest globular clusters: G272 and G280. In my 16-inch at 366x, oval C410 is elongated 2:1 and exhibits a slight central brightening; a glimmer of a star appears repeatedly on its eastern side. G272 is magnitude 14.7 and 3.4" in diameter. I called it the Blinking Globular--I can only see the stellar core with direct vision, and I can only see the halo with averted vision. G280 is magnitude 14.2, but is only 2.7" in size. Its stellar core is brighter than G272's. Only on the steadiest nights did G280's tiny halo make it seem larger than the similar-magnitude star beside it.
At 4.9" in diameter, G279 is second to only G1 in size, but it's only magnitude 15.4. This is rather faint to be teasing out detail with my 16-inch, but at 366x G279 appears larger than adjacent stars. Direct vision shows an extremely faint stellar core, but the stellaring is invisible with averted vision.
The galaxy's northeastern end features an open cluster trio, C311/312/313. Together they form an extremely faint band, elongated 6:1. There's a hint of mottling with my 16-inch at 366x. My club's 25-inch Dobsonian working at 227x separates out C311, the southernmost cluster, as a large knot.
A group of globulars lies near M31's core: relatively easy G235 and G222; fairly difficult G230; and difficult G229, G257, G205, and G217. The bright core looks daunting on photographic guide charts, so I avoided hunting these globulars for decades. However, I've learned that this area looks far brighter on images than it does at the eyepiece. All of these globulars appear starlike except G229. The 25-inch at 318x revealed a very faint core in G229's fairly large dim halo.
There are fewer noteworthy objects on the M110 side of the Andromeda Galaxy. G73 is interesting because it may be in orbit around M110 rather than around M31. G58 is rather faint and small--only magnitude 15.5 and 2.3" in diameter--but on two nights I logged it as "soft" in comparison with very close G59, which is much brighter but stellar. On a superb night during 2008's exceptionally quiet Sun (which should decrease airglow, thus giving a darker sky), G35 and G72 also looked slightly swollen in comparison to similar-magnitude stars.
Very easy G78 is part of an asterism resembling the Greek capital letter sigma ([SIGMA]). G78 is the same magnitude as G76 and G280. In my 16-inch at 366x, G78 has a stellar core in a substantial halo. Its neighbor, 15.9-magnitude G70, is the faintest globular revealed by my 16-inch.
The many other clusters plotted on the charts all appear stellar in my scope, but I viewed most only at 203x or 229x. A few might blossom into fuzzies on an excellent night that enables a high-power view.
On a very memorable night (September 14, 1999), the rare combination of excellent seeing and transparency allowed me to glimpse blue-supergiant stars in NGC 206 with my 16-inch and an orthoscopic eyepiece giving 261x. I had been waiting for the perfect night to attempt these massive young stars, whose intrinsic luminosity is 250,000 times that of the Sun!
Four or five brighter Milky Way stars lie in front of NGC 206. But behind them perhaps eight stars flickered in and out of visibility at the limit of vision. I carefully checked four similar-sized areas immediately surrounding NGC 206, and I didn't see any stars at the edge of vision in those areas. So I'm confident that most of these glimmerings were indeed some of the 70-odd blue supergiants that are so prominent in color photographs of NGC 206. After my observation I carefully examined a photograph that showed eight particularly bright blue stars, matching my observation. So the brightest stars in the Andromeda Galaxy are within the grasp of a 16-inch telescope on a perfect night!
RELATED ARTICLE: M31's Large-Scale Structure
M31's spiral arms are quite difficult to discern because the galaxy is tilted just 12.5[degrees] to our line of sight. The strongest clues to a visual observer are the two dust lanes that are particularly prominent between M31's core and the satellite galaxy M110. See how far you can trace them in each direction--an exercise that depends more on the quality of your skies and your skill as an observer than on the aperture of your telescope.
Guy Mackie's sketch below shows how much is possible with a 12.5-inch scope; he traced the inner arm looping back toward M32 and discerned intricate structure in the outer arm southwest of NGC 206.
The gegenschein and zodiacal band are occasionally visible from Alan Whitman's backyard observatory in British Columbia's Okanagan Valley.
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|Title Annotation:||Going Deep|
|Publication:||Sky & Telescope|
|Date:||Nov 1, 2013|
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