September Skies 2018

Viewing almost straight up in Denver at 9:00 PM on September 15th. Telrad circles are shown positioned to put NGC 6871 near the center of a finderscope field, after centering on Eta (η) Cygni and slewing toward Sadr. From here, a 1° slew to the southeast (perpendicular to the swan’s “neck,” or the line from Sadr to Albireo) will get the cluster in or near your low-power eyepiece field. Note position of Eta Cygni at trailing edge of the 4° Telrad circle. Also note the dotted line showing the alignment of M29, Sadr, and Omicron1 Cygni.
The Cygnus Rift, illustrated above, is an area of dust and gas that obscures the Milky Way’s stars and nebulae. It is the same type of structure as the “dust lanes” often seen in edge-on galaxies.
–Object positions, constellation and meridian lines charted in SkySafari, and then enhanced. (Tap on image above for larger version.)

© Zachary Singer

The Solar System

If you’ve been watching the sky after sunset in August, then you’ve likely noticed the striking vista of four bright planets—Venus, Jupiter, Saturn, and Mars—lined up from southwest to southeast. Even without a telescope, their sweep makes a memorable view, and the arrangement will continue well into September; if you haven’t seen it you should—get outside about a half-hour after sunset and look for the planets to start coming out. As the month progresses, Venus will get lower in the west, so you’ll need a clear horizon.

Mercury continues as a pre-dawn object in early September, rising about 1½ hours before the Sun. Around 5:30 AM, an hour before sunup, Mercury sits 4° above the eastern horizon, rising to about 7° some 15 minutes later. Look soon though, because the planet’s on its way to superior conjunction (where it lines up with the Sun in the sky) on the 20th, and it will be lost in the sun’s light well before then.

Early in the month, Venus appears only 10° above the horizon 30 minutes after sunset, and as noted above, it will get lower daily. Though this will make telescopic observations increasingly challenging, the upside is that by mid-month, the planet will also appear as a stunning, large and bright crescent, reaching magnitude -4.6 at its peak, according to SkySafari software (the folks at Sky and Telescope say -4.8!).

The dust storms on Mars are reported to be abating, but visibility of the planet’s surface from Earth remains sub-par: In a moderate telescope in the penultimate week of August, the southern ice cap could be seen, with only a hint of detail around dark surface features in the mid-latitudes. At that time, the Martian disk spanned about 22”; by early September, it will be down to about 21” and will shrink to roughly 16” by the end of the month. That’s still large enough to see detail clearly if the planet’s atmosphere continues to clear, so keep an eye (and a ’scope) on the red planet.

Jupiter starts off September only 20° up in the southwest, 45 minutes after sunset, and gets far lower by the end of the month—in short, conditions are already less than great for observing this planet and will worsen dramatically by month’s end. Jupiter disappears into the sunset in November.

Saturn, though, still looks great! It’s now a little farther from us than a few months back, but the difference is negligible to visual observers, and the planet will remain a good target all month. As a plus, leaving Saturn behind us (as Earth moves quickly in our smaller orbit) means that sunlight no longer strikes Saturn straight-on from our point of view, so we see a large shadow on the rings from the planet itself, creating a deeper sense of dimension. The effect was already noticeable in a 4-inch Maksutov ’scope in late August and will intensify this month.

Uranus is now well-enough up at midnight, about 27° above the eastern horizon, to be a reasonable target. By the end of the month, you can get the same altitude about two hours earlier, but midnight views will instead offer a much higher altitude of almost 49°. Look for the planet about 4° northeast of Omicron (ο) Piscium mid-month—about a half-degree farther from the star in early September and about a half-degree closer at the end. (Uranus will appear closer and closer to Omicron through late January, making the planet easy to find.)

Neptune is at opposition on September 7th, so it’s visible all night, and highest in the south around 1:00 AM on that date—a prime opportunity for Neptune fans, especially before the weather turns cold. (By the end of the month, Neptune is at its highest before 11:30 PM.) Look for the planet about midway between Hydor (Lambda [λ] Aquarii) and Phi (ϕ) Aquarii throughout September—closer to Phi early on, and right in the middle at month’s end.

Stars and Deep Sky

At the time of writing, we’ve had several months of smoke and haze, making targets with low surface brightness, like some nebulae and galaxies, much more difficult or impossible to see. Similarly, carbon stars, known for their deep orange or red coloration, won’t be appreciated as much when even the Moon looks like a pumpkin, as we’ve seen so often lately. For these reasons, I have chosen two open clusters this month, instead of other targets I’d had in mind. They’re interesting objects, to be sure, but it is hard to ignore the underlying reason for choosing them—much of the West, including the Canadian West, has been ablaze this year—as it was last year, and the year before

First up then, is M29, at 20h 25m, +38° 34’, in the constellation of Cygnus, the Swan. Unlike some open clusters we’ve seen, this one is quite young, with an age of about 10 million years. That’s long enough for almost all of the hottest and brightest, O-class stars to have burned themselves out already (they die the quickest) but the still-very-hot B stars are still with us. These stars vastly outshine the others in the cluster, and most of M29’s visible light comes from them.

Many write-ups for observers suggest there’s not much to see here in a telescope. Perhaps so, but visual observing is highly subjective, and I disagree with the common view for a number of reasons. For one, M29 is easily visible in the city, even in a 4-inch Mak at low power (50x) and with less-than-perfect skies. At about 100x, the inner “butterfly” shape is easy; 10-15 stars will be visible. To me, M29 in a ’scope is much nicer than, say, the Pleiades in a binocular field, yet many folks are unimpressed with the former and thrilled with the latter…

Many common sources say the number of visible stars (under dark skies) is about 50, but that’s only half the story—M29’s actual structure is more complex, with vast tracts of dust and gas from the Cygnus Rift concealing the cluster’s true nature. A deeper investigation reveals studies counting 100 to 250 members, and there are more stars hidden behind the dust; they become readily apparent on infrared images, such as those taken during the 2MASS survey.

M29 is actually a very bright cluster, intrinsically, but the intervening dust obscures and reddens the light, dropping its apparent brightness by roughly three magnitudes—more for bluer stars, less for redder ones. Without this light loss, M29 would be a fairly easy naked-eye object; as it is, it still manages magnitude +7.1, just beyond the limits of vision for most of us.

Now, here’s something to think about—the Cygnus Rift, mentioned above, is part of a much larger system of gas and dust running all the way to the center of our Galaxy, in Sagittarius. As much as we might consider this large, dark area an inconvenience for hiding stars, there’s another way to look at it—it’s the same kind of structure that we go out of our way to see in other galaxies—a dust lane! In short, here’s a chance to see a would-be brilliant cluster, dimmed by a nearby dust lane, and all within our own galaxy and the grasp of a small ’scope—or even binoculars… (The Rift is marked on the chart above.)

To find M29, first look for the bright star Sadr, aka Gamma (γ) Cygni; it’s where the “wings” of Cygnus meet its “body.” (If you’re not familiar with Cygnus, or you don’t know how to find it, it’s part of the Summer Triangle, a great landmark and an area rich in targets—for an overview of this region, check out “Getting Your Bearings,” on page 4 of the August 2015 Observer, at Note that the chart is set for August; it will be the same for any given September too, but just shifted westward about 30°.)

Next look for even brighter Deneb, aka Alpha (α) Cygni, and then Delta (δ) Cygni—the latter is where the Swan’s wings bend. Now imagine a star centered between them. If you can see it, you’ll conveniently find a star almost in that location—it’s Omicron1 (ο1) Cygni—but if you can’t, just keep its would-be position in mind. M29 lies directly on the opposite side of Sadr from Omicron1; since a straight-through, inverted finderscope will flip its image 180°, centering Sadr in your finderscope’s crosshairs should include M29 in its field, in the same direction from the center as Omicron1 (or its imagined position) is when viewed naked-eye.

If you can’t make out M29 in the finderscope, use your Telrad instead—center it on Sadr, then move it away from Omicron1 until Sadr is almost at the trailing edge of the outer, or 4°, Telrad circle.

Our second cluster, NGC 6871, lies farther down the “neck” of the swan, at 20h 07m, +35° 49’. To be sure, this is a less well-known cluster; it’s of some aesthetic and scientific interest though.

Aesthetically, it has a few bright stars at its center, arranged in a somewhat “butterfly-shaped” figure, rather like M29 (they’re mostly hot and bright, too), but with a lovely Milky Way background.
In small ’scopes in the city, the cluster itself is very small but easy to see, and arguably best at around 100x, where the extra magnification helps brighten the stars. The central stars become even more obvious and easily separated at about 150x. In the city, naturally, the Milky Way won’t be visible, but out in the country, especially in a larger telescope, it can be impressive—I got my first look at this cluster years ago, and I still remember it even though I made no observing notes. (The cluster had a simple grace to it.)

Scientifically, the main star in the brightest of the cluster’s two brighter pairs is a rare Wolf-Rayet star, WR 133. (The star might be easier to find by its other names, V1676 Cygni, HD 190918, or SAO 69402.) Reputedly, WR 133 is the brightest Wolf-Rayet visible in the northern hemisphere; at magnitude +6.78, though, it pushes the edge of naked-eye visibility. Still, that’s pretty impressive for a star estimated to lie about 5,000 light-years from us—our own Sun would appear about that brightness when just 75 light-years away, and that’s not accounting for any dimming of WR 133 by intervening dust.

Essentially, Wolf-Rayets are “old” class-O stars. They’ve blown off their outer envelopes, so their underlying helium, oxygen, carbon, and nitrogen show in spectra, with little or no hydrogen. Those elements glow as emission lines in the spectrum, instead of the more usual dark absorption lines—that’s because these stars are so hot that they radiate a lot of UV light, causing those gases to light up like a fluorescent tube.

Wolf-Rayet stars are nearing the end of the line before they explode as supernovae. These stars are rare because the massive, O-class stars they came from have short lives, as noted earlier, and the “WR” part of their evolution is even shorter—just a few hundred such stars are catalogued, out of the billions of stars in our galaxy.

Start the approach to NGC 6871 by looking for Eta (η) Cygni; it’s a moderately bright, magnitude +3.9 star about halfway between Sadr and Albireo (the latter is the bright star marking the “nose” of the Swan). You shouldn’t have any trouble spotting Eta under dark country skies, but on a very good night in southern Denver, light-pollution leaves it right at the edge of visibility—if you can’t see Eta, guesstimate the halfway point between Sadr and Albireo, and you should see Eta as the brightest star in your finderscope’s field.

Once you have Eta centered, slew your telescope about 2° towards Sadr. If you can see Eta in your Telrad, stop slewing when it touches the trailing side of the outer, 4°, circle, and look in your finderscope—you’ll see a few bright stars perpendicular to the center, about 1° towards the southeast side—that’s the same side of the swan’s body as M29 or the Veil Nebula, but remember your straight-through finderscope’s view is inverted! 

If Eta wasn’t visible in the Telrad, and you had to use the finderscope to find it, slowly slew your telescope in the direction of Sadr, and start to watch the incoming field for noticeable stars—you’ll see just a few pinpoints, including WR 133, instead of a whole cluster. These should come near to the finderscope’s midpoint (offset from the very center as mentioned above), around the time Eta begins to approach the trailing side of your finderscope view.

A quick note for the Dobsonian folks: At our chart’s time of 9 PM (after the first week or so of the month), our targets will lie close to the zenith, making pointing your telescope difficult. It’s best to wait for an hour or two, until you’re comfortably past the need to point your ’scope straight up.

—See you next month.

Leave a Reply