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Public Night

Lunar Eclipse, M11, Binary Stars, Veil: September 2015

by Zach Singer


Western Veil Nebula, copyright Alan Erickson, June 2009

On the “to-see” list for this month, we have a total lunar eclipse, a relatively easy-to-find example of a rare carbon star, two different sets of binary stars, a well-known and brilliant open cluster, and an equally well-known supernova remnant, as well as some notes on other happenings in the solar system.

The big headline, of course, is the total lunar eclipse on the evening of Sunday, September 27th; the first subtle (penumbral) darkening begins at 6:12 p.m. For full details of the eclipse’s timing, as well as the Denver astronomical Society’s public eclipse viewing event at Chamberlin Observatory, please see “2015 Lunar Eclipse!

Next to an eclipse, the motions of the planets might seem anticlimactic, but there are still sights to be seen. Among the first is Venus as a very thin, bright crescent, large in a telescope at dawn as September begins. Look for the (-4.4)-magnitude planet about 14° above the eastern horizon at 6:00 a.m.; it should be easy to spot in the pre-dawn sky. By month’s end, Venus will have “fattened up” to a lemon-wedge appearance, and will be nearly 30° above the horizon in a darker sky. (The planet’s eastern bearings and later sunrise are, of course, a harbinger of fall’s arrival.)

Saturn is now low in the west in the evening; by the end of the month, it will be close to setting as the sky becomes dark, and will reappear this winter as a morning object in Scorpius. Meanwhile, Neptune comes to opposition on the night of Aug. 31/Sep. 1, and is therefore at its best around midnight at the beginning of the month—and at 10 p.m. at month’s end. Unlike a planet nearer to us, Neptune’s size and appearance won’t change much as Earth’s orbit begins to move our planet away from Neptune. The disk will shrink from a tiny 2.4” at opposition to a slightly tinier 2.3” two months from now…. (For the newbies, Neptune should be a detectable disk at high power; if space permits next month, there will be some help with finding the planet then.)

Unfortunately, September’s meteor showers are worth noting only to say that they’re hardly worth noting: The Aurigids, a minor shower, will be obscured by a nearly full moon. Another minor shower, the September Epsilon Perseids (not to be confused with the well-known August shower known simply as the Perseids) occurs from the 5th to the 21st, peaking on the 9th, according to Guy Ottewell. As he notes, the peak rate is only 5 per hour, but they are “often bright.” The same source says the shower should be visible all night, with its radiant highest around 4 a.m.

Leaving the solar system this month, our first stop is at the “bottom” of the Summer Triangle, near the southernmost part of the constellation Aquila. (The September “Getting Your Bearings” has a detailed map of the Triangle—for its general location compared to other constellations, see the August article.) We have two targets in this area, and they’re “next door” to one another, which will make finding the second one—the carbon star—a little easier.

M11 – The Wild Duck

The first one, the Wild Duck Cluster, also known as M11 (Messier 11), is just over the border of the constellation Scutum, southwest of Aquila, at 18h 52m, -6° 15’. M11 is a very large, bright and dense open cluster—it’s visible in a 6-inch telescope in the city, on a decent night. (If you’re going to try for that, use 80X or higher to help separate the cluster from the light-polluted background.) Out in the country, M11 is among the finest objects you can aim at with a small telescope, and in larger instruments, like a 10- or 12-inch, its bluish stars are dazzling. Either size gives a great view: The 6-inch scope allows the cluster to stand out more from the background, as the dimmer stars behind M11 fade away with the smaller aperture; on the other hand, the raw power of a bigger scope renders M11 with remarkable brightness, and the stellar Milky Way background is a wonder in itself.

To find M11, start at Altair, the bright star in the northern part of Aquila. If you look carefully under dark skies, you’ll see that Altair, or Alpha (α) Aquilae, is roughly in the middle of two close-by, dimmer stars, Beta (β) and Gamma (γ) Aquilae. (See maps in the September Denver Observer ) Look at the latter (it’s listed as Tarazed on some maps), and you’ll see it’s also the “topmost,” or northernmost, of three stars forming the long, diagonal “backbone” (or centerline) of Aquila’s “eagle” outline—the other two are Delta (δ) and Lambda (λ) Aquilae, in that order as you progress along them to the southwest.

When you get to Lambda, you’re close—a careful look at the area reveals two more stars: i (or 12) Aquilae, at mag. 4.0, about a half magnitude dimmer than Lambda; and Eta (η) Scuti (of the constellation Scutum), dimmer still at mag. 4.8. As you can see from the star chart, they curve away from Lambda, in hops of about 1½°. Centering the last one, Eta Scuti, in your Telrad will put M11 into your finderscope (if you can’t see Eta Scuti for whatever reason, then roughly estimating its position should get you close enough).

To get to V Aquilae, our carbon star, at 19h 05m, -5° 39’, just center your Telrad (or finderscope’s crosshairs) midway between Lambda and i Aquilae, which we just passed on the way to M11—a notably red star should be in the finderscope’s field. V Aquilae is of the same rare type as R Leporis (“Hind’s Crimson Star”), an especially cool, and therefore red, supergiant with a twist—large amounts of carbon and carbon compounds near the star’s surface cause even deeper reddening of its light, much as soot from fires reddens our skies here on Earth.Since carbon stars accumulate more and more carbon over time, they become increasingly dimmer and redder—until they blow off the carbon shell and start over again. Some of the stars with the best reputation for deep color, like R Leporis, can vary by as much as 6 magnitudes.

Though the numerical measures of V Aquilae’s spectrum suggest that it’s not quite as red as some of its carbon-star cousins (some of which are described by various authorities as “scarlet”), it’s “very red” nonetheless. It’s also a less variable star than the others, dimming less than two magnitudes during its roughly yearlong cycle. In practice then, V Aquilae offers a practical trade-off: In exchange for somewhat weaker color, the star’s limited variability allows the star to remain visible in a finderscope—even at minimum, when it’s most colorful. The combination of V Aquilae’s relatively high brightness and its easily found “Telrad companions” (Lambda and i Aquilae) makes this star one of the easiest carbon stars to locate visually, and a great starting point for beginning the observation of carbon stars in general.

Binary Star System

Our next stop is the binary system of Gamma (γ) Delphini; at 20h 47m, +16° 11’, it’s the “nose” of the dolphin-or kite-shaped constellation Delphinus. (See “Getting Your Bearings” in the Observer or your star chart for Delphinus’ location.) The two stars here, with masses 50% and 70% greater than the Sun’s, both started out somewhat hotter and bluer (or perhaps better, “less yellow”) than our home star is. The larger one, though, has already run through the hydrogen fusing part of its lifecycle, having cooled and expanded to become a sub-giant, and the smaller star will do the same eventually.

The result is two stars of somewhat disparate color temperature and brightness, leading to a “95 Herculis” sort of condition, in which some observers describe one star as having a significantly different color than the other, even though the two aren’t greatly different. In Gamma Delphini’s case, the historically described colors are yellow for the one, accompanied by “green,” “blue,” and even “lilac” for the other, when both stars are pretty much white or yellowish. Still, they’re both reasonably bright at 4th and 5th magnitude, an easy split even in small telescopes, and lovely to look at.

Professor James Kaler (Univ. of Illinois) states that the system’s orbit is highly eccentric, with an average value of 330 Astronomical Units (AUs)—about 11 times the Neptune-Sun distance. At the extremes, they’re as close as 40 AUs or as far as 600 AUs apart. While these two stars aren’t “monsters” in size or luminosity, they are as a pair some 24 times brighter than the Sun, enough to make them easily visible to the naked eye from more than 100 light-years out.

In contrast, the pair of stars we know as 61 Cygni are together less than 1/4 of our sun’s intrinsic brightness—dim enough that were they just 5 or 6 light-years farther than their actual distance of 11 lightyears, the individual stars would be on the edge of invisibility to unaided eyes. The reason for this “half-hearted” performance is that these two relatively cool dwarf stars, both of class K, are each only about 2/3 the Sun’s diameter and less than 2/3 of our star’s mass—so there’s less surface area to radiate energy, and (because of the low mass) less energy from fusion to radiate in the first place.

Because of 61 Cygni’s close proximity, though, the pair is wonderfully visible, even in small telescopes, with a beautiful red-orange tint that comes from its coolness (try comparing 61 Cygni to Gamma Delphini, and you’ll see what I mean). 61 Cygni’s actual separation of about 120 AUs, together with the system’s nearness, gives a wide apparent separation of 32 arc-seconds as seen here on Earth, making the pair a very easy split, even at low power.

To find 61 Cygni, at 21h 8m, +38° 49’, look for the two brightest stars on the southeastern “wing” of the Cygnus “swan”: Gienah/ Epsilon (ϵ) Cygni, and Zeta (ζ) Cygni (see detailed map of Cygnus, above). Imagine these two stars as the base of an equilateral triangle, and look northeast (“up and left” when facing southward around 9:30 p.m. mid-month) for the third star, Tau (τ) Cygni, which is just half a magnitude dimmer than Zeta. With a closer look, you’ll see that Tau is half of a duo, with its sidekick, Sigma (σ) Cygni, about 1½° away, roughly to the north. Tau’s pairing with Sigma makes a great landmark, ensuring you’re in the right place.

When you center Tau in your Telrad, your finderscope’s view will include both Sigma and our target, 61 Cygni. 61 Cyg will be just slightly farther from Tau than Tau is from Sigma, and 61 Cyg will be the dimmest of the three. Once you have it in your telescope, 61 Cygni should appear as two stars, regardless of the eyepiece in use.

The Veil Nebula

Our last object, the Veil Nebula, is a huge, glowing shell of gas, about 100 light-years across. It’s the left-over debris from a supernova that exploded some 5,000-8,000 years ago. In that regard, you can also think of it as a still-expanding shockwave from that long-ago explosion. (Robert Burnham, in his Celestial Handbook, points out that because this shock wave has swept away much of the interstellar dust in this area, low-brightness stars are more visible behind the wave than in front of it—where the dust still remains, for now. This effect is visible on deep photographs of the region, and worth an Internet search for images on a cloudy night—Burnham also has examples in his book.)

The Veil’s looping outline is more than 2½° across, so you won’t get all of it into a telescope’s field—in fact, it’s so large that early astronomers failed to realize that its various sections were all part of the same object. Because of all this, the two areas most visible in amateur telescopes are now known as the “Eastern” and “Western” Veil Nebula. The former is somewhat brighter and easier to see in a telescope, while the latter is perhaps easier to locate visually, as it runs behind the star 52 Cygni. Both sections of the nebula, though, have a very low surface brightness, which led to this object’s description, in years past, as very difficult to see. There’s also an intense star field behind it (since the Veil is found along an arm of the Milky Way), which helps “camouflage” the nebula.

To get a higher-contrast view that “pops out” the nebula, you’ll need either a UHC or O-III filter. With either of these, the Veil is a striking object! (Ironically, I also like the “difficult” view, without the filter, after I’ve seen the area with the filter. It’s easier to perceive the nebula’s subtle form once you’ve seen where it is, and the filter-less, star-filled view is more easily appreciated if you know you’re not stuck with it.)

To find the Eastern Veil, at about 20h 57m, +31° 48’, go back to Gienah and Zeta Cygni (the stars we just used to find 61 Cygni), center the Telrad between them, and then sweep your scope perpendicularly to the two stars, in the opposite direction from 61 Cyg.  A bit less than a 1° sweep should do it. The Eastern Veil is at least 1° across, so use lowest power to get it all in.

The Western Veil requires less guesswork—just center 52 Cygni (20h 46m, +30° 47’) in your Telrad or finderscope, and there you are. 52 Cygni is the first reasonably bright (4th-magnitude) star west of the Eastern Veil, and it makes a skinny version of the Summer Triangle with Gienah and Zeta Cygni—it’s easy to see, once you know where to look. The expanse of the Western Veil is even greater than that of the Eastern, so once again, start off with low power here.
—See you next month.

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