December Skies 2018

Chart of area around Triangulum.
Chart of the area surrounding the constellations Andromeda, Triangulum, and Aries, as seen from Denver in mid-December at 9:00 PM, viewing just south of the zenith (nearly “straight up”). Telrad circles are shown centered on 6 Trianguli (6 Tri), one of this month’s targets. As an aid to finding 6 Tri, note how the circles center on the imaginary line running from Almach to Beta Tri, and how the outermost circle touches another imaginary line running between Gamma and Alpha Tri. –Object positions, constellation and meridian lines charted in SkySafari, and then enhanced. (Tap on image above for larger version.)

The Solar System

Mercury ended November lost in the solar glare, but as it sweeps rapidly through its orbit, it will become increasingly visible as a pre-dawn target. After about the first week of December, the planet will sit almost 10° above the southeastern horizon at 6:30 AM, roughly 40 minutes before dawn. (By then, it will look like a fat crescent in telescopic views.) A week later, Mercury reaches its widest angle, as we see it, from the Sun (known as “greatest elongation”), and sits slightly higher at the same hour, brightening by about a half-magnitude as well. After that, the planet will appear closer to the Sun each day. Look for a close conjunction with Jupiter on the morning of the 21st, when the two planets will lie within a degree of each other.

Late-November views of Venus were spectacular, even with the naked eye—the planet’s sheer brilliance in a dark sky made a 5:30 AM rise worthwhile. (Venus was so bright, it made it hard to recognize a nearby, seemingly wan star for what it really was: 1st‑magnitude Spica.)

Happily, the views continue in December—though the planet’s brightness diminishes slightly and the disk appears a bit smaller, it still presents a terrific target, for naked eyes and telescopes alike. In the latter, you’ll still see a “fat crescent” through about mid-month, broadening to a half-disk or “lemon wedge” by month’s end. You can also sleep a bit later than last month, because of the later sunrise—and the planet will be higher, more than 25° up at 6:30 AM.

Mars remains with us as December begins, transiting in a dark sky just after 6:00 PM. It’s still fairly large and bright, at about 9” across and magnitude 0. By New Year’s, though, Mars will shrink to just 7” and dim by about a half-magnitude….  We’ll have Mars in our sky for quite a while yet, but the best of this apparition is now behind us—well, except for one thing: On December 6th and 7th, Mars and Neptune have a very close conjunction.

The two planets will appear just 2.1 arc-minutes apart on the morning of the 7th, an astonishingly close pairing, around 7 AM Mountain Standard. Here in Denver, though, the two planets will be well below the horizon at that time, but before you think of shooting the messenger, here’s what we get in return—not one, but two opportunities for what are still very close conjunctions. On the night of the 6th (i.e., the night before the “official” event), the planets are just over 20 arc-minutes apart, or a third of a degree, around 6 PM. The sky at that time will be dark, and our targets more than 40° up in the south. Neptune will appear to the east of Mars (to our left, in unreversed views); if you don’t see Neptune at first, center Mars in a low-power eyepiece and let your telescope drift a while (i.e., turn off your ’scope’s tracking if you’re using it); look for a tiny, pale bluish dot—there won’t be many other stars in the eyepiece field, so Neptune should be noticeable.

By 9 PM, Mars and Neptune will have drawn noticeably closer, to 16½ arc-minutes, or just over ¼° apart; they’ll still sit almost 27° above the southwestern horizon then. By 10 PM, they’re less than ¼° apart, though only 17° above the horizon; the two will draw ever closer until they set just after 11:30 PM our time.

The situation reverses on December 7th—at 6 PM, you’ll find Neptune to the southwest, instead of the east, and less than 18 arc-minutes from Mars. As the evening progresses, the gap between the two planets will widen, so best viewing will come early rather than late—by 10 PM, they’ll be almost 24 arc-minutes apart.

Depending on your eyepiece’s angle of view, it’s possible that you may not have enough power to really appreciate Neptune as a disk (that is, as a planet, with a visible diameter) when you view it together with Mars. If that happens, take a moment to “zoom in” on Neptune using a higher-powered eyepiece (200x should do it), and when you’ve really seen Neptune, go back to the lower-powered eyepiece and enjoy the pairing with Mars (the color contrast should be great).

Like Mercury, Jupiter was lost in sunlight in late November, and will remain so for much of December—but it will become a noticeable pre-dawn object by mid-month. By the end of December, Jupiter will sit 12° above the horizon, 45 minutes before sunrise—it’s not a great telescopic target yet, but it will become one early next year. (In the meantime, don’t forget the conjunction with Mercury on the 21st!)

So long for now, Saturn! The ringed planet starts December low in the southwest, only about 10° up, just 45 minutes after sunset. By the end of the month, it’s lost in the sun’s glow, and we’ll see it again as a pre-dawn object early next year.

Uranus is a lovely and convenient target. By 6:30 PM in early December, it’s at least 45° up, and crosses the meridian (i.e., it’s highest in the south) at that hour at New Year’s. Look for it 1½° degrees northeast of Omicron (ο) Piscium in early December, and just 1¼° north by month’s end. If Uranus was “easy-peasy” last month, it’s …er… “easy-peasier” now. 

Slow-moving Neptune sits about where we left it last month, halfway between Hydor, aka Lambda (λ) Aquarii, and Phi (φ) Aquarii. It’s near the meridian around 6:00 PM in early December, but sinks into the southwest, only 35° up at that hour, a month later. As you might expect because of the conjunction, you can use Mars to find Hydor early in December, and Phi through mid-month.

Stars and Deep Sky

Leaving our solar system behind, we have “a tale of two binaries.” (For the beginners, a binary system is one where the two stars within it are in orbit around each other—in contrast, two unrelated stars moving separately through space are merely “optical doubles.”)

Neither target this month should be difficult if you can find Mirach (Beta [β] Andromedae) and Almach (Gamma [γ] Andromedae). If you can’t, don’t worry—they’re part of the constellation Andromeda, and lie to the east of the Great Square of Pegasus, which is easy to recognize; the whole area is described in the December 2017 Observer, at (Read the section under the subhead, “Getting Started with Navigation,” on page 6, and check out the wide-view map—you’ll see our current map is just a closeup of the area in and below Andromeda!)

First up, then, is 6 Trianguli—“6 Tri” for short—at 02h 13m, +30° 23’. It’s a beautiful and interesting system, and not just binary, though that’s what it looks like. Each main star also has an extra companion star that astronomers detected through spectroscopy—these companions orbit too closely to become visible themselves. The main stars (the ones you can see) are indeed in orbit around each other, as we’d expect from a binary system. What we really have, then, is a four-star system, organized in pairs, and each pair is its own binary system!

In his Celestial Handbook, Robert Burnham, Jr. depicts this star as “strong yellow and blue,” though noting that it has also been seen as “a bright golden yellow pair.” (If you go looking for Burnham’s full description, you’ll find it listed under “Iota [ι] Tri,” another name for this star. Though the Handbook was last updated some 40 years ago, Burnham noted that each of the visible stars had unseen companions, as above, and described them in detail. His distance information, though, is dated; Prof. James Kaler, of the University of Illinois, uses a more modern data set for his description, with a new distance estimate of about 300 light-years, about half-again as far as Burnham had noted. That also means Burnham’s estimates for the size of the system itself are off a little bit, as well.)

Simulated image of 6 Trianguli.
6 Trianguli, as it might appear in a high-power telescopic view. Illustration © Zachary Singer.

Looking through a 5-inch Maksutov not long ago, I saw a close but pretty duo, Creamsicle-orange and dim white. It looked best between 125 and 200x, though the pair was quite tight at the lower magnification. In my 12-inch Newtonian a year ago, the primary appeared butter-yellow, and the secondary grayish at 125x—but very pale lavender at 200x and with more distinct colors at 300x. 

Although we often say that a binary’s stars “are in orbit around each other,” the deeper truth is that both stars circle their common center of mass, much the same way children playing “Ring Around the Rosie” circle their common center. If the children are of equal size (mass), then the circles they make around that center are also of equal size. But if one child is replaced with a bigger (more massive) kid, then both will still “orbit” their common center, but the smaller child will wind up making a larger circle than the bigger kid—the relative size of their “orbits” is proportional to their mass.

Illustration of the 6 Trianguli system.
Above: The 6 Trianguli system. The two main components, A and B, are visible in small amateur telescopes (simulated in inset, above left). Note how A and B orbit their common center of mass, shown with a “+,” and that B’s orbit is larger, because it is less massive than A.
Each component is itself a binary system, detectable only with special instruments. The “zoomed-in” view of each component (bottom of main drawing), shows the true makeup of 6 Tri: Component “A” contains two stars, “Aa” and “Ab,” and component “B” does too, with “Ba” and “Bb.” Each of these sub-systems in turn orbits its own center of mass, in a manner similar to the A-B system—a “wheels turning inside of wheels” scenario. (In the detail of the B component, Ba and Bb have similar-sized orbits, because the stars are of similar mass.)

The same is true for stars orbiting in space—in 6 Tri’s case, the more massive orange giant and its companion (together, they’re the orange- or yellow-looking “A” component) have a greater mass than the class F star-pair that we see as the white or blue “B” component. So component A has a smaller orbit than the B component does—and we can see a similar situation within this system’s individual pairs, too. There, we see the unequal-mass A components in orbits of unequal size, while the matched stars of the B component have orbits of the same size. (See the illustrations above for an overview of 6 Tri’s arrangement.)

To find 6 Tri, we’ll need to find the small constellation of Triangulum itself—to get started, first look for the stars we mentioned above, Almach and Mirach, in Andromeda (see star chart, on page 1.) Imagine the line between these two bright stars as the base of an upside-down equilateral triangle, with its third vertex “below” the line, towards the south—under dark skies, you’ll soon see a moderately bright (3rd– and 4th magnitude), skinny right-triangle sitting in that area—as you might guess from its name, that’s Triangulum! 

Once you’ve got Triangulum, look at the star marking the right angle—that’s Beta (β) Tri. Then look at the triangle’s “far” star, at the constellation’s narrow end—that’ Alpha (α) Tri; the third star, closer to Beta, is Gamma (γ) Tri. Next, visualize a line between Almach in Andromeda, and Beta Tri. Now, to hit 6 Tri, slowly slide the center of your Telrad down that line, away from Almach, and towards Beta—keep going, past Beta, until the trailing edge of your Telrad lines up with the imaginary line between Gamma and Alpha Tri—6 Tri should be in your finderscope, and if you were careful, in or near your telescope’s low-power eyepiece field. Our chart shows the Telrad’s positioning.

One last note: At 5th magnitude, 6 Tri is more than bright enough to appear in a 7×30 finder (even under city lights) but it might be a bit dim to show up in your Telrad, even in the country—I find 5th magnitude stars difficult through a Telrad’s plastic, even though I can see the same stars naked-eye. You don’t need to see 6 Tri in your Telrad to find it with this approach.

Our second binary is Gamma (γ) Arietis, aka Gamma or γ Ari for short, or Mesarthim on some charts. Located at 01h 55m, +19° 23’, Gamma Ari is worth seeing, “just because.” For one thing, unlike 6 Tri, it’s not at all difficult to split its components—the stars are of roughly matching brightness and appear about 8” apart—easy even in small ’scopes. In the 5-inch, I saw two softly glowing pearls at 100x, an enchanting view—but the pair split cleanly enough even at half that power.

Even “simple” systems like this one have their fascinations: Since Gamma Ari’s two stars are of similar spectral type, and they’re within about 10% of each other’s mass, you might not be too surprised that their brightnesses are somewhat similar, too. The difference is only 0.1 magnitude, so your eyes won’t notice, but it’s worth mentioning, because the dimmer star is the more massive one.

Normally, the more massive the star, the hotter (and thus brighter) it is. But hotter stars’ light is also bluer in color—and in Gamma Ari’s case, that drives the light of its more massive star enough towards the blue end of the spectrum to push it into ultraviolet. We don’t see ultraviolet light, so the more massive star’s visual luminosity seems lower than it is. (Its total luminosity, counting the ultraviolet, is brighter than its smaller companion’s, as expected.) Appearances—and physics—can be deceiving.

If you peruse our chart, you’ll see the constellation Aries, home to Gamma Arietis, on the opposite side of Triangulum from Almach—Triangulum is almost centered between Andromeda and Aries. Look for bright, 2nd-magnitude Hamal, or Alpha (α) Arietis, just west of the extended line running from Almach through Beta Tri and into Aries. Now follow the arc of stars to the southwest—you’ll encounter mag. +2.7 Beta (β) Ari, aka Sheratan, next (it’s about 4° from Hamal), and then you’ll arrive at Gamma, our target, just 1½° south of Beta.

With a little practice, you’ll be able to pick up Gamma directly with your naked eyes under a dark sky, but at magnitude +3.9, it’s barely on the edge of visibility on a good night in the city, and often washed out. That’s no problem; just center your finderscope on Beta, and Gamma should be obvious within its field. (If it’s a lousy night and you can’t see Beta either, center on Hamal, and guesstimate Beta’s position—follow the same angle as you see between Almach and Mirach—it’s crude, but it will put Beta in your finderscope, which you can center as above to get Gamma.

One last note for Dobsonian users: We’re looking very close to the zenith, or almost straight up, for our main targets this month. Make life easier for yourself, and observe at least an hour earlier (or an hour or more later, if that’s better), when the angles aren’t as difficult for your ’scope.

Have a Happy Holiday!

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