October Skies 2018

Looking just west of south in Denver at 9:00 PM on October 15th. Telrad circles are included for scale; their center is about 30° above the horizon. Note that with 3rd- and 4th- magnitude stars in its outline, most of Capricornus will likely not be visible to the naked eye under city lights. –Object positions, constellation and meridian lines charted in SkySafari, and then enhanced. (Tap on image above for larger version.)

© Zachary Singer

Welllllll… We had a planet-rich summer, but many of the planetary observational opportunities are going away or will do soon. At the same time, we’re in a great position for deep-sky targets, with late-summer objects still in play, and winter targets, like Orion, becoming visible to observers in the wee hours. The earlier onset of night helps, too. Here’s what’s up for October:

The Solar System

Mercury is in solar glare this month, but may be a binocular target, very low in the west, at month’s end. (If you’re going to try, look for it about 3½° above the horizon half an hour after sunset. It’s directly below Jupiter on the 28th and “down and left” of Jupiter on the 31st.)

At the beginning of October, Venus is also very low in the west—just 2.2° up—half an hour after sunset, and it will get even lower as the days pass. The planet will then be lost in sunlight, re-emerging in early November as a bright, pre-dawn crescent.

As for Mars, there’s Murphy’s Law: Now that Mars is well past opposition and the planet’s apparent size shrinks by the day, the Martian dust storm is at an end. On the bright side, the planet is still nearly 16” across at the beginning of October, large enough to show off detail. At that point, Mars is also highest in the south at 9 PM, a very convenient hour for observing. By month’s end, Mars transits an hour earlier, at 8 PM, but its disk will shrink to just 12”. Enjoy this planet while you can.

Jupiter starts off October just 14° above the western horizon 30 minutes after sunset, leaving it a blurry target at best. By the end of the month, it will be only 5° up. Superior conjunction, when the planet lines up behind the Sun and is lost in the star’s glare, occurs in late November; after that, Jupiter, too, will become a pre-dawn object.

Saturn remains a good target through at least mid-October. By the end of the month, though, you’ll only have a short while to observe—in twilight—before the planet gets low over the horizon. (Saturn will be only 16° up, 90 minutes after sunset on Halloween, and just 12° up 30 minutes later that evening.)

Uranus, on the other hand, is an easier target in October, almost 28° up by 10 PM on the 1st, and 49° up at that hour on Halloween. Opposition is on the 23rd; on that night, Uranus is highest in the sky and due south just before 1 AM. The planet is easy to see in a 4-inch Mak, so don’t pass up Uranus for lack of bigger apertures (try about 100x in small ’scopes). Look for the light-blue planet about 3½° northeast of Omicron (ο) Piscium early in the month, and about 2½° northeast at month’s end.

Neptune is well up after 9 PM at the beginning of October, though 10 PM might be a bit better. By the 31st, the planets transits (sits highest in the south) at 9:20 PM. Look for it about halfway between Hydor (Lambda [λ] Aquarii) and Phi (ϕ) Aquarii all month. Neptune is also about ½° from 81 Aqr, a 6th-magnitude orange star near that halfway point.

Stars and Deep Sky

Our targets this month lie in the constellation Capricornus, which lies in the south around 9 PM in mid-October. Most of Capricornus’s stars are somewhat dim at 3rd and 4th magnitude, but its outline is easily recognizable in a dark sky once you know where to look. Though its historical shape is supposed to be a “sea goat,” a more modern observer will find it easier to look for a “giant bikini bottom,” and that is how the outline will be referenced—especially since referring to “bikini strings” at the top corners will get you oriented more quickly than trying to point out the “nose” or “tail” of a mythological beast. If you’re not familiar enough with Capricornus to find it, see “Getting Your Bearings,” on page 4 of the October 2015 Observer, at https://www.denverastro.org/xobserver/october2015_denverobserver.pdf.

Our first target, M30, at 21h 41m, -23° 06’, is a large globular cluster—that is, a round, or “globe-ular,” ball of 100,000 stars or more (perhaps double that, in M30’s case). While reasonably bright, M30 isn’t a “showpiece” globular like the more famous M13 in Hercules or M5 in Serpens. In fact, it appears about 1½ magnitudes dimmer; that’s partly due to M30’s distance from us of about 28,000 light-years (some sources say 26,000), and also because M30 is intrinsically a bit dimmer, too. Still, it’s more than bright enough to be seen even in small ’scopes, and it’s an interesting object for other reasons.

If you’re familiar with globulars, you likely have a mental image of a “classic” one as a big ball of stars, as suggested above; typically, they’re somewhat brighter near the center and darken gradually out to the edge. M30, like a few others, departs from that—it’s significantly brighter at its core, because the population of stars there is much more tightly concentrated than the already-dense groupings of stars in most globulars, and the core is relatively small.

Numerous sources, including Prof. James Kaler’s The Cambridge Encyclopedia of Stars and Jeff Kanipe’s Annals of the Deep Sky, discuss the likelihood that M30 has undergone core collapse. Such an event is thought to result from the sorting of stars in the cluster; interactions between massive stars and lighter ones tend to send the former towards the cluster’s center, and the latter ones away from it—some of these lighter stars get flung out of the cluster entirely. The grouping of the massive stars near the cluster’s center leads to the collapse, and the star density there climbs dramatically.

In M30, this effect is clearly seen in small telescopes, including the 4-inch Maksutovs I had a chance to observe through in early September. One night, the skies were especially “soupy” and I could only barely see Capricornus’s “bikini” naked-eye, but M30’s small inner core was easily visible in a 4-inch with direct vision at 50-85x. Averted vision showed the cluster’s halo glowing quite a way out from the center.

An important aspect of amateur astronomy for some folks is that along with our visual observations, we bring our own minds and understanding to bear on the targets—in M30’s case, then, there are the “blue stragglers.” This type of hot, blue star is found in other globulars, but M30’s are often mentioned, and they used to be something of a conundrum. On the one hand, the population of stars in globulars is old—typically nine billion years or older, and about 12 billion in M30’s case. Blue stars, though, ordinarily have much shorter lives than that—they shine brightly, but burn out quickly. If all the stars in the cluster presumably formed at about the same time, why are there still blue “stragglers” alive in M30 (and elsewhere)?

The answer, astronomers now understand, involves M30’s core collapse. The great density of stars near the cluster’s center means that stars are frequently interacting with each other, and in some cases, grabbing hydrogen from each other. (One way is a binary star with a tight orbit siphoning gas from its companion, and another is by collision.) The different methods produce subtle differences in color and spectra in the stragglers—if you’d like to become a “blue straggler connoisseur,” there’s plenty of material about that in the books above, and on the web…

One last observational note about M30 before moving on is that in 8-inch ’scopes and up, the cluster is known for streams or chains of stars sprouting from the north or northwest side. Observers with 12-inch ’scopes and up often report three streams, and those with smaller apertures, just two.

To get to M30, have a look at the left (eastern) side of Capricornus’s bikini shape; about a third of the way down along the outline, you’ll see a noticeable pair of stars less than a degree apart, just where the outline bends—the lower and brighter of the two is 4th-magnitude Zeta (ζ) Capricorni. Look about 3½° roughly eastward of Zeta, and you’ll see an unassuming 5th-magnitude star, 41 Capricorni (“41 Cap” for short); M30 is less than ½° west of it. Though far from bright, 41 Cap sits out there almost alone when viewed naked-eye, and shouldn’t be hard to recognize (there are only two other naked-eye stars within two degrees of it, and they’re over a magnitude dimmer at magnitude 6.4 or so).

Centering 41 Cap in your Telrad would put M30 near the middle of a finderscope field, if the finder is strong enough to show the cluster—it should faintly appear as a fuzzy object in a 9×50, but if it doesn’t, that’s no big deal. Just nudge your ’scope gently from 41 Cap towards Zeta—a ¼° bump will put M30 into your telescope’s eyepiece, and a ½° bump (the same as the innermost circle on your Telrad) will roughly center it. For folks with equatorial mounts, centering 41 and slowly slewing westward (i.e., with your Right Ascension control) will put M30 in your eyepiece.

On a lousy night, when 5th-magnitude stars like 41 Cap aren’t naked-eye visible, put Zeta and its pair-mate, b Cap, a bit north of the western edge of your finderscope field. 41 Cap should be visible on the eastern side, or almost directly opposite Zeta and b. Once you center 41, you can nudge the telescope as described above.

Our second target, Dabih, or Beta (β) Capricorni (β Cap for short) lies on the other side of Capricornus, at 20h 22m, -14° 43’, just below the bikini’s top-right (northwestern) corner. It’s a very easy multiple star, with its brightest components about 205” apart. That’s wide enough for them to be a naked-eye split, at least in theory, and far enough for the companion to have its own name, Dabih Minor, or Beta2 Cap (the brighter component, as you might guess, is Dabih Major). They’re also easy because they’re relatively bright, at magnitudes +3.1 for Dabih Major and +6.1 for Dabih Minor—in a dark sky, either star would be visible on its own to the naked eye. (You might not actually be able to see Dabih Minor without optical aid, because of the glare from Dabih Major.)

Dabih lies about 330 light years from Earth, so the fact that either star in the pair is naked-eye visible at that distance tells us that both of them must be intrinsically bright—at the same range, our own Sun would be a feeble magnitude +10, too dim to be seen, even in a 6×30 finderscope.

Taken together, Dabih’s distance and the components’ angular separation also tells us how far apart those components are in space, and the answer is at least 21,000 AU. (One AU, or astronomical unit, is the distance from Earth to our Sun.) 21,000 AU is also about a third of a light-year, so light takes four months to travel from Dabih Major to Dabih Minor, or vice versa—in contrast, light takes just hours to go from the Sun all the way out to Pluto.

According to Prof. Kaler, the stars’ distances apart and their masses suggest an orbital period of roughly 1,000,000 years. He also notes that the main star, a cool giant about 35 times the diameter of our Sun, has a companion in a (relatively) small orbit around it—you won’t see it, but the companion can be detected with a spectrograph, and it orbits the main star in just under four years. “But wait!” It’s binary too—so Dabih Major is a three-star system. Dabih Minor also has an unseen companion, so the entire Dabih system contains at least five stars.

Ironically, even though the complexity of the system will exist mostly in your mind’s eye, with only the two main components visible (that is, Dabih Major and Minor), they look like they have company in a telescope eyepiece. A third star lies about the same angular distance from Dabih Major as Dabih Minor does, making for the appearance of a triangular star system. As it happens, this third star lies more than 100 light-years farther from us than Dabih, so it’s just a line-of-sight coincidence, but it makes for a beautiful arrangement nonetheless. (At magnitude +8.8, this third star is almost three magnitudes dimmer than Dabih Minor, so you shouldn’t have much trouble telling the Dabih members from the interloper.)

Illustration of eyepiece view of the Dabih system (north is up). Dabih Major is at center, and Dabih Minor is to its right.
Image: Z. Singer

Viewing the system was lovely at just over 50x using a 25mm eyepiece in a 4-inch Mak, and the split was easy. Doubling the power with a 12mm eyepiece, the third star was more visible and Dabih’s colors stood out a little better – a pale wheat yellow for the primary and a subtly blue-tinted secondary.

Finding Dabih is simple—as mentioned above, it’s near the “top-right” of Capricornus’s bikini. If you look at that corner, you’ll see a reasonably bright star there—sharp-eyed observers might see a pair, about 6’ apart. Whether as a single star or a pair, that’s Algedi, aka Alpha (α) Capricorni. Dabih is the next bright star you’ll see when you glance down the right side of the bikini, roughly where you’d imagine “bikini strings,” if there were any. Easy-peasy!

My warmest thanks to Sorin of Mile High Astronomy, and to July Candia, for the opportunity to do extended observing with their respective 4-inch Maksutovs.

—See you next month.

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