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Center of the Milky Way: August 2015

by Zach Singer

From the author: With the aim of making this column useful to everyone, beginner and experienced observer alike, we’ll be looking at fewer targets beyond the solar system, but spending more time describing each one—just like we did last month. We’ll continue the separate “Getting Your Bearings” section (see the pdf version of the DAS Newsletter) whenever space permits, so beginners can find that month’s stars and constellations without slowing us down here. I hope that regardless of your level, you’ll give each target due consideration—the experienced observer may rediscover forgotten objects, while the beginners can get a feel for what’s out there—the “range of possibility”— even if they can’t yet get there on their own.

Planets

Compared to June and July, when Venus and Jupiter put on a show, August promises to be relatively laid-back for planets… The two just mentioned are deep in solar glare, and will reemerge as morning objects this fall. Saturn, though, can be found right where we left it last month—almost exactly, in fact, since its slowing retrograde motion kept it from shifting very far. Our distance from Saturn has been increasing since opposition a few months ago, but the planet is only a little smaller in the eyepiece, and good views can still be had. Keep in mind that by September, Saturn will be both smaller and lower in the sky, so enjoy the views while you can.

Then again, “lose a planet, gain a planet”— if you’re up early on the morning of the 20th, look for Mars in M44, the Beehive. The spectacle will occur low in the brightening pre-dawn sky, with the still-faraway planet just 4° above our eastern horizon at 5 a.m., rising to almost 6° by 5:10. If that all seems a bit marginal, well… Mars is definitely coming, and will be more and more of a fixture in the sky as it approaches opposition next spring. Uranus, too—it rises around 11 p.m. at the beginning of the month, and two hours earlier at month’s end. Look for the tiny blue orb in Pisces (at 1h 16m, +7° 20’ midmonth), about ½° from Zeta (ζ) Piscium.

As always in August, there’s the Perseid meteor shower, and we’ll have favorable conditions: During the shower’s peak, after midnight on the 13th, the moon will be absent; when the pencil-thin crescent finally clears the horizon at 5:22 a.m., it will hardly matter. (If you’re still up by then, though, the moon will be worth a look.)

Deep Sky Objects

Before we start on deep-sky targets, there is our galaxy itself — at 10 p.m. on a mid-August night, the very center of the Milky Way glows brightly in the south—an unmistakable landmark in dark skies, and an amazing sight on its own. Let your eyes dark-adapt properly, away from city lights, and they’ll show you a detailed, yet panoramic view of a galaxy (ours!) that no actual telescope can match with any deep-sky galactic competitor. Right in front of you, as you face south, is the central “bulge” within the Milky Way’s flying-saucer shaped cross-section, a view which includes dark lanes, clusters, and nebulae—details you’d find difficult or impossible to see in other galaxies with a telescope.

We’ll start in the “teapot” of Sagittarius and work our way upward — if you’re not familiar with the constellation, though, check out this month’s “Getting Your Bearings.” These past few months, we’ve been looking at globular clusters, because as summer progresses, Earth’s night sky faces the center of our galaxy—where globulars are most commonly found. Since the Sagittarius “teapot” is seen in front of the center of the Milky Way (from the Earth’s point of view), you’d expect to find a great number of globulars in this area—and you’d be right. There are at least 10 of them in the teapot or directly adjacent to it, and at least 20 more scattered in the bright patch of the Milky Way north and west of the asterism.

M22 (18h 38m, -23° 53’), our first telescopic target, can be glimpsed with the naked eye under good conditions. It’s the glow you’ll see to the northeast of the star at the top of the teapot, Lambda (λ) Sagittarii (also known as Kaus Borealis on some maps). In poorer conditions, or if your eyes aren’t fully dark-adapted, you can center the star in your finderscope, and find M22 by moving your scope gently eastward and “up” (northward) a touch. A short hop will put M22 in your finderscope well before Lambda exits the view.

Compared to last month’s showpiece, M13, the visual impression of M22 is that it’s more “spread out”—almost like an “oval open cluster.” Individual stars are easily resolved, but somewhat “mushy” looking, because M22’s southern declination brings it low in the Denver sky, where our turbulent atmosphere scatters its light. Don’t let that discourage you, though—M22 is impressive, and it offers something that M13 can’t—a beautiful, star-laced Milky Way background. To see detail in the cluster, try 125X to 200X magnification, but don’t forget to back the power down and get a wide-field view, too—M22, with our galaxy’s center behind it, is very much worth seeing!

Unlike M22, M54 (18h 56m, -30° 27’) isn’t a showpiece at all—as we see it here on Earth, it’s much smaller looking and less distinct than M22, with stars remaining unresolved in my 12-inch scope. It is an interesting object, though, for what and where it is—a very large globular, boasting a physical diameter more than double M22’s (and nearly double M13’s), and packing an absolute magnitude that makes M54 intrinsically more than 6 times brighter. If M54 were located where M22 is, it would be an easy naked-eye object glowing brightly at about magnitude 3.1, compared to M22’s mag. 5.1.

The discrepancy between M54’s actual characteristics and the way we see it is due in part to its great distance—some 86,000 light years— and in part to the scattering of light by inter-vening dust particles at the center of our galaxy. Ironically, although M54’s line-of-sight projects it onto Sagittarius and our galactic center, M54 lies well beyond—in another galaxy! That’s right—modern measurements suggest that M54 is a member of the Sagittarius Dwarf Elliptical Galaxy, which though closer than the Large Magellanic Cloud, still lies a considerable distance “behind” the center of the Milky Way from our point of view. Looking through the eyepiece while considering this puts M54 in a whole new light—and M22, too, as a now“ foreground” object at a “close” 10,000 light years!

In theory, M54 is easy to find, as its 7.6-magnitude brightness (and surface brightness about the same as M22’s) should make it visible in a finderscope. If so, you’ll see it right away by centering Ascella, Zeta (ζ) Sagittarii, in your finderscope (the star is where the “base” of the teapot meets the “handle,” towards the southeastern part of the asterism).If visible, M54 will be near the western edge of the finder’s field. Considering the conditions we’ve had in the Denver area these last many months, you may not be so lucky—I wasn’t, when I observed this globular a few weeks ago. To find it while “flying blind,” you’ll need finely calibrated setting circles, a go-to system, or more precise Telrad techniques than I can fully describe here. For the latter, though, pay strict attention to the angles from Ascella to M54, and how they apply to the season and time of night. It is possible to find M54 with a Telrad without seeing it first—I hit the globular that way on my second try.

Moving northward, or pretty much “upward,” at our stated time, you’ll see brighter patches within the Milky Way’s glowing band. Among the first you’ll encounter after leaving the teapot is the Lagoon Nebula, M8, a huge star-forming region containing a beautiful cluster of the stars it gave birth to, and which now sets the nebula itself alight.The whole complex is about 1½° across, so rich-field telescopes—and even finderscopes—will give you a great view. Going up to moderate power (125X) boosts the contrast so the nebula becomes clearer, but I’ve found a good UHC filter to be worthwhile, too. At the same time, moderate power (without the filter) brings out the cluster itself, which is worth an extended look on its own.

Don’t let the suggestion of the filter throw you—M8 is to summer skies what the Orion Nebula, M42, is to the winter — not just a showpiece, but a sure entry on most observers’ “Greatest Hits” lists. M8 isn’t just bright enough to be seen naked-eye in dark skies, it also remained visible with moderate haze in the “semi-dark” skies south of Chatfield Reservoir, and still showed nebulosity in the telescope there.

Navigation to M8, at 18h 05m, -24° 30’, is straightforward; since it’s bright enough to be seen and pointed at directly, the only problem is making sure you’re actually pointing your scope at the right bright patch among the many. To get to the Lagoon, just imagine the teapot’s “spout” steaming straight up (relative to the teapot’s base), and follow the steam up to the nebula. You’ll find it about the same distance from the tip of the spout, as the spout itself is long. If you’re in the right place, the Lagoon Nebula’s stars will show as a more-or-less horizontal “bar” in your finderscope; when you center the Lagoon, the Trifid Nebula will share the finderscope field to the north (note that the Trifid would then be near the bottom of most finderscopes’ inverted view).

As both a crosscheck on finding M8, and the way to the next object, look north and slightly eastward from M8 and you’ll find Polis, or Mu (μ) Sagittarii—it’s the first reasonably bright star in that direction (for reference, it’s about the same brightness as the star at the tip of the teapot’s spout). Polis is about the same distance from the teapot’s top, Lambda (λ), as the teapot’s top is from M8.

Now that you’ve got Polis, our next stop, M24, the Small Sagittarius Star Cloud, is a quick hop away—to get to M24, at 18h 19m, -18° 23’, imagine a line from M8 to Polis, extend past the star for about the same distance, and look for the bright patch just above (north of) where you land. You’ll know M24 immediately when you see it—a huge, dense field of stars 1½° across. Use the lowest power you’ve got to start with, both to ease navigation and to get the widest view — but when you’ve taken it all in, try sweeping the area at high power and lose yourself in the expanse.

Realistically, this column could remain in Sagittarius, touring object after object, for months — so the objects we have covered here are more of a “taste” than a comprehensive list. Like every “group tour” you’ve ever been on, we’ll move on now, before we’ve really seen it all.

Unlike the deep-sky objects we saw in Sagittarius, of which the closest (M8) was about 4,700 light years from us, our next target is a multiple-star system in “our local neighborhood,” at a mere 162 light years from Earth: Epsilon (ϵ) Lyrae. Also known as “The Double-Double,” it’s a fascinating system at 18h 45m, +39° 41’, near enough in the sky to the bright star Vega to make finding it simple. To get to Epsilon Lyrae, just center Vega (see this month’s “Getting Your Bearings”) in your finderscope—Epsilon will share the field, and with the finder’s modest magnification, it will be clearly split in two.

It’s when you have it in your telescope, though, that the magic really happens—the “single” star, which became a “double” in the finder, begins to separate yet again, until at high power it’s clearly revealed as two close pairs—four stars in all! These pairs aren’t just line-of-sight coincidences, they’re actual binary pairs, with each pair orbiting its own center of mass—while both couples orbit the whole system’s combined gravitational center.

With the tight pairs’ separations of 2.2-arcseconds and 2.8-arcseconds, my 6-inch reflector can split both at 150-200X in Denver’s choppy skies, though on worse nights, the closer pair may only hint at separation. According to Professor James Kaler (Univ. of Illinois), the average orbital distances within the pairs are about 145 and 235 astronomical units, respectively (but with very eccentric orbits leading to much larger and smaller values at the extremes). The distance from one pair to the other is at least 1/6 of a light year, with an orbital period estimated at 400,000 years or more. If you’d like to add an extra touch of wonder or complexity, though, consider this—data now suggest that the easternmost star of the four may also be double….

—See you next month.

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