July Skies 2018

July 2018 Skies as seen from Denver Colorado.
Viewing due south in Denver at 10:30 PM in mid-July. Note position of M19 (shown just right of center) slightly above the imaginary line that runs between 36 Ophiuchus and Sigma (σ) Scorpii. (Some labels use “Oph,” the standard abbreviation for Ophiuchus/Ophiuchi, for clarity.) –Object positions, constellation and meridian lines charted in SkySafari, and then enhanced. (Tap on image above for larger version.)

© Zachary Singer

The Solar System

The big news for July is that Mars comes to opposition on the 27thmeaning that it will be at its highest in the south on that date around 1 AM, and also more or less at its largest as seen from Earth. Now, don’t let that fool you—Mars is already very good as July begins, showing a disk 21” across, which is better than we got two years ago, and only slightly smaller than the 24” expected at the end of the month. (Observations in my 6-inch reflector at the end of June showed an impressive disk at just 150 power.) Realistically, Mars will maintain its size and brightness at similar levels through the end of August, so don’t restrict yourself to observing it only at opposition!

For what it’s worth, we usually turn our ’scopes towards the planets when they’re highest in the sky on a given night, to get the sharpest image—but Mars is worth viewing naked-eye when it’s rising. Surprised? The red (well, orange) planet appears even redder when rising, making for much deeper color. It’s a guilty pleasure on an aesthetic level, if not a scientific one. If you want to indulge yourself, Mars rises around 10:30 PM at the beginning of July, an hour earlier mid-month, and about 8:20 PM at month’s end.

Meanwhile, Mercury is an evening target, low in the west in early July as it approaches its greatest elongation (largest angular distance from the Sun as seen from Earth) on July 11th. After passing that mark, the planet appears increasingly closer to the Sun until getting lost in sunlight about mid-month. While Mercury remains visible, telescopic views will show a roughly half-illuminated disk 7” or 8” across. Look for a challenging binocular pairing with a very thin crescent Moon on the 14th; 0.7-magnitude Mercury will lie just over 2° below and slightly rightward.

Venus remains a bright and beautiful object, obvious in the west after sunset, at better than magnitude -4. Its disk grows from 16” to 20” at the end of the month, as its phase progresses from gibbous to roughly half-illuminated, and will appear even larger and brighter next month. Venus has a pairing with the Moon a day after Mercury’s, on the 15th; the duo will fit inside a 2° field.

Jupiter is high overhead as July begins, transiting soon after sunset. Its apparent diameter shrinks from about 41” to 38”, but the planet will be a wonderful target throughout July. Next month, Jupiter will be noticeably lower in the southwest after sunset, so enjoy it while it’s still well-placed.

Because Saturn was at opposition in late June, it appears highest in the south just before 1 AM as July begins; by month’s end, that transit comes at 10:35 PM, so the ringed planet will be in a terrific position for observing all month. The 0-magnitude planet appears just above the top of Sagittarius’s “teapot.”

If you’re new to observing Saturn, a moderately sized (5- or 6-inch) telescope will give you a tiny, jewel-like, and bright view at 60x, with two moons, Titan and Rhea, likely visible. Under good conditions at 100x, you might see a bit of the shadow between the planet and the rings, and perhaps a hint of the bands in Saturn’s atmosphere. At 150x, Saturn won’t look “small” in the eyepiece anymore and details, like the shadows mentioned above, should be more easily visible.

Uranus is about 20° up in the eastern sky around 3:30 AM in early July, and much better, at more than 40° up at the same hour at the end of the month. You’ll find the planet about 4½° northeast of Omicron (ο) Piscium mid-month—a little less in early July, and a little more towards the end.

Neptune, like Uranus, is a late-night object, but you’ll find it 20° up an hour earlier than Uranus, at 2:30 AM. (Similarly, Neptune will be more than 40° up at 2:30 by month’s end.) Look for Neptune a little over 1° west of Phi (ϕ) Aquarii all month—closer in early July and farther at month’s end.

Stars and Deep Sky

This month, we’re going to have a look at some objects that might take a little experience to appreciate—they’re not “showpieces.” Beginners are welcome to come along for the ride, and these targets shouldn’t be too difficult to find. Still, if you’re looking for a better starting place for a beginner, check out this column’s previous July issues, in the Observer on the web, at www.denverastro.org/?page_id=13. If you look at the column’s “Stars and Deep Sky” section from the last three years (usually starting on page 6), you’ll see enough to keep you observing for hours, and if you want more, check out the June and August issues too—the objects will still be up; just offset a little to the east or west….

Our targets this month officially lie in Ophiuchus, the Serpent Bearer, an ancient constellation. It doesn’t have the name recognition of flashy Orion or Scorpius, so it’s sometimes overlooked by newer observers, but its position just above Scorpius makes it easy to become familiar with.

Our first target is the multiple-star system, 36 Ophiuchi (36 Oph for short), at 17h 16m, -26° 37’. 36 Oph is unusual, in the sense that it’s made up of class K orange dwarves—that is, each of its three stars has slightly lower mass than our own Sun, and therefore, burns cooler and less brightly.

Ironically, such stars are numerous in our galaxy, but we don’t get to see many of them naked-eye, because they’re not luminous enough to be visible beyond a moderate distance—in 36 Oph’s case, the system is just under 20 light-years from us, and the brighter two of its three stars are each visual magnitude +5.1 (our dark-adapted vision, away from city lights, can usually take us down to magnitude +6, or perhaps +7 under exceptional circumstances). Stars like these would be near the limits of human vision at around 40 light-years from us, a short distance as far as stars are concerned—for comparison, many of the bright blue stars we see may lie hundreds of light-years from us, but appear brighter.

The K-stars’ cooler outer atmospheres also mean a more orange-looking appearance than most naked-eye stars (orange giants, a type of dying star, are one notable exception). Most naked-eye stars are hotter and thus bluer; because they’re inherently brighter, they remain visible over longer distances, and so we see more of them.

The two +5.1-magnitude stars, 36 Oph A and B, appear about 5” apart as seen from Earth, and form a binary pair (they’re in orbit around each other). According to Professor James Kaler, University of Illinois, they’ve been observed for centuries, but astronomers are still working out the exact parameters of their orbit (some aspects of their expected masses and orbital periods are inconsistent with each other, implying an error somewhere). In spite of that, we do know that the orbit is highly elongated, and that the pair’s average distance from each other is about 80 astronomical units (AU), or about twice Pluto’s distance from the Sun. The system’s third star, 36 Oph C, is slightly cooler (and thus slightly dimmer and redder); it orbits at least 4,400 AU from A and B, about 110 times Pluto’s distance from the Sun. Prof. Kaler estimates that 36 C takes 180,000 years or more to circle the inner pair.

In my 6-inch Newtonian, the 36 Oph system is a beautiful trio; as suggested by the stars’ color classes and brightnesses, the inner pair appears wheat-colored, while the outer star displays a more orangey hue and is a touch dimmer. (Colors were distinct even though there was a nearly full moon beaming down the barrel of my ’scope.)

Under good seeing conditions, the inner, A-B pair remained tight but split clearly at 100x. It also split at 60x with just a hairline between the two stars, but it will take a practiced observer to pull this off. Under a steady sky, a 150x view didn’t add much, but it might provide an easier split for beginners; I also found it took 150x to separate the inner pair when the seeing wasn’t ideal. The outer, “C” companion is an easy split, even at very low power—you’d hope so, since it’s over 12 arc-minutes away! At that distance, though, it still shares an eyepiece field, even in a narrow Plössl, at 150x.

36 Oph is the westernmost of an arc of four naked-eye stars which includes 3rd-magnitude Theta (θ) Oph—36 is the next-brightest star “down and to the right,” or southwest of Theta (the other two stars lie to Theta’s northeast, 4th-magnitude “b” and “c” Oph). If you’re familiar with Ophiuchus, you can get to Theta by looking first for 2nd-magnitude Sabik, aka Eta (η) Oph, the “leftmost” or easternmost star at the bottom of Ophiuchus’s “coffin.” You’ll find Theta about halfway between Sabik and the “tail” of Scorpius (see chart).

If you’re not familiar with Ophiuchus, don’t worry—look for Antares, the bright, orange star in the body of Scorpius, due south around 10:30 PM in mid-July. (If you’re not familiar with the constellation Scorpius, you’re missing a lot—it’s covered in the July ’16 and ’17 Observer.) Looking upward, or northward, about 15° from Antares (that is, just a bit less than the distance from Antares to the stars in the scorpion’s stinger), you’ll see three bright stars making a wide, gentle arc—the arc’s span is about the same as from Antares to the stinger. The highest and westernmost of them is magnitude +2.7 Delta (δ) Oph, and it’s about 45° up at our stated time; the middle one is Zeta (ζ) Oph, directly “above” Antares, and the lowest, at left (or easternmost) is Sabik, which is also the brightest of the three. (From Sabik, of course, drop down to Theta, as described above, and you’re on your way.)

Our second target, the globular cluster M19, lies less than 3° westward of 36 Oph, at 17h 04m, -26° 17’. To fully appreciate this object, it helps to know that a globular cluster is supposed to be globe-like, or roughly spherical. Since these star-clusters rotate, they may appear subtly bulged at their equators, for the same reasons as the Earth or the other planets in our solar system do. M19, though, appears conspicuously oval-shaped, even in small telescopes—it’s among the most oblate globular clusters in our galaxy.

Some sources propose that M19’s shape comes from its proximity to the center of our galaxy, implying that tidal forces (the uneven force of gravity across an object) are the cause. (If we could view our Milky Way from the outside, edge-on, we’d see M19 floating “above” and close to the galaxy’s central bulge.) Interestingly, the cluster’s long, or “major” axis, lies perpendicularly to a line drawn between the centers of the cluster and our galaxy.

Unlike the “showpiece” globulars, like M13 or M5, M19’s individual stars are difficult to resolve, and the cluster isn’t as bright-looking. In part, that’s because M19 is actually a tad dimmer in the first place; its light also travels a slightly longer distance to reach Earth, thus dimming the light further.

However, when we look at M13 or M5, we are also looking more directly “outward,” or away from the plane of the Milky Way, so there is much less obscuring dust and gas to penetrate. The effect is similar to what we encounter when we look through our Earth’s atmosphere—we see objects more sharply when we look towards the zenith than we do when looking towards the horizon; in the latter case, the view lacks contrast and is dimmer and redder. In M19’s case, we’re looking much closer to the Milky Way’s axis, and thus through a much thicker slice of the galaxy, and so, predictably, the view is dimmer, redder, and lacks “punch.” Ironically, then, the same closeness to the galactic center that may give M19 its interesting shape also prevents us from seeing it as well as we’d like.

(Sadly, I must note our especially dismal conditions in Colorado as June ends, due to both inclement weather and several large wildfires. At magnitude +6.7, this object should be reasonably bright, but with our murky transparency these days, you could well find M19 dim even in a large ’scope. Don’t give up; just wait out the conditions.)

To find M19, start by centering your Telrad on 36 Oph (it’s already done if you were just observing it!). Then slide the Telrad 3° toward Sigma (σ) Scorpii, the bright star just west of Antares. As a guide, stop when the trailing edge of your outer (4°) Telrad circle lies midway between 36 Oph and the nearest part of the inner (2°) circle. If M19 isn’t in your telescope’s low-power field, try nudging the ’scope towards Sabik, in Ophiuchus; if still no luck, spiral around the area carefully. As a further cross-check on positioning, M19 lies almost exactly on the imaginary line between Mu (μ) Scorpii and Sabik, and about midway between the two; use the line’s intersection with the 36 Oph-Sigma Scorpii line to improve your positioning, if necessary.

(Note that Sigma Scorpii shares its Arabic-derived name, “Al Niyat,” with a similar-looking star just on the other side of Antares, so it’s easy to become confused—go by position and Greek letter [Bayer designation] instead. If it helps, the “other” Al Niyat is Tau [τ] Scorpii.)

—See you next month.

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