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April 2019 Skies

by Zachary Singer

For April, we’re looking at a beautiful binary in Leo, and some galaxies in a tight grouping—but perhaps not the one you’re guessing! First, though, we have the planets….

The Solar System

Technically, Mercury will be up before the Sun in early April, but even at greatest elongation on the 11th (when the planet appears farthest from the Sun on this orbit), it will remain very low over the horizon, less than half an hour before sunrise.

Venus, on the other hand, is a bright and easy target. It was gorgeous with naked-eye viewing, just before dawn recently. Telescopic views will present a waxing gibbous phase in early April, becoming more fully illuminated as the month progresses. By month’s end, though, Venus will be less than 6° above the horizon, a half-hour before sunrise—about 2° lower than at the beginning of the month.

Mars is in the constellation Taurus throughout April. Now at magnitude +1.5, dimming as its distance from us increases, the planet is outshone by the adjacent orange-giant Aldebaran. This Mars pass is slowly drawing to a close—the planet is down to a measly 4” or so and shrinking. Still, there will be some good views before Mars loses itself in sunlight—for example, a twilight pairing of Mars and Mercury in mid-June.

Happily, Jupiter is already transiting (crossing the Meridian, so highest in the south) before 6 AM, or 45 minutes before sunrise, as April begins. (It’s striking in a pale aquamarine sky in morning twilight!)

By the end of April, the planet crosses the Meridian at 4 AM, so early risers now have an increasingly favorable setup for observation—especially since Denver’s skies are typically calmer in the wee hours, and more likely to provide a sharp telescopic view. Look for it halfway between the bright stars of Scorpius and Sagittarius.

Early on the 23rd, Jupiter and the Moon’s center are less than a degree apart—it will be a great view in binoculars or a wide-field telescope, and naked-eye too.

Saturn is also well up, 45 minutes before dawn, in early April, sitting 24° above the southern horizon at that time. Look carefully for Saturn, though—at magnitude +0.6, it’s no match for Jupiter’s showy magnitude of -2.2. By month’s end, Saturn is high in the south, approaching the Meridian 45 minutes before dawn.

Uranus is very low on the western horizon in evening twilight as April begins, sinking into the solar glare this month. It will become a reasonable pre-dawn target in early June.

Neptune is also lost in sunlight this month. It should emerge as a pre-dawn object in the wee hours, late in May or in early June.

Stars and Deep Sky

Chart of the constellation Leo.

Viewing south and up in Denver at 9:30 PM in mid-April. (Telrad included for scale.) Note the imaginary line running from Mu (μ) Leonis through Algieba, and on to M105. –Object positions, constellation and meridian lines charted in SkySafari, and then enhanced.  Click on image for enlarged version.

Moving on to the deep sky, we’ll be looking at the western side of the constellation, Leo, the Lion. If you’re not familiar with Leo, check out the beginning of the “Stars and Deep Sky” section of the April 2017 Observer (pp. 6-7), to get yourself running up at speed. (In mid-April, Leo’s head is due south at 9:30 PM; with its bright stars and distinctive shape, it’s not hard to find.)

Our first target is the bright, golden-yellow binary called Algieba (from “the lion’s mane” in Arabic, according to Robert Burnham Jr.’s Celestial Handbook); it’s just as commonly listed as Gamma (γ) Leonis, or Gamma Leo, for short. Located at 10h 21m, +19° 45’, Gamma Leo sits right where its Arabic name suggests—in the curve of the “sickle” or “question-mark” shape that suggests the lion’s mane. At magnitude +2.2, it’s easily found, even under southern Denver’s light-polluted skies. (It’s worth becoming familiar with this star, not only for its own interest, but also because we’ll use it to point towards our second, more-challenging, target.)

Gamma Leo has been one of my favorites for years—it’s tight, bright, and gorgeous. I’ve often shown it to the public at star parties (and the Denver Astronomical Society’s Open Houses) around this time of year, and they marvel at its appearance (as well as the idea that the two stars are actually in orbit around each other).

Gamma’s components appear just 4.4” apart, so you’ll need at least moderate power to split the pair—I’ve found that 60x looks very tight, but it’s doable; 125x is still tight but not difficult; and 200x is easy. A 4-inch ’scope won’t have much trouble hitting these magnifications, and the pair remains quite bright with this small aperture. Since the companion is less than 1½ magnitudes dimmer, there’s no problem with the glare of one component washing out the light of the other—in the eyepiece, they’re an “almost-matching” pair.

Because of their roughly equal brightness and “tight-but-not-too-tight” separation, this system is a good target for beginners looking to improve their skills in observing double stars—try 120x or 60x, then work your way down, to lower powers, just to see how close you can get these two and still split them. While Gamma isn’t usually difficult to separate, all things considered, it can be tough or impossible on a night with poor seeing—if the stars appear to twinkle with naked-eye viewing, or look “choppy” in your telescope’s eyepiece, don’t be surprised if Gamma doesn’t split—just try on a better night.

As always, observing reports differ, and I’ve seen many that suggest that Gamma’s stars have slightly contrasting colors, but I’ve never noticed that myself. In my own experience (as well as the comments that guests have made), Gamma is a matching golden pair, color-wise. That fits with the stars’ color temperatures and spectral classes, which aren’t vastly different from each other—Prof. James Kaler, of the University of Illinois, lists the main component as a class K1 giant with a color temperature of about 4400 K, and its companion as a class G7 giant glowing at about 4900 K.

Ironically, the smaller star here is reminiscent of the larger component in Iota Cancri, which we toured last month (see “Stars and Deep Sky” for March 2019). Gamma Leo’s stars appear brighter to us, because they’re closer—just 130 light-years from Earth, compared to some 330 light-years for Iota Cancri.

If we could put Gamma Leo at Iota Cancri’s distance, we’d also see the Gamma system’s separation of 4.7” narrow down to roughly 2.5”—a rather tougher target, and one that contrasts sharply with Iota’s wide, 30” gap! Iota Cancri’s pair lie about 2,800 astronomical units (AU) apart; in contrast, Gamma’s are “just” 240 AU apart at this time. Still, this smaller orbit is about six times Pluto’s average distance from the Sun.

The exact parameters of Gamma Leo’s orbit are still being worked out, because a full cycle takes five centuries, and we don’t yet have enough data for exact numbers. We do know that the orbit is highly eccentric—according to Prof. Kaler, it varies from 26 AU out to 313 AU, but some of the data we have is inconsistent with other data points, or expected data points. Prof. Kaler has a wealth of information about Gamma Leonis—including an explanatory diagram and details of a large planet orbiting the larger component—you can visit that page here, and I suggest you do!

Directions

Once you’ve become familiar with Leo’s overall shape, finding Algieba is straightforward—look first at the “sickle” or “backward question mark” shape that distinguishes Leo’s head, and find the brightest star, Regulus, at the question mark’s base (roughly where the “dot” would be on a real question mark). From there, look at the next two bright stars to the north. The first one, Eta (η) Leo, lies “straight up,” relative to the question mark shape (and also straight up from the horizon at 9:30 PM in mid-April); it’s also where the question mark bends sharply to form a curve—Algieba is the next bright star along that path, and it’s noticeably brighter than Eta. If you look at the chart, you’ll notice that Algieba is about the same distance from Eta as Eta is from Regulus, making a distinctive pattern.

Next up, the elliptical galaxy M105 and its “nearby” companion, NGC 3371—the former is at 10h 49m, +12° 29’, and the latter is just over 7’ to the northeast (NGC 3371 is also commonly listed as NGC 3384). Over the last several years, we’ve toured a good number of galaxies, but most of them have been spirals (NGC 404 in Andromeda, aka Mirach’s Ghost, is one notable exception). While “a change of pace” is one reason to look here, these two galaxies are also quite close together—even closer than the famous “Leo Trio” farther to the east—even a somewhat narrow, ½° eyepiece field will easily show M105 and NGC 3371 together, while the Trio (or “Triplets,” as they’re sometimes called) needs double that to avoid a cramped view.

Elliptical galaxies are a bit like the central part of a spiral galaxy—not just in shape, but in terms of their stellar population—both have a much older, “Population II,” collection of dim, yellow stars. (As we’ve seen in other tours, the stars here are yellow because they exhaust their nuclear fuel far more slowly than hot, blue stars—and thus survive over the long haul, well after the blue stars, and the more intermediate white stars, are gone.) Generally, ellipticals (and the center of spirals) lack the gas needed to form new stars, so just the old (yellow) ones are left. In contrast, stars in a spiral’s arms continue to form from the gas that’s usually abundant there—because there’s a ready supply of new stars, and many of them are hot, blue, and brighter than the others, spiral galaxies’ arms are distinctly bluer than their cores.

M105 has an absolute magnitude, or intrinsic brightness, roughly that of our own neighbor, M31, the Great Andromeda Galaxy—that is, they’d appear about equally bright if they could be viewed from the same distance. (M105 is much more compact, with a diameter less than half of M31’s—M31 is a spiral, though, and its arms account for much of its diameter.) M31 is famously close to us, at about 2.5 million light-years from Earth—it’s near enough to be seen naked-eye under a sufficiently dark sky. M105 lies much farther away, about 37 million light-years distant. At that range, M105 needs a telescope to become visible.

M105’s neighbor, NGC 3371, lies relatively close to M105—they’re about 6.2 million light-years apart, or a little over double the distance between our Milky Way Galaxy and M31. Lying about 31 million light-years from us, NGC 3371 is actually closer than M105 is—but it looks about three-quarters of a magnitude dimmer to us, because NGC 3371 really is dimmer. It’s also a bit smaller—and it’s a lenticular (“lens-shaped”) galaxy.

Lenticulars are generally described as a form between those of ellipticals and spirals. They’re flatter-looking, or more disk-like when seen from the side, than ellipticals, and thus actually a touch more like the core of a spiral—NGC 3371 may even have a central bar, like some spiral galaxies do. Like ellipticals (and spirals’ cores), they’re made up of old stars, with little or no new star formation. While lenticulars are known to have reserves of dust, much of the literature on them suggests they’re lacking gas for star formation, like ellipticals. There are at least some exceptions though, like NGC 404 (“Mirach’s Ghost,” mentioned above). Recent UV imaging shows a ring of star formation there—see page 7 of the November 2015 Observer.

M105 and NGC 3371 are part of the Leo I Group of galaxies, which is also sometimes called the “M96 Group.” (M96, a spiral galaxy, appears less than a degree to the south of M105, and its neighbor, the spiral M95, is just 42’ west of M96. Both spirals lie about 32 million light-years from us, and are of similar apparent brightness as M105.) The beforementioned Leo Triplet galaxies are also part of this group, and the group itself is a part of the Virgo Cluster of galaxies.

Both M105 and NGC 3371 won’t be difficult in a 6-inch ’scope, under a clear, dark sky—and they should be dim but visible in a good 4-inch under the same conditions. Low powers, around 6x per inch of aperture, are a good place to start—in the 6-inch, try about 35x. In an 8-inch or larger instrument, both targets will be reasonably bright, and you should observe brightening towards the galaxies’ centers. (Unfortunately, these objects become more challenging under the haze we’ve seen so much of lately—if you’ve got a “milky” night, give it a shot but be ready to try again another time.)

In larger ’scopes, you’ll get a bonus: 12th-magnitude NGC 3373, a spiral galaxy, appearing less than 10’ east of M105; it shares the same ½° field with M105 and NGC 3371. NGC 3373 lies 63 million light-years out, about twice as far as the other galaxies—it’s probably not part of Leo I, but it does make for an interesting view. On a practical basis, this galaxy “should” be visible in an 8-inch ’scope, if we have a good sky—on lesser nights, it may prove difficult.

Directions

To get to M105, look first to the star at the very top of Leo’s head, Ras Elased Borealis, more conveniently known as Mu (μ) or 24 Leonis. From there, imagine a line to our last target, Algieba, and extend it beyond Algieba—M105 lies exactly on this line, slightly farther from Algieba than Algieba is from Mu.

Detail Chart of area surrounding M105, in Leo.

Detail Chart of the area around M105, in Leo. Note how M105 and the adjacent galaxies NGC 3373 and NGC 3371 sit inside (or nearly so) the Telrad circles’ innermost, 1/2° ring, when the stars 52 and 53 Leo are distributed evenly from the circles’ center (or from a finderscope’s crosshairs).  —Object positions, constellation and meridian lines charted in SkySafari, and then enhanced.  (Click on image for larger version.)

To dial things in a little better, notice 5th-magnitude 52 Leo (aka k Leo), which lies along a line from Regulus to Chertan (the latter is one of the bright stars in the triangular group forming Leo’s haunches, and the closest to Regulus). Because 52 Leo also lies near the Mu Leo-Algieba-M105 line, you can use 52 to position your Telrad more accurately when searching for M105—as you slew along the line between Mu Leo and M105, stop when 52 Leo lies about halfway between the outer (4°) and the mid-sized (2°) Telrad circles, and you should be on target—putting 52 Leo a bit closer to the outer circle will be even better.

If 52 Leo is hard to see in your Telrad, there’s an alternative: Estimate M105’s position along the line to Mu Leo as best you can, position your ’scope there, and then look in your finderscope. 52 Leo will appear near the northern edge of the finderscope’s field (so near the top of an unreversed image, at our stated date and time; and near the bottom of the field in the more usual inverted image in a straight-through finderscope). You’ll also see 53 Leo (aka l Leo), of similar brightness to 52 Leo, near the opposite edge—if you don’t see both stars in the finderscope, slew around the area gently. When you put your finder’s crosshairs midway between these two stars, M105 should fall inside a ½° eyepiece field, or very near to it. On a clear night, M105 might also be visible in a 50mm finderscope, near the crosshairs; if so, centering the galaxy in your eyepiece will be even easier.

—See you next month.

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