by Zachary Singer
Intro
We start the first month of the New Year off with a splash—a total lunar eclipse on the night of January 20th. Along with that, we have planets and two targets in Eridanus—one is an important multiple-star system, and the other a striking planetary nebula.
Lunar eclipses happen when the Earth, Moon, and Sun align so that the Earth is between the other two bodies, preventing the Sun from shining onto the Moon. A total lunar eclipse occurs when all of the Sun’s direct rays are blocked; these are mesmerizing phenomena to observe, and while a telescope or binoculars would be great to have, you’ll be riveted even with your naked eyes. (Unlike solar eclipses, there’s no need to worry about eye protection.)
The Solar System
Lunar Eclipse
Here in Denver, the eclipse’s first penumbral contact occurs just before 7:40 PM, Mountain Standard Time (MST). The penumbra is the outer part of a shadow; it has a soft edge and can be difficult to detect at first. That’s because the penumbra is the area that corresponds to an object being partially lit—in this case, the Earth is beginning to block the Sun’s light—but since the light source (the Sun) is only slightly obscured at the outset, the effect is negligible. As the Moon slides deeper into the penumbra, more of the Sun’s light is blocked, and the shadow gradually deepens. You should see noticeable darkening by around 8:10 PM (MST), roughly when the Moon is halfway into the penumbra.
The first hard “bite” of shadow appears, as the Moon enters the umbra at 8:36 PM. The umbra is the deepest part of the shadow, resulting from the total cutoff of light coming directly from the source. Unlike the penumbra, you’ll see a fairly sharp line between the umbra and the surrounding penumbra—at first, the umbra will look like someone took a little nibble out of the lunar surface. This dark area will grow quickly and become increasingly obvious in the first few minutes, and it will be halfway across the lunar disk around 9:10 PM. As the area within the umbra grows and the relative glare of the penumbra lessens, your eyes will be better able to see into the umbra, and you may begin to notice the umbra’s reddish or copper color, if it’s there.
We’ll see totality, when the entire Moon is within the umbra, starting at 9:42 PM. At this point, the direct light from the Sun is completely eliminated. The lunar surface would appear black, except that sunlight still passes through Earth’s atmosphere, and its path is bent towards the Moon, in very much the same way that sunlight is bent at sunset.
As with sunset, it’s mostly the red light that makes it through—the blue light is scattered by our atmosphere—so the lunar surface takes on a reddish tinge. The exact color and brightness of this light depends on what’s happening in our atmosphere at the time—the more dust there is (from volcanoes, fires, or pollution), the redder and dimmer the light gets.
Totality for this eclipse will last just over an hour, and the Moon will begin to enter the other side of the penumbra around 10:45; the Moon exits the umbra at 11:52 PM. Roughly 15 minutes after midnight, the Moon will be about halfway through the penumbra (and heading back into full sunlight), so this is a good guide for when the eclipse is “over,” visually speaking. As a matter of astronomical timing, the Moon fully exits the penumbra at 12:49 AM on the 21st, but by then, you won’t be able to tell the difference.
Planets
Mercury is lost in sunlight after the first days of the new year. It becomes an evening-twilight object next month.
Venus remains a spectacular target in January, starting out the year at about magnitude -4.5, with a half-lit phase. By month’s end, Venus will be slightly gibbous, a touch dimmer (but still bright), and a bit lower in the sky.
On the morning of January 31st, the Moon and Venus will appear close together in a binocular (or finderscope) field, with Jupiter off about 7° to the west; the view with the Moon’s thin crescent should be impressive. If you can manage a 2/3° field in your telescope (or better, 1° or more), then a daylight view will show the Moon and Venus in the same field around 10 AM. (Venus will be about ½° above, or northward, of the Moon’s bright limb—it should be easy to find optically, once you’ve aimed at the northern point of the Moon’s crescent.)
Mars shrinks from 7½” to just 6” during January, and dims by half a magnitude. As seen from Earth, Mars isn’t at its best anymore, but it still clearly displays a disk with moderate magnification.
Jupiter is now a pre-dawn object, rising around 5 AM at the beginning of January, and 3:40 AM at the end of the month. Though low for good telescopic viewing early in the month, the giant planet will sit 22° above the southeastern horizon 45 minutes before dawn at month’s end—so if you’re a Jupiter fan, things will be looking up soon!
Technically, Saturn is making its reappearance from the Sun’s glare this month, but it won’t be anything to shout about. Though the planet increases its apparent angle from the Sun each day, it will still only be about 7° above the horizon, 45 minutes ahead of sunrise, at the end of January.
Uranus appears close to Omicron (ο) Piscium, making the pale blue planet relatively easy to find. All month, centering Omicron in your finderscope will put Uranus there, too; look for it just 1° 15’ and just slightly east of north from the star in early January, and less than 1½° (and slightly more eastward) at the end of the month.
On the evening of the 20th, the planet’s alignment with Omicron will act like a pointer to help you see the magnitude +12.3 asteroid, (674) Rachele—remember, this is the same night as the lunar eclipse. From about 6 PM to 11 PM, you’ll find Rachele almost directly opposite from Uranus, on the other side of Omicron Piscium, or almost due south from the star. This is an unusual opportunity to see a less-well-known asteroid. Under real-world conditions, you’ll want a fairly dark sky and likely at least a 5- or 6-inch ’scope to pick up the asteroid; an 8-inch would be a great help. (Astrophotographers, this even is made for you—by the following night, Rachele will have shifted visibly, so you can get a good comparison shot.) Next month, Uranus will have a close conjunction with Mars; we’ll keep you posted.
As for Neptune, it will begin to sink into the sunset by the end of the month, when it will be just 21° above the horizon, 45 minutes after sunset. Earlier in the month, though, it will still be high enough to make it a reasonable target. (If you’ve never seen Neptune, try using 200x or more to glimpse the planet’s tiny-looking disk.) Look for the planet about 2½° east of Hydor, aka Lambda (λ) Aquarii, early in January, and about 3° east of that star at month’s end.
Stars and Deep Sky
This month, we look at two interesting, less-traveled objects in the constellation Eridanus, “The River.” If many amateur observers think of the adjacent constellation Orion as being on a celestial “interstate highway,” easily cruised with big road signs and famous sights, then they might say that Eridanus is a back road. Great! Eridanus is a little slower-going as it meanders faintly across the sky, but there are wonderful things to see that others might miss.
Before we get started, a note for those unfamiliar with the area around Orion: It’s OK if you don’t know your way around Eridanus—I’ll show you how to get around the northeastern part, where we’ll be looking. Orion will be our starting point for finding our targets, though, and if you’re not familiar with Orion, then you’ll have a much harder time with navigation. If you have a go-to telescope, that would get around the navigation issues, but you’ve likely been missing some great targets in Orion! For these reasons, I suggest that you check out “Getting Your Bearings” on page 4 of the January 2016 Observer (https://www.denverastro.org/xobserver/january2016_denverobserver.pdf) so you can navigate, and the “January Skies” from that issue so you see some of Orion’s targets. More objects in Orion are featured in the “January Skies” for 2018 at https://www.denverastro.org/xobserver/january2018_denverobserver.pdf.
Keid, aka 40 Eridani
Now that we’re squared away, our first target, Keid, is also known as Omicron-2 (ο-2) Eridani, or 40 Eridani; you’ll find it at 04h 16m, -07° 36’. At magnitude +4.4, it’s no Orion-style showpiece, but it’s a fascinating little system! There are two main attractions here—the first is its orange main component, which we’ll get to in just a moment—and the second is something very unusual, a white dwarf that you can see in a small telescope. (There’s a third component, too!)
Keid’s main star, 40 Eridani A, or 40 Eri A among friends, is an “uncommon common star.” It’s an orange dwarf, cooler than our Sun, and therefore both more orange in color and comparatively dim—if we could put our own Sun next to it in space, our star would outshine 40 Eri A by nearly a magnitude.
In practice, there’s only so far away that our Sun could be from us and still remain visible to the naked eye, and since 40 Eri A is inherently dimmer, it would disappear from human vision at a shorter distance. For our Sun, the cutoff is about 50-70 light-years (depending on sky conditions and how good an observer’s vision is, but it’s a reasonable range); for a star like 40 Eri A, it’s rather less. Any similar stars lying farther away would be invisible to us without a telescope or binoculars.
And that’s why 40 Eri A is uncommon—at a distance of just over 16 light-years from Earth, it’s one of a relative few that is close enough to be seen naked-eye (under a dark country sky). Ironically, stars like 40 Eri A are as common in our galaxy as sand grains on a beach—but most of them are simply too far away to be seen easily.
(As luck would have it, there’s another star like 40 Eri A, just “down the river” in Eridanus—it’s Epsilon [ε] Eri, which is actually a touch closer to us and thus slightly brighter than 40 Eri A—but that’s another story. There’s also the beautiful binary, 61 Cygnus—see the September 2015 issue of the Observer, at https://www.denverastro.org/xobserver/september2015_denverobserver.pdf.)
In a 4-inch Mak, under a moonless but somewhat hazy sky, 40 Eri A appeared moderately bright (“meh”) and honey-colored—but on a similar night in an 8-inch ’scope, it was a bright, deep-orange dazzler, somewhat reminiscent of Aldebaran. On one hand, knowing the star is intrinsically somewhat dim gave me a deep impression of how close it must be to shine like that. And then it struck me, still looking at this star in the 8-inch, that Aldebaran is still brighter visually, and much farther away—and suddenly I could sense how much more powerful that giant star must be in comparison to our next-door neighbor, 40 Eri.
40 Eri A has recently been suggested to have a roughly Neptune-sized planet of about seven or eight Earth-masses orbiting it, in about 40-odd days. Astronomers are still working through the data, and there is some debate about whether the signal truly represents a stellar “wobble” caused by a planet. Regardless of the ultimate confirmation of any orbiting planets at 40 Eri A, our little star is indeed the home of the fictional planet Vulcan, dear to Star Trek fans. When you observe 40 Eri A, then, you can consider that it’s the real home of a made-up planet, and the possibly imagined (because of error) home of a real planet…
If the 40 Eri system has a calling card, though, it’s its white dwarf, 40 Eri B. This star is among the very few white dwarfs that are relatively easy to observe; it’s discernable, under moderate conditions, in a 4-inch ’scope.
Briefly, a white dwarf is what you get after stars of Sun-like mass (or a bit bigger or smaller), run out of their usable hydrogen fuel. At first, they shrink under the force of their own gravity, and the extra pressure allows them to fuse helium for fuel instead—the stars grow much hotter internally and expand to become giants. (The helium is the byproduct of the original hydrogen fusion.) After they burn through their helium, these moderate-mass stars can’t squeeze tightly enough to fuse elements much heavier than carbon—so they eventually use up their fuel again, becoming unstable in the process.
At that point, they eject their outer layers—creating a “planetary nebula,” like the famous Ring Nebula (M57), or for that matter, our next target! The hot core of the star is exposed—when such stars were first observed, the color of their light suggested a large, fairly hot star which should be very luminous. (In Main Sequence stars—that is, normal, hydrogen-fusing stars—the hotter the star is, the bluer its light, and the more massive and intrinsically bright it must be.) But astronomers realized that these stars weren’t putting out anywhere near enough light to be normal, hot stars, and that to be as dim as they are, they must have far less surface area—that is, they must be tiny. That and other clues told astronomers that these are “spent” stars, compressed by gravity down to about the size of the Earth—they radiate light dimly as they slowly cool, over billions of years.
If you think about it, all you should have to do to find such a star is go looking towards the center of a planetary nebula—and such stars are indeed found there. The problem, as far as visual observation goes, is that even the closest planetaries are quite a way off, and these stars are then dimmed not just by their inherit faintness, but by distance as well—the nearest, the Helix Nebula, NGC 7293, in Aquarius, is perhaps 650 light-years from us, with a 13.5-magnitude, white-dwarf central star. The next one, by distance, is the aforementioned Ring, lying at least 1,400 light-years from us (distances to planetary nebulae are approximate at best). The Ring Nebula’s central star is 16th magnitude—too faint for most amateur ’scopes.
Along with being a rare, visually observable white dwarf, 40 Eri B is a good example of the breed. It has just over half of our Sun’s mass, as expected by theory, and an absolute magnitude of 11—that is, it would appear roughly 250 times dimmer than our Sun if they could be viewed at the same distance. Because the 40 Eri system is fairly close to us, 40 Eri B’s apparent magnitude (its brightness as seen from here on Earth) remains doable at +9.5.
In the 4-inch Mak, this star was at its best at about 130x using averted vision, improving after a few visual sweeps of the area and after becoming better dark-adapted. Interestingly, at 52x, 40 Eri B “blinked” like some nebulae I’ve seen—it appeared and disappeared sharply as vision alternated from averted to direct and back again. My observing partner experienced this, too. At either power, the white dwarf was a dim and difficult little speck, even with averted vision—on many nights this past December though, we had somewhat milky skies, and I could only just make out 4th-magnitude stars naked-eye. You should do a little better in a darker or clearer sky.
There are other nearby stars that famously have a white-dwarf companion, like Sirius, on the other side of Orion from 40 Eri in the constellation Canis Major. Sirius is the brightest star in our sky, and very easy to find—and its white dwarf companion, Sirius B, is a full magnitude brighter than 40 Eri B—but the glare from Sirius A (the bright main component) all but washes out its white dwarf companion. Now that Sirius B is nearing its farthest point from the bright main component, we might be able to see it, under very good seeing conditions, in a large instrument. Maybe. (Sirius B was first discovered by Alvan Clark in an 18-inch refractor not unlike our own 20-inch at the Chamberlin Observatory—that should tell you something!)
To bring this all home, Keid is only my second observed white dwarf, and I’ve been at this a while now. The first was the central star in the Ring Nebula, almost 20 years ago. As I mentioned, that star is 16th magnitude—beyond the range of even my 12-inch Newtonian. I saw it easily in the 60-inch reflector on Mt. Wilson, in California. (At the time, I was pretty new to observing, and did not understand all the wonder-filled “ooohhhs and aaahhhs” coming from my observing mates as they were treated to this rare view.) I haven’t seen the central star since—observing reports on the web suggest it can be glimpsed in a 17- or 18-inch ’scope under good conditions. Unless you’ve got large optics, then, Keid/40 Eri B is your best bet!
The 40 Eri system has one last attraction, 40 Eri C. This star is a red dwarf—a Main Sequence star of such little mass that its low temperature makes it look reddish and dimmer than the other stars we’ve just discussed—even its white-dwarf sibling outshines it. These stars are the most common in the galaxy, but because they’re so faint, they’re mostly out of our range—11th-magnitude 40 Eri C failed to appear in the 4-inch, but an 8-inch should pull it in under the same conditions.
Well, that’s if this star remains at 11th magnitude. It’s a variable also known as DY Eri, and some sources suggest that it can dim another two magnitudes, to +13. If so, 40 Eric C would become more challenging, because along with being dim, the “glare” from its +9.5-magnitude white-dwarf companion, just 8” away, won’t help. In moderate ’scopes, then, it might be best to consider 40 Eri A and B your targets, and 40 Eri C a bonus…
Under a dark country sky, getting to Keid/40 Eri involves just a touch of star-hopping, in a few easy jumps. Don’t let the unusual length of these directions dissuade you—the path to success here is much easier to follow than describe.
Look first to Rigel, the bright star making up Orion’s western foot. About 1½° northeastward from Rigel (that is, looking about a fifth of the way towards the western star in Orion’s belt), is +3.6-magnitude Tau (τ) Orionis. (See chart.) If you make a perpendicular turn from Tau to the northwest (“rightward,” when is Orion is vertical), you’ll quickly notice Cursa, or Beta (β) Eridani. Cursa is fairly bright at magnitude +2.8, and marks the starting point for following Eridanus’s “river” to our target.
Once you’re at Cursa, you’ll see a pair of 4th-magnitude stars about 2° apart to Cursa’s northwest, Mu (μ) and Nu (ν) Eridani. Nu, the westernmost of the pair, is about the same distance from Cursa as the span between Orion’s two feet—about 8°. Nu is also roughly the same direction from Cursa as Rigel is from Saiph, Orion’s other foot. In a clear sky, this is a minor point, just helping to confirm you’re in the right place—but in a light-polluted or hazy sky, you can use the similarity in span and direction to estimate Nu’s position. (This can be a handy trick, as we’ll see shortly.)
If you look at the line tracing the curves of Eridanus’s “river,” you’ll also notice that Mu and Nu are at the first curve from Cursa, and that the river bends again, by about the same amount, right after leaving Nu. If you jump 7° (happily, about the same distance as our first hop) in the new direction from Nu, you’ll notice another pair of stars—these are oriented almost perpendicularly to the river, and lie about 1° apart. The brighter one, at magnitude +4.0, is Beid—and the other, to the southeast, is our target, Keid/40 Eri. (Keid and Beid are within a half-magnitude of each other, but the difference will be quite noticeable in a finderscope, aiding identification. (Another star that helps confirmation in a finderscope is 5th-magnitude 37 Eri, which lies close to Beid, at about a third of the Beid-Keid distance.)
**If you study the star chart carefully before heading to dark skies, you’ll find Keid without much trouble. If the sky is a bit light-polluted or murky though, it will wash out most of the stars in Eridanus—the majority are only 4th-magnitude. In that case, have another look at the chart, and follow the next, similar bend in the river from Beid, for about the same distance (7½°, in this case), to find +3.0-magnitude Zaurak (Gamma [γ] Eri). If Zaurak isn’t visible in the sky, give it up for the night—but if it is, guestimate Nu’s position (as described above); then put your Telrad halfway between Nu’s location and Zaurak, and nudge it slightly northwest. You should have Keid, Beid, or both in your finderscope. If not, circle the area; you won’t be off by much. (We’ll use Zaurak to find our next target, too, so it’s worth getting familiar with.)
NGC 1535/Cleoptra’s Eye
Our last target is the striking planetary nebula NGC 1535, at 04h 15m, -12° 41’. Also known as Cleopatra’s Eye, this nebula will remind experienced observers of the Eskimo or Clownface Nebula, NGC 2392, in Gemini. (For a full description of the Eskimo, see the February 2018 Observer at https://www.denverastro.org/xobserver/february2018_denverobserver.pdf.) Both of these nebulae have overlapping series of gas shells produced by a dying star on its way to becoming a white dwarf—the same kind of star that we explored above, 40 Eri B. (Both of these nebulae have a central star visible in moderate ’scopes, but neither of these is a white dwarf—such stars would be much dimmer than the ones seen here, especially since the estimated distances to both nebulae are at least 4,000 light-years.)
Like the Eskimo, this nebula has high surface brightness, so you won’t have any trouble seeing it, even in the city—from southern Denver, a 5-inch Mak will show it nicely at moderate power, and without benefit of a UHC filter. It’s easily detectable as a “fuzzy, non-stellar object” around 50x, and improves with increasing magnification. (I haven’t tried 1535 in a 4-inch, but you won’t have a problem under a dark sky, and it’s a good bet for the suburbs.)
In an 8-inch Schmidt-Cassegrain under better skies, NGC 1535 is a blue ball at 80x (essentially, “low power” with a standard 25mm Plössl). At 220x, the inner and outer nebula were clearly differentiated and appeared with visible mottling, especially with averted vision. In between, at about 135x, Cleopatra’s Eye was almost a “blinking nebula”—direct vision showed only the inner part, while averted vision showed the whole nebula, and they would alternate back-and-forth as you moved your eye. As it happens, no UHC filter was used for this observation either, but given my experience with the Eskimo, I’d expect a noticeable gain in contrast, improving details still further—this nebula’s a beauty!
NGC 1535 lies just 4° east of Zaurak, the 3rd-magnitude star we used above to help find Keid in a hazy sky. Conveniently, this position puts the nebula just south of a line between Zaurak and Rigel—just set up your Telrad so the distance from Zaurak to the nearest edge of the Telrad’s outermost (4°) circle is the same as from that edge to the Telrad’s center—then give your ’scope a slight nudge so that the top of the Telrad’s innermost (½°) circle rests on that line. This approach should put NGC 1535 into a ½° eyepiece field, or close to it—if it’s not in your eyepiece, spiral carefully around the area. If your eyepiece shows a wider field, you’re more likely to put the nebula in there, but be careful using lower powers—1535 may appear more star-like, and you could miss it if you’re not alert.
Finally, equatorial-mount folks get an easy ride—NGC 1535 is almost due south of Keid. If you center the star and then slew westward a little over ¼°, a 5° slew southward from that point will put the nebula into your eyepiece. Use setting circles, if your mount has them, to bring you close, say 4°—and then go the rest of the way, in declination only, watching through the eyepiece to avoid overshooting. Because of the short hop from Keid, an approximate polar alignment should be fine.
—See you next month.