by Zachary Singer
In March, we have a relatively quiet month for planets: Most of them are now early-morning objects, but they are at a greater angle from the Sun, allowing better observing. In the “Stars and Deep Sky” section, we’ll look at two stars in the constellation Cancer—the first is a wonderful binary, and the other, a lesser-known carbon star.
The Solar System
Mercury is low in the west, about 8° above the horizon, in early March. It’s dimming rapidly, though, and will be lost in sunlight sometime after the first week of the month. The planet will become a pre-dawn target in early April.
In early March, Venus sits about 16° above the southeastern horizon at 5:45 AM, 45 minutes before sunrise. At the end of the month, “45 minutes before” is 6:00 AM, Daylight Time—and the planet will have sunk to a mere 6° up. Venus remains bright all month, but its disk shrinks from more than 15” to just 13” across, becoming a bit more gibbous as the days progress.
Mars won’t be showing you any detail worth mentioning, as its apparent disk dwindles from an already small 5.3” to just 4.6” this month. On the other hand, a high-power view still shows the red planet as a disk, and not just a point of light. Look for it about 45° above the western horizon, 45 minutes after sunset in early March, or about 6:40 PM—and a bit lower (and later) at the end of the month.
Jupiter is now an easy target for early risers. Early in March, it’s more than 20° up at 4:45 AM, and 26° above the horizon an hour later—still about 45 minutes before sunrise. By month’s end, Jupiter transits, 28° above the southern horizon, 45 minutes before sunup. A recent naked-eye view in pre-dawn twilight was filled with quiet grace.
Though still low in the sky at the beginning of March, Saturn is also becoming a practical morning target. By the end of this month, it will appear about 24° above the southeastern horizon, 45 minutes before the Sun breaks the horizon.
Uranus remains high enough in early March for a credible observation as soon as it gets dark outside. Look for the light-blue planet about 2° northeast of 4th-magnitude Omicron (ο) Piscium. For those of you relying on star-hopping instead of computerized “go-to” systems, this is your last opportunity to find Uranus with a relatively easy landmark any time soon! By month’s end, Uranus will be close to slipping into the light of sunset, and it’s “curtains” for sure in April—superior conjunction, when Uranus lines up almost exactly behind the Sun as seen from Earth, is April 22nd.
Neptune is lost in solar glare this month. Technically, it will reappear as a pre-dawn object in April, but the planet won’t really become a practical target again until May.
Stars and Deep Sky
As mentioned above, we have two stellar targets, and the first one is 4th-magnitude Iota (ι) Cancri, or Iota Cnc for short. Iota, located at 8h 48m, +28° 41’, is a beautiful and easy binary pair—their 30” separation is wide enough to be split even in small telescopes. Sitting at the “top” of the constellation Cancer, Iota is also a “gateway” of sorts to the “UFO Galaxy,” NGC 2683, which we toured a few years back (see page 7 of the March 2016 Observer).
Along with the wide separation, both components are fairly bright—at 6th magnitude, even the dimmer companion would be visible on its own to the naked eye, under a dark country sky. In practice, all these factors mean that observers won’t have to try to hit an “optimum” magnification to split the pair. Instead, you can change eyepieces just to improve the aesthetics.
If you look in the literature for Iota Cnc, you’ll often see it described as a “yellow and blue” pair, but in my 12-inch Newtonian, I saw a lovely deep butter-yellow main component, with a lavender secondary. Iota looked best to me, color-wise, at 120x, moderate power for this ’scope. As “conventional wisdom” would dictate, color contrast declined noticeably at 200x—the yellow primary was still yellow, but the secondary was just an off-white. Interestingly though, colors were also very subtle at the other end of the scale—at about 50x, with a 32mm Plössl. Color improved with greater magnification up to the beforementioned 120x.
(The conventional wisdom says that the colors of star-pairs appear richer in smaller ’scopes, and at lower power. It isn’t always so, though.)
On my most recent observation, I also used a high-grade 0.90 neutral-density filter, reducing the light-flow on the 12-inch to about that of a 4½-inch ’scope. Color contrast increased with magnification while using the filter—I actually liked 200x best. (Sticklers will note that this approach isn’t a perfect simulation of a smaller ’scope, because the image still has the resolution, or sharpness, of the larger instrument—consider the comparison a “rough sketch.”)
Since the colors we’re seeing through the ’scope are partly a product of our nervous system, instead of actual reality, you might wonder what the true nature of these stars really is. According to Professor James Kaler, University of Illinois, the main component—the bright “yellow” star in the ’scope—is a large, class G7.5 giant, with a surface temperature of about 5,000 K. Its “blue” companion is actually a white, class A3 star with a surface temperature of 8,800 K.
If you’ve studied basic astronomy, you might be surprised that the lower-temperature (yellow) star here is the brighter one—generally, the hotter a star is, the bluer and brighter it is (and more massive, too). Indeed, the yellow star’s color and temperature aren’t much different than our own Sun’s (the Sun is a G8, at about 5,500 K). The trick for Iota’s main star is that it’s a giant—that is, it used to be an even hotter (and bluer) star than its “blue” companion, but after running through its hydrogen and then its helium fuel, it’s now greatly expanded, and cooling.
Although Iota Cancri A, the bright component, roughly resembles our Sun color-wise, its mass is about 3.5 times that of our Sun (and almost double that of its companion). Compared to the Sun, Iota A is some six magnitudes more luminous—it would appear more than 200 times brighter than the Sun if they were side-by-side. It shines at magnitude +4.0 (naked-eye visible in outlying suburbs), even though it’s about 330 light-years from us—at the same distance, you’d need large binoculars, at minimum, to see our Sun.
As another indication of this star-system’s scale, the pair’s 30” separation works out to a physical distance of about 2,800 astronomical units, or about 70 times the average distance from the Sun to Pluto.
Iota Cancri is fairly obvious in a dark country sky, since it sits at the top of the constellation Cancer’s simple outline, and Cancer itself lies halfway between two other bright and easy constellations, Gemini and Leo—see chart. In mid-March, you’ll see Cancer just to the east of the meridian (the imaginary line running between north and south, across the top of our sky) at 9:30 PM, Daylight Time. Iota sits very high in the sky at that point, and when it transits (crosses the meridian) just over a half-hour later, it will only be 10° south of lying directly overhead.
Cancer, Iota included, isn’t visible to the naked eye in the city, because of light pollution—but it is visible even in small finder ’scopes. A suburban observer with a spirit of adventure (or at least, a sense of humor), can still find this star by roughly “guesstimating” its location. Just aim your Telrad halfway between the bright star Pollux (in Gemini) and the “topmost” star in the curve of Leo’s head. Just to make it a sport, the latter star may not be visible in the city, either, but just use that general area for a guide—you should see the other stars in Leo’s head, unless the skies are poor. (If you don’t find Iota at first, try slewing slightly back toward Pollux, and then spiral gently around that area while looking through the finderscope.)
Using this approach, I was able to put Iota right into my finderscope. It was easy, except for the high altitude—the star was pretty near the zenith at the time. Iota is the brightest star for many degrees, so if you see a noticeably bright speck in your finderscope after your “rough guess” with the Telrad, it’s probably Iota. The star’s distinctly yellowish appearance in finderscopes (or if you’re into it, binoculars) also aids in confirmation.
Whether you’re “guesstimating” Iota’s position or can see it directly, it’s a good idea for owners of Dobsonian ’scopes to observe this area well after our stated times, because of the altitude issue alluded to above. By waiting, you let this part of the sky move downward from its highest point, so it will be easier to point a dob—it’s also easier on any observer with a sore back or a straight-through finderscope!
Our second target is the carbon star, X Cancri, at 08h 56m, +17° 09’. It’s a variable star, with a magnitude range of about 5.9-7.9—but like other carbon stars, what makes it interesting is the mechanics of its structure, and the resulting color.
Carbon stars are dying giants; they have relatively cool exteriors, that on their own would make their light appear noticeably orange, like Betelgeuse’s or Aldebaran’s. Carbon stars, though, have an additional trick up their sleeves—they dip into the carbon formed near their cores, and bring this material to their upper layers.
This “dredged up” carbon blocks some of the light, particularly in blue and violet, making these stars’ light noticeably deeper in orange and red hues. The more carbon builds up, the redder-looking a star gets—until that star’s outer envelope gets blown into space. After this “reset,” the star brightens, and its color isn’t as deep—but more carbon builds up again, and the cycle begins anew.
All carbon stars are not alike, however, and one result of this is that their cycles of carbon buildup occur at different intervals—in contrast to X Cancri’s roughly six-month period, R Leporis in Orion, for example, takes about 430 days. These stars also differ in their ranges of intrinsic brightness (absolute magnitude); R Lep generally appears dimmer to us on Earth, even though it’s some three times closer than X Cancri is—so X Cancri would appear much brighter than R Lep, if the two stars were viewed from the same distance.
The few reports I’ve seen for X Cancri suggest a deep red color (presumably at the most carbon-intense part of its cycle), but my own observations in late February showed a “smoky orange”—you could also say “dusky.” While this wasn’t the deep color I’d expected, it was noticeably different than more common cool (orange-hued) stars. Viewed in the 12-inch from southern Denver, its color was somewhat subtle, and looked best at low power. Part of the star’s appearance on that observing run was doubtless due to hazy atmospheric conditions—they were a bit “soupy,” as they’ve often been in Denver, lately.
X Cancri appeared quite dim with my 0.90 ND filter (approximating a 4½-inch ’scope), but its color was nonetheless better at low powers. The star was challenging to see in this configuration—but a thin layer of clouds or haze, and light pollution, were likely the problem. In short, while you can see this target in the city with small instruments, you’ll have a much better experience under darker skies. Remember that X Cancri has a roughly half-year period, though—if you don’t like the way it looks, come back again soon. The star remains observable into May, though it sets early by then, and reappears in the wee hours during fall.
X Cancri can be found a little over 2½° southeast of Delta (δ) Cancri (aka Asellus Australis on some charts)—if you think of Cancer’s shape as a “mandolin,” then Delta is the star where the mandolin’s neck meets its triangular base.
First center your Telrad on Delta, and then imagine a line from Delta to Regulus, the bright star at the bottom of the “question mark” in adjacent Leo. Slide your Telrad towards Regulus until the trailing edge of the outermost (4°) Telrad circle rests on Delta—X Cancri will lie in your finderscope’s field, a bit toward the east (and a touch south) from center.
X Cancri will appear in a 50mm finderscope (and should appear in a 30mm), but when I last looked, the star’s color wasn’t apparent. Instead, you’ll need to recognize it as the center star of three stars in a row—they’re all of 6th– and 7th magnitude, making a pronounced pattern. In turn, two additional stars just to the north create a trapezoid, with the first three (including X Cancri) forming the base. (See detail chart.)
For users of “go-to” ’scopes, note that X Cancri may be listed by other names, like SAO 98230 or HR 3541.
—See you next month.