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Cosmic Challenge: Zeta (ζ) Cancri
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Cosmic Challenge: Zeta (ζ) CancriMarch 2021
Phil Harrington
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Target |
Type |
RA |
DEC |
Const. |
Magnitude |
Size |
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Zeta |
Quadruple |
08h 12.2m |
+17° 38.9’ |
Cancer |
5.6/6.0/6.1/10.0 |
see |
Although it is one of the faintest constellations along the zodiac, Cancer the Crab hosts a variety of targets to test our mettle during the early spring. Spotting M44, the Beehive Cluster, by eye alone may prove very challenging for suburban observers, while the Crab's underappreciated second open cluster, M67, may also reach naked-eye visibility from more rural environs. While the constellation boasts a variety of challenging galaxies, in the test here, we will try our luck with one of the constellation's prettiest binary stars, Zeta (ζ) Cancri.
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Above: Early evening star map. Credit: Map adapted from Star Watch by Phil Harrington. |
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Above: Finder chart for this month's Cosmic Challenge. Credit:
Chart adapted from Cosmic Challenge: The Ultimate Observing List for Amateurs
by Phil Harrington.
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Many "best of" lists include Zeta Cancri as a spring showpiece target, so there is a good chance that you have already crossed paths. Zeta's two brightest suns, known as Zeta-1 and Zeta-2, were discovered in 1756 by German physicist/astronomer Johann Tobias Mayer. The Zetas are separated by 5 arc-seconds, which is wide enough to be resolvable through just about any amateur telescope with an aperture 2 inches (5 cm) or larger.
Fifteen years later, the exacting eyes of William Herschel noticed that Zeta-1 was not a solo act, but rather was a tight stellar duet. Known today as Zeta Cancri A and Zeta Cancri B, these two yellow-white main sequence stars have roughly equal luminosities and masses. They shine at magnitudes 5.6 and 6.0, respectively, and take 59.6 years to complete an orbit about their common gravitational center. During that time, their separation varies between 0.6" at periastron (closest separation) and 1.2" at apastron (widest separation).
The last apastron occurred just last year, so now is the time to catch them. Given steady seeing, a 6-inch instrument at 200x or more can resolve Zeta Cancri A and Zeta Cancri B as identical yellowish headlights nearly touching one another. As a hint, the stars are oriented almost exactly north-south at present, although this will change as the stars continue their orbits.
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Above: CN member recorded this image in late April 2014 through his 6-inch (15cm) Celestron C6 using a Tele Vue 2x Barlow and a Canon T4i DSLR. You can find other images of Zeta Cnc that he took in this thread from a few years back in the Double Star Observing forum. |
By 1831, Herschel's son John noticed that Zeta-2 was wobbling ever so slightly in its orbit around Zeta-1. Although it was assumed this behavior was caused by a second star orbiting Zeta-2, this unseen companion remained unconfirmed until 2000. That year, photographic observations made with the Canada-France-Hawaii Telescope by J. B. Hutchings, R. F. Griffin, and F. Ménard finally resolved the elusive companion. (Direct Observation of the Fourth Star in the Zeta Cancri System; J. B. Hutchings, R. F. Griffin, and F. Ménard; Publications of the Astronomical Society of the Pacific 2000 112:772, 833-836).
Subsequently, Zeta-2's two components have been designated as Zeta Cancri C and Zeta Cancri D. The case is still not fully closed, however. Although spectroscopic studies of Zeta-D reveal it to be a red dwarf, its brightness suggests we are looking at not just one, but two dwarf stars that remain too close to resolve even with today's best equipment. A second investigation conducted during a lunar occultation of Zeta in 2000 confirmed that D is in fact a close-set binary. That investigation, appropriately titled "An Investigation of The Multiple Star Zeta Cnc by a Lunar Occultation," was published by A. Richichi in Astronomy and Astrophysics, v.364, p.225-231 (2000).
Can any amateur telescope possibly glimpse Zeta D? Zeta C, a yellow main sequence star, shines at magnitude 6.1, while the newly discovered Zeta-D is a weak 10th magnitude. They are separated by just 0.3 arc-seconds and have an orbital period of 17 years. That challenge may exceed even the largest backyard telescopes, although knowing the persistence of amateur astronomers, I suspect it may only be a matter of time.
Good luck with this month's challenge! And be sure to post your results in this column's discussion forum.
Remember that half of the fun is the thrill of the chase. Game on!
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About
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Phil Harrington's Cosmic Challenge is copyright 2021 by Philip S. Harrington. All rights reserved. No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder. |
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- Dave Mitsky, Jon Isaacs, John Huntley and 10 others like this
62 Comments
Thanks, everyone, for the great comments about this challenge. There was more chatter here than any previous challenge to my knowledge! Keep 'em coming!
Thanks, DoctorD. The honor is mine.
Phil,
I still can't get over how long you've been at this. Many years ago in the early 90's I acquired your first edition of Star Ware which I think was about 1994. It was an absolute treasure-trove and I still cherish it to this day as well as the other revisions. I picked it up at a local bookstore and was amazed at all the information you had about so many telescopes.
I guess this shows both how underrepresented double stars are today and how much love there still is for them, at least the brighter, showier pairs.
Clear skies!
Thomas, Denmark
I am a relative newbie but am coming to like these double stars in light pollution (I can't see any of Cancer, even M44, naked eye from my viewing area) because most nights I have come to realize I just am not going to see many galaxies and these doubles can be a nice challenge. I have an 8" dob but my highest mag available right now is 150x (8mm). I can get a clean split of the components at 34x but haven't been able to split the much tighter pair. There is some elongation at 150x. Haven't done any back of the envelope calculations, but appears I will need a bit more magnification to split the tighter pair. Thank you Phil for a fun challenge.
Spot on! Hunting and splitting multiples is a thrilling challenge (as this whole Tegmine endeavour showed me), extremely instructive and rewarding, and I've come to regard it as my "moonlit night" DSO activity. Faint fuzzies and even bright clusters just suffer too much under the combined effect of the Moon and light pollution. It's now cloudy here and I'm putting together a nice "beyond Messier" observing program. Today I'll work on doubles!
From memory: I had to go up to ≈250-280x to split Zeta1. You need a Barlow my friend ;D – and a steady sky.
Guess I'm late to the party but I've been enjoying reading the responses to Phil's challenge while I waited for an opportunity to see Zeta Cancri for myself. It appeared as a creamy white double at low power. Zeta-1 appeared elongated at 163x and 198x and was resolved (according to David Knisely's definition posted above) at 326x. I suspected a notch at 244x but had to go to 326x to confirm.
It might be cheating to use a 14" scope, but the seeing wouldn't allow me to go any higher to see the split. In brief moments of settled air I thought I could see the split at 355x and 391x but it might have been averted imagination so I didn't include that view in my sketch. The seeing made the stars appear somewhat bloated at higher magnifications.
Thanks for the challenge, Phil! This is the first time I've looked at Zeta Cnc. Btw, I have the 3rd and 4th editions of Starware--it's a very good reference.
Very nice sketch. And thanks also for the kind words about Star Ware. Of all the books I have written, I must admit that was my favorite one to research.
Ha, yes I really am that old! First book was Touring the Universe through Binoculars in 1990, while my first published article was in 1985 ("A Messier Marathon" in S&T's January 1985 issue). Wrote that one on the old Smith-Corona. Yikes!!
Sorry to go OT but I juuuust got Touring the Universe through Binoculars, and it's become instantly the indispensable companion to plan binocular (and even telescopic) forays!
Thanks, radiofm74. I am happy that you find TUB useful. Don't know if you know, but >20 years ago, a buddy named Dean Williams and I collaborated to create companion software Touring the Universe through Binoculars Atlas (TUBA, for short). Although originally designed to work with Windows 98, it works fine with all subsequent Windows OSs, including Win10. It's free for downloading, if you're interested. (Sorry, no Mac version is available.) You'll find info on my web site.
Thank you Phil! I am a Mac user but I'll hack my wife's PC to use it ;D