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Cosmic Challenge: Zeta (ζ) Cancri

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Cosmic Challenge: Zeta (ζ) Cancri


March 2021



Phil Harrington


This month's suggested
aperture range

Medium scopes: 6-inch (15cm) to 9.25-inch (23cm)











08h 12.2m

+17° 38.9





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.

Above: Early evening star map. Credit: Map adapted from Star Watch by Phil Harrington.

Above: Finder chart for this month's Cosmic Challenge

Credit: Chart adapted from Cosmic Challenge: The Ultimate Observing List for Amateurs by Phil Harrington.
Click on the chart to open a printable PDF version.


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.


Above: CN member evan9162 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!

About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy, including Cosmic Challenge: The Ultimate Observing List for Amateurs.  Visit www.philharrington.net to learn more.

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.


  • Dave Mitsky, Jon Isaacs, John Huntley and 10 others like this


I'm all for Jon's great idea.  I am also in a poor viewing area with no really dark skies, and it's about a Bortle 6/7 zone (suburbs of Cincinnati).  The nearest two good dark sky sites are both about 1-1/2 hrs away from my home, so I haven't even taken the C8 Edge to either site yet.  I have been observing planets, Moon, a few globular clusters and doubles/multiples mostly.  Since the planets aren't in ideal positions (for me that is, as I don't like getting up early in the morning), I have been focusing (pun intended) on some of these great Winter and Spring doubles.  I find them very interesting and I'm always up for a good challenge!  

    • Jon Isaacs, PhilH and Lewis Cason like this
Mar 10 2021 06:06 PM

Here is my take on Zeta Cancri with my Nexstar 8SE OTA taken the other night under good conditions:

Attached Thumbnails

  • Attached Image: zeta cancri_00001 03_27_12Z_Zeta_Cancri.jpg
    • PhilH, Lewis Cason, Sasa and 3 others like this

Zeta Cancer is also known by the name TEGMAN...and is found on most Celestron telescope keypads (Handcontrollers)---you select...




Using your arrow keys ...select TEGMAN


It is high in eastern sky an hour after sunset...


Thank you Phil for for being the topic starter---looking forward to future topics from you.  


BTW  Iota Cancer is nearby...it is an easy yellow and blue star double...kind of interesting to view it and then slew over to Almach to compare them.  I find it fascinating to do comparisosns of smiliar objects as it forces me to be a more discerning observer.  Iota Cancer is also on the Celestron Double star list.  I find it interesting to slew between different d-stars to appreciate them more.  

    • PhilH likes this

I did get out last night again with my Nexstar 11 GPS.  The CSC showed "poor" seeing and it was a tad better than that, but not by much.  I slewed over to Tegman (Zeta CNC); using a 13mm TV Plossl (215X) I noticed that one of the double stars looked odd: it was actually two stars, separated by a thin ribbon of dark sky.  It wasn't easy to see as the stars were shimmering.  I have a 6" APO; I dare say that it probably would have shown it "better".   I then viewed numerous objects in the late winter sky...mostly Open Clusters. But I did go back to view Tegman a no. of times.   I also viewed Mars using 215X... just a salmon fuzzy ball of light showing gibbous phase.   Also had to take a peek at nearby Uranus; a soft light green disk.

    • PhilH, Sasa and Eclipsed like this

The sky finally cleared off in the early evening, so I decided to target Zeta Cnc in the backyard tonight.  Seeing was mediocre.  I started with the 127 Mak at ~233x.  The triplet showed nicely, with the tighter pair forming a nice ~N/S touching duo, the northern component clearly dimmer.


I then tried the AT72EDII.  This made the closer binary, Zeta 1, unresolvable, although at 144x there were some indications of the N/S elongation, more of a blurring on the north side.  At 216x this was somewhat more apparent.  It clearly wasn't round like Zeta 2.


With the ED80 there was clearly more elongation of Zeta 1 at 200x than with the AT72EDII;  and at 300x there was a hint that there might be a weak notch, more of an unresolved overlapping ghost "double image."

    • PhilH, Lewis Cason, Sasa and 2 others like this

I resolved Zeta Cancri with my 6 inch Starblast last night. The seeing was good, and so I could clearly see it as 'split' at around 150X, but the best view was at around 240X. It's a great triple star, and I think it should be resolvable in even a 4 inch telescope. 


Is there a required "amount of power" needed to split a double for a given separation? That is, for Zeta CNC with a separation of 1.1", does one need 240x to split the double regardless of how much aperture one uses? Seems to me that would be the case... but I could be wrong. 


Obviously, the magnitude of the stars plays into the equation. hmmm. 

Even though this question wasn't meant for me, but still.

I think that the relevant yardstick here is exit pupil, not magnification. Even if you have perfect(20/15 or even better) eyesight, your eye is typically not diffraction limited above about 1.5-2.5 mm. You can see this by putting in a eyepiece that gives you a 5 mm exit pupil in your telescope. All the stars appear spiky(the effect vanishes after a while because your pupil contracts) and this is not due to the scope, but due to the eye. A smaller scope, however, will be operating at a smaller exit pupil at the same magnification, and thus the image will look 'better'.

I'll give you a real life example. My 6 inch F/5 Newt gives a 5 mm exit pupil at about 30X.

I have a homemade 40 mm finderscope with interchangeable eyepieces. At around 30X, the image appears cleaner, if very dim as compared to the 6 incher.


So, I think that you need at least a 1-2 mm exit pupil in any size scope to resolve a near-diffraction-limit double. Of course, I may be wrong, but this has certainly been my experience while observing.

    • Jon Isaacs, PhilH, Sasa and 2 others like this
So, I think that you need at least a 1-2 mm exit pupil in any size scope to resolve a near-diffraction-limit double. Of course, I may be wrong, but this has certainly been my experience while observing.





My own thinking goes like this:


For most doubles, the most aesthetically pleasing view is in a scope where the split is about twice the Dawes limit.  The double-double is about 2.3", twice the 1.15" Dawes of a 4 inch.  The best view will be with an exit pupil of 1mm or less.


This is based on experience but there's rational to support it. The first minimum of the Airy disk is the Rayleigh criteria, the first minima of one star passes through the center of the second star. Under these circumstances, the disks are overlapping though it may not seem that way.


Double the Rayleigh Criteria and now the minimums are aligned, an optimal condition, doubling the Dawes limit is somewhat less optimal but its in this range where clean splits can be made without pushing the scope too hard. 


With a scope that's say an 8 inch rather than a 4 inch, seeing and cool down can be more of and as you say, the eye likes smaller exit pupils so the eye will require more magnification just get a clean image.


For a 1.1" double, an 8 inch might be about optimal aesthetically.


Thursday night I was able to see the closer pair elongated in my 80 mm F/6 ED doublet at about 200x.




    • PhilH, Sasa, DAG792 and 2 others like this

Zeta Cnc has long been a favourite  spring double for me. This season I used an 80mm SW ed instead of the usual 6-inch refractor. At 200x using a 3 mm Delite the elongation was obvious especially when compared with the C component. Time to try  60mm.


I would like to know of those who can see Zeta as a naked eye star.

    • PhilH and Sasa like this


I would like to know of those who can see Zeta as a naked eye star.

Anyone should be able to see it naked eye in rural sky.  At ~4.6 magnitude combined, I detect it naked eye in the suburbs (red zone.)  

    • PhilH likes this

Was out again tonight w/my Nexstar 11 to view Zeta Cancer,,,,the seeing was so bad that I couldn't split.  Beta Mon is also  available and is a delightful triple that is similiar to Zeta Cancer.  It is included in the Nexstar double star list. 

    • PhilH likes this

Here is a picture of Beta Mon......all the stars are "B" and are white.



    • Jon Isaacs and Eclipsed like this

Here is a picture of Beta Mon......all the stars are "B" and are white.

I also had a go at Beta Mon (sorry to step away from thread topic!). 
edit: I’ll repost in the Double Star thread!

    • Wallyl likes this

There is an entire forum dedicated to Double Stars in the Observing Forum. If someone wants to post pictures of Beta Mon it's probably more appropriate to do it there, under a new thread. 

    • PhilH likes this

Thanks for a great challenge, Phil!  I'm mostly a DSO guy, and often overlook things like this.  Thanks for providing a nice first view.  I was able to split all three components with my 12.5" dob and mediocre seeing from the greater DC area the other night.  Keep up the great work!

    • PhilH, Lewis Cason and Sasa like this
Mar 17 2021 03:03 PM

Just had an hour or so under fairly good skies. Zeta Cancri AB was *just* split at 187x  (6mm Zeiss ortho) in my 102/1122mm ED. I could just barely see the thinnest black line imaginable between them. 



Clear skies!

Thomas, Denmark

    • PhilH, Lewis Cason, Sasa and 6 others like this

I’m going to have another look tonight (if the sky stays clear).  It’s well worth another look and I’m going to try more eyepieces to see what looks the best.  If it’s super clear and stable, another photo is worth a go.  

    • PhilH likes this

Last night was clear and not too bad as far as seeing conditions.  A little breezy, which causes some jitter when viewing at higher magnifications.  Visually, I was able to just split the tighter pair of Zeta Cancri 1a and 1b.


1. Celestron 40mm Plossl (51x):  If I looked just right, I could barely split Zeta 1 and Zeta 2.

2. APM 30mm 2" Ultra Flat Field (68x):  Clearer split of Zeta 1 and 2 but no sign of Zeta 1a/b split

3. Baader 17.5mm Morpheus (116x): Great clear view of Zeta 1 and 2.  Zeta 1 showed a slight elongated shape but I couldn't see a clean split.

4. Baader 17.3mm plus 2x Orion Barlow (232x):  Finally I could clearly see all three stars.  Quite shaky and some atmospheric issues but from time to time a very nice view.


Then I snapped a short video using my SvBony camera and the Orion Barlow.  The video had a quite a lot of smudgy images but there were enough very decent images in there to allow some simple processing to get an image.  This was a quicky and I didn't spend much time finessing the image quality:


Attached Thumbnails

  • Attached Image: Zeta Cancri Image Details.png
    • PhilH, Lewis Cason, Stargazer3236 and 4 others like this

I finally managed it! Average seeing and above the roof of my house, and yet with my 6" Newtonian at my highest power (6mm eyepiece + 2.25 Barlow = 281x) Zeta 1 and 2 Zeta A and B were a clean split. It was a lovely sight, and the challenge taught me a lot about observing doubles: being patient, identifying their tiny Airy disks when at high power, getting help from color differences (not on Tegmine… but on Izar it was key to spotting the secondary). I also think this challenge gave me full confidence in my scope and in my ability to collimate it properly.  It's certainly not a fancy model but given average conditions it managed to split stars not too far from its theoretical Dawes' limit of 0.8".

After Tegmine I've gone for more doubles and had lots of fun – loved Izar and Xi Boo. K Leonis resisted me. The bright moonlit sky may have had a role, as the secondary is so dim.


Tonight seeing is forecast to be great and I'll go for another doubles round! Thanks Phil, and thanks to all who contributed to the discussion!

    • PhilH, Astrojensen, Sasa and 1 other like this

EDIT: I wrote Zeta 1 and Zeta 2… I meant the two components of Zeta 1 were a clean split… DOH! ;D

Renewed thanks to Phil! I'm just dropping one more message to say that Tegmine has now become a favorite of mine and every time I'm in that corner of the sky I always pay a visit. And what a goldmine of interesting multiple stars that corner of the sky is! Bright Castor right above, blue and white Iota Cnc and lovely 57 Cnc above the Beehive, colorful 38 Lyn just a bit farther North, with its two Struve companions right and left… not to mention impregnable k Leo (my 6" hasn't cracked that one yet!).


I am looking forward to the next Challenge!

    • PhilH likes this

Thanks Phil for this article. Somehow I missed this very nice triple. I checked it tonight with my 82mm refractor. The wider pair was already visible in 40mm eyepiece at 42x. Switching to 16mm eyepiece (104x) showed the Airy disc of main component slightly elongated in PA~0 in contrast with circle shape of the second component. The 6mm eyepiece (278x) showed then beautiful tripple system. In calm moments I could clearly see that the main component was in fact two slightly overlapping discs. Here is a quick sketch with rough orientation from my logbook



    • Special Ed, PhilH, Astrojensen and 4 others like this

Excellent sketch Sasa.  I wish I was more artistic as there are some things that my camera just does not show well (like splitting Rigel).

    • PhilH likes this
Mar 31 2021 02:13 PM

Last night I observed Zeta Cancri in my Stellarvue 105mm triplet F/7 using a TV 3-6 Zoom. 


Observing from my back yard in a Bortle 8 zone on a 450 foot hill about 13 miles from the ocean I often get good seeing conditions.  Last night was about 3.5 / 5.  Using the zoom at 3mm (245x) I was getting a hint of a split.


Using a quality 2x barlow with the zoom at 250x showed the tiniest of a split, so I zoomed to 300x and the view was better, and another click to 375x for the best view of the night, three perfect airy disks with the tiny space between the A and B components.


You got to love the perfect star images in a quality refractor! 

    • PhilH, Lewis Cason, Sasa and 1 other like this
Daniel Mounsey
Mar 31 2021 07:29 PM

Ahhhh my favorite. Phil, it's always an honor having you around. 

Steady skies!


    • PhilH likes this

Finally had clear skies recently here in the Pacific Northwest, and this was one object I wanted to get a good look at. It split very nicely in my 5” Mak-Cass with a 7mm Nagler 

(220x). Seeing was a bit iffy, but when things settled down it was a very clean split that was easier than I expected.


Nice challenge object this month! If you haven’t tried it yet, give it a try.

    • PhilH and Sasa like this

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