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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)

 

Target

Type

RA

DEC

Const.

Magnitude

Size

Zeta
Cancri

Quadruple
star

08h 12.2m

+17° 38.9

Cancer

5.6/6.0/6.1/10.0

see
article

 

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


62 Comments

Photo
John Huntley
Mar 01 2021 08:28 PM

This is an excellent challenge for the small to medium aperture scope !

 

Under good conditions this year I have managed to split the closer pair, A & B, with my Skywatcher ED120 refractor at 257x. The split appears as a very fine thread of black sky between the almost touching airy disks of the A & B stars.

 

With my Takahashi FC100-DL refractor, I cannot quite split this pair but they are "resolved" as a notched pair.

 

I use David Knisely's definitions when observing double stars:

 

post-3169-14074143332622_thumb.jpg

 

Thanks for the supply of challenges Phil !

    • Dave Mitsky, Jon Isaacs, PhilH and 6 others like this

I heard from a couple of CN'ers that two errors crept into the article this month.  They have since been corrected.  Thanks, Dave and Loretta!  Keep those cards and letters coming.

    • Dave Mitsky and Voyager 3 like this

If it's clear here in Southern Ohio tonight (fingers crossed), I'm going to have a go at this using my Celestron 8 EdgeHD.  I love doubles and if all goes well, maybe I'll have good enough seeing to catch this as a triple!  If I read things correctly, the spacing between Zeta 1 and Zeta 2 Cancri is 5.06 arcseconds and the spacing between Zeta 1A and Zeta1B is currently still close to its maximum of 1.2 arc seconds.  I'll keep you posted of results.

    • Jon Isaacs, PhilH, John Huntley and 1 other like this

I was out last night with my 13.1 inch F/5.5  Starsplitter. I'd left it out the night before covered.. 

 

I was looking forward to looking at Zeta Cancri as my number one target/goal for the evening. The seeing was what I consider so-so but I was able to split the 1.2" pair, not the prettiest but definitely split. I'll be looking at a lot more.

 

Phil picked a good one this month, Zeta CNC will be part of my permanent repertoire. It's always amazing, how many fine doubles there are and just how many I haven't seen.

 

Jon

    • Dave Mitsky, PhilH, John Huntley and 6 others like this

Tonight was decent and although it was cold I managed to view Zeta Cancri with my Celestron 8 EdgeHD.  Using my 2" 2X Orion Barlow plus the Baader Morpheus 17.5mm (approx 232X magnification), I saw a slightly shaky but clear image of all 3 components.  Then I attached my SvBony 305 camera to the Barlow and captured about a 3,000 frame SER video.  Converted to AVI using PPIP, aligned and stacked with AutoStakkert and lightly processed with Registax.  The lucky imaging process certainly cleaned up the video, as I only used about 20% of the best frames.

 


Attached Thumbnails

  • Attached Image: 19_29_10_pipp_g4_ap3_Drizzle15  2021_03_02 Zeta Cancri.png
    • Dave Mitsky, Jon Isaacs, PhilH and 8 others like this
Photo
TelescopeBah
Mar 04 2021 09:55 PM
Good job there Eclipsed, I was able to resolve the second component at 58× with my 12"Dob. Then I cranked up the power to 508× and was able to split the third component. Thanks PhilH! Fun challenge!
    • Jon Isaacs and PhilH like this

I had the DZ and DL out tonight pointed at Zeta. A big thanks to John Huntley for posting the different definitions above. I wasn't able to split Zeta1A and B, but I was able to resolve the pair. Quite easily, in fact, with both scopes, using a Vixen 3.4HR. I might have to try a barlow the next time and see if I can actually split them. But maybe that's not possible with a 4" scope?

 

DZ and DL taking in Zeta Cancri
    • PhilH and Magnus Ahrling like this

I had the DZ and DL out tonight pointed at Zeta. A big thanks to John Huntley for posting the different definitions above. I wasn't able to split Zeta1A and B, but I was able to resolve the pair. Quite easily, in fact, with both scopes, using a Vixen 3.4HR. I might have to try a barlow the next time and see if I can actually split them. But maybe that's not possible with a 4" scope?

 

 

The Dawes limit for a 4 inch is 1.14" 

 

Stelle Doppie lists Zeta Cancri at 1.10".

 

https://www.stelledo...icerca= Search

 

Sky Safari Pro lists it at 1.1"

 

Sky Tools 3 lists it at 1.13".

 

Both Sky Tools and SkySafari compute the separation based on orbital parameters. 

 

So, it may be possible with a 4 inch but it would be a very difficult split.

 

That said, last night I gave it a look in my 120 mm ED refractor and I was able to make the split though it was not wide, it seemed wider than 1.1".  That was at about 250x, I tried 360 and 510x but it wasnt any better. I saw some separation at 180x.

 

Around 8 pm, I pulled out my 10 inch Dob. After it had cooled for an hour, it made the split quite nicely at 240x on up.  It's very rare that I setup a Dob after sunset let alone more than two hours after sunset, I can't remember the last time I did it. I wanted to compare the 120 mm and the Dob.

 

The seeing here tends to be quite good and the 120mm often runs out of gas before the seeing does. 

 

Give Zeta CNC a try, it's an almost perfect Dawes limit split in a 4 inch.

 

Phil picked a good one.

 

Jon

    • Dave Mitsky, PhilH, Magnus Ahrling and 3 others like this

The seeing here tends to be quite good and the 120mm often runs out of gas before the seeing does. 

 

Give Zeta CNC a try, it's an almost perfect Dawes limit split in a 4 inch.

 

Phil picked a good one.

 

Jon

Here's a silly question, Jon. 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. 

    • PhilH likes this

By the way... I'm impressed that the OP was able to put a Greek Zeta in the thread title. That's awesome! 

    • Jon Isaacs, PhilH and Magnus Ahrling like this

I had another look at Zeta Cancri last night and couldn't split the closer pair this time.  It was colder and a bit windy and with the scope jiggling around, it wasn't a good night at all. Seemed easy 2 days earlier.  What a difference the seeing conditions make!!

 

Going back to images I took on the better evening of March 2nd, I zoomed in greatly on a stacked/processed image and attempted to measure the relative spacing of the three components Zeta 1A, 1B and Zeta 2.  Using a spacing of 5.06" for the A-B distance I created a bullseye scale.  Looking at my diagram, I'm seeing a spacing between Zeta 1A and 1B of around 1".  Of course there's plenty of room for error in my basic comparison measurement but I thought it would be interesting to try anyway.

 

 

 


Attached Thumbnails

  • Attached Image: Zeta Cancri diagram jpg.jpg
    • Dave Mitsky, Jon Isaacs, PhilH and 4 others like this

Here's a silly question, Jon. 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. 

It's actually somewhat complicated.  I am basing this on my experiences.. 

 

There are two competing factors:

 

- The Airy disk of the larger scope is smaller.

 

- At larger exit pupils, the eye does not resolve the Airy disk.  

 

The minimum magnification needed to resolve a 2.5 arc-second double in a 10 inch will probably be greater than in a 4 inch because the double is well within the reach of the 4 inch, that would be under 100x on a good night.  A 2.5 mm exit pupil is probably too large in the 10 inch.

 

But as the separation decreases, the disks begin to overlap in the 4 inch which means very high magnifications will be needed, usually about 80x/inch for a Dawes limit split.

 

Meanwhile, the 10 inch is just coming into it's own, the exit pupil is in the region where it's small enough to resolve the Airy disks so the smaller Airy disks take over.

 

Jon 

    • Dave Mitsky, PhilH, Voyager 3 and 1 other like this

Here's a silly question, Jon. 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. 

I found Rigel very hard to split with my 8" telescope even though the separation between Rigel A and Rigel B is about 9.5 arcseconds.  The issue here is the extremely different apparent brightness of the two stars (A is 0.13 and B is 6.7).  Rigel A is about 440x brighter than B. The glare of the brighter star washes out he dim companion star.  I think in this case even a 100X magnification would split the pair but the quality of the optics (and your eyes) comes into play too.  I successfully saw Rigel B using my F/10 scope (2032 mm focal length) using a Baader Morpheus 17.5mm.  This is a magnification of 116x.  

Back to Zeta Cancri.  The stars are all almost the same magnitude and this certainly makes the observation of the 1.1" pair a lot easier.

    • PhilH and StarAlert like this

I found Rigel very hard to split with my 8" telescope even though the separation between Rigel A and Rigel B is about 9.5 arcseconds.  The issue here is the extremely different apparent brightness of the two stars (A is 0.13 and B is 6.7).  Rigel A is about 440x brighter than B. The glare of the brighter star washes out he dim companion star.  I think in this case even a 100X magnification would split the pair but the quality of the optics (and your eyes) comes into play too.  I successfully saw Rigel B using my F/10 scope (2032 mm focal length) using a Baader Morpheus 17.5mm.  This is a magnification of 116x.  

Back to Zeta Cancri.  The stars are all almost the same magnitude and this certainly makes the observation of the 1.1" pair a lot easier.

I agree. Rigel is an easy split at 100x with my 4” Taks. 
 

I’m not so sure about the same-magnitude Zeta Cancri being easier. Thanks to John’s post, according to Knisely a split is when one can see a thread of black space between the two stars. It seems to me that two very close (1.1”) bright stars make this more difficult because the amount of “darkness” between them is less than if the stars were dimmer.

 

I don’t know exactly how separation is measured, but if it’s measured as the distance between the center point of the two stars, dimmer stars would have more dark space between them, yes? Wouldn’t this make them easier to split? 

    • PhilH likes this
Photo
Astrojensen
Mar 06 2021 11:02 AM

I've had a couple of rather decent nights with good seeing. March 4th I was observing with my APM 152 ED and could easily resolve Zeta Cnc AB at 255x (4.7mm ES82). Two distinct, sharp dots, with a thin line of blackness in between. The next night, March 5th, I only had my 63mm Zeiss out, which couldn't split AB, but did show it as a little elongated. The elongation was noted already at 93x (9mm ES100), and distinctly seen at 179x (4.7mm ES82). 

 

 

Clear skies!

Thomas, Denmark 

    • PhilH, Lewis Cason, Pete W and 4 others like this

I don’t know exactly how separation is measured, but if it’s measured as the distance between the center point of the two stars, dimmer stars would have more darkness between them, yes? Wouldn’t this make them easier to split? 

I'm sure the apparent separation between two stars in a double are based on the center points of each.  In any case, no Earthbound telescope can really accurately resolve these stars as discs due to their immense distance.  Ideally, I think any star should be just a pinpoint of light.  I reality, they aren't of course, due to atmospherics, telescope optical limitations (Dawes Limit as well as unwanted artifacts like coma, chromatic aberrations, lense/mirror imperfections, collimation, etc.)  On top of that I suppose our eyeballs are also not at all perfect ! (I know mine aren't, as I've already had cataracts removed and "so-called" multifocal lense implants inserted).

    • PhilH likes this

I found Rigel very hard to split with my 8" telescope even though the separation between Rigel A and Rigel B is about 9.5 arcseconds.  The issue here is the extremely different apparent brightness of the two stars (A is 0.13 and B is 6.7).  Rigel A is about 440x brighter than B. The glare of the brighter star washes out he dim companion star.  I think in this case even a 100X magnification would split the pair but the quality of the optics (and your eyes) comes into play too.  I successfully saw Rigel B using my F/10 scope (2032 mm focal length) using a Baader Morpheus 17.5mm.  This is a magnification of 116x.  

Back to Zeta Cancri.  The stars are all almost the same magnitude and this certainly makes the observation of the 1.1" pair a lot easier.

 

I find Rigel B easily seen in larger scopes at relatively low magnifications, even in poor seeing. I have fond memories of Rigel B just clearing the backyard fence at about 50 x in my 12.5 inch.

 

These days, I have cataracts and they seem to result in some glare and less sharpness around bright stars but Rigel still comes clean at low mags most the time. 

 

Jon

    • PhilH, Magnus Ahrling and argonbeam like this

Took a peek at zeta Cnc last night with my old C5.  At 300X (4-2mm Nagler zoom) the tight pair was a clean split - dark gap between the Airy disks - during fleeting moments of good seeing.  Thanks for posting it as a challenge object!

    • Jon Isaacs, PhilH, Sasa and 3 others like this

Just back in. Thanks Phil for the challenge!

I was enjoying the Beehive and decided I'd give it a try.

I came in thinking that I had split it, but now I think I only split A/B (together) and C… I was at 125x in a 6" f/5 Newtonian, seeing not exceptional and some cloud. Tegmine showed like a very close pair of evenly matched, yellow stars with a noticeably dimmer star farther down. Boosting power at 281x, the image started falling apart but I could still distinctly separate the three elements – not more – and they occupied a great portion of my FoV (0°11’06”).

The arrangement looks very much like that posted by Eclipse above, but the third star was noticeably dimmer and the maths don't compute (Phil says at least 200x, and 6" should not look so big in the eyepiece even at high power…).

Anyway, it was a lovely sight and I still have some weeks before the challenge's over!

    • PhilH likes this

Just back in. Thanks Phil for the challenge!

I was enjoying the Beehive and decided I'd give it a try.

I came in thinking that I had split it, but now I think I only split A/B (together) and C… I was at 125x in a 6" f/5 Newtonian, seeing not exceptional and some cloud. Tegmine showed like a very close pair of evenly matched, yellow stars with a noticeably dimmer star farther down. Boosting power at 281x, the image started falling apart but I could still distinctly separate the three elements – not more – and they occupied a great portion of my FoV (0°11’06”).

The arrangement looks very much like that posted by Eclipse above, but the third star was noticeably dimmer and the maths don't compute (Phil says at least 200x, and 6" should not look so big in the eyepiece even at high power…).

Anyway, it was a lovely sight and I still have some weeks before the challenge's over!

According to what I found the brightness of these three stars are very similar:

Zeta1 Cancri A: +5.58

Zeta1 Cancri B: +5.99

Zeta2 Cancri: +6.12

    • PhilH and radiofm74 like this
Photo
Lewis Cason
Mar 10 2021 08:34 AM

I was able to observe Zeta A and B last night, 3/09/21, with my 8" f6 dob and 5mm LVW ep, 240x. I live at the beach in South Carolina where turbulent skies are the rule rather than the exception, and last night was no exception. However, during moments of steadiness, I could see a clean split of 2 yellow stars of almost equal brightness. It was a truly splendid sight. Thanks for the article Phil!

 

Lewis Cason

Kiawah Island, SC

    • PhilH, Sasa and Magnus Ahrling like this

Great reports, everyone!  Keep 'em coming.  I am glad to see there is so much interest in binaries.  I'll have to feature more in future months.

    • Jon Isaacs, payner, Magnus Ahrling and 2 others like this

Great reports, everyone!  Keep 'em coming.  I am glad to see there is so much interest in binaries.  I'll have to include feature more in future months.

 

Phil:

 

Binaries are great because they don't require dark skies so they're available to urban observers. I spend about 7-14 days a month at our place in the high desert where the skies are quite dark. The rest of the time I'm in San Diego observing from my light polluted backyard. The seeing is often quite good so double stars are an important part of the menu.

 

Here's a thought:  Every month, along with the challenge object of the month,  include an interesting double so there's something for those who can't get out to dark skies.

 

Jon

    • PhilH, Astrojensen, Magnus Ahrling and 5 others like this

Great reports, everyone!  Keep 'em coming.  I am glad to see there is so much interest in binaries.  I'll have to include feature more in future months.

waytogo.gif waytogo.gif  

    • PhilH and radiofm74 like this

Here's a thought:  Every month, along with the challenge object of the month,  include an interesting double so there's something for those who can't get out to dark skies.

 

Jon

waytogo.gif waytogo.gif +1 .

    • Jon Isaacs, PhilH and radiofm74 like this


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