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Mu (μ) Cygni Musings

When talk turns to challenging double stars in Cygnus, Delta (δ) Cygni usually comes up quickly in the conversation.  It dominates the western wing of Cygnus at a very noticeable magnitude of 2.9, which makes it easy to find.  Over on the Swan’s opposite wing, at the very tip, so far out that a powerful flex of those mighty muscles would extend it across the border into Pegasus, is another difficult double, known to admiring Star Splitters as Mu (μ) Cygni.  Noticeably dimmer than Delta (δ) Cygni at a visual magnitude of 4.6, it forms a neat little triangular asterism with its two cross-border mates, 14 and 15 Pegasi.

Mu (μ) is found flapping over in the upper left hand corner of this chart, at the eastern tip of the Swan’s wing. With its cross-border companions 14 and 15 Pegasi, it forms an obvious triangle. Careful here — it’s very easy to mistake either one of those stars for Mu (μ). I speak from experience! (Stellarium screen image with confusing Greek labels added, click to make them larger)

And I’ve been pursuing that little devil for over a month with no success whatever.  For a few weeks, people in this little coastal town actually crossed to the other side of the street to avoid hearing my mutterings about Mu (μ).

You see, the problem is the seeing.  In fact, it’s not just the seeing, it’s the ocean.  Even more in fact, it’s not just the seeing and the ocean, it’s the prevailing wind patterns — which come in off of the ocean most of the time.  But eventually all three of those things held a climatic conference and decided to cooperate for a couple of nights.  And when they did, I was ready and waiting to finesse that beady little pair of stars out of the inky darkness of interstellar space.

Mu (μ) Cygni  (Σ 2822)  (H III 15)       HIP: 107310    SAO: 89939
RA: 21h 44.1m   Dec: +28° 45′
*****                Magnitudes      Separation      Position Angle        WDS Data
AB:                      4.8,   6.2                1.6″                    318°                      2012
AC:                     4.8, 11.5              72.6″                   290°                       1999
AD:                     4.8,   6.9            197.5″                     44°                        2011
DE (ES 521)     6.9, 13.3              16.9″                   284°                       1999
Stellar Classifications      A: F6    B: G2   C: F6   D: F2   E: A5
Distance:  73 Light Years
Status: AB gravitationally linked (orbit can be seen here), AC & AD are optical pairs

My intention had been to bag this one — and by “this one” I mean the AB pair —  with my Meade AR-5, a five inch achromatic refractor.  Actually, Neil English had called my attention to this star, and was curious about whether I had seen it yet with a five inch scope.  I hadn’t, but I quickly put it on my list.  However, on that first night of improved seeing, I was sitting behind my six inch f/10 instead.  So I swiveled it up and around to the tip of Cygnus’ eastern wing and got to work.

I started with an 18mm Radian (84x), which gave me a great view of both the primary and 6.9 magnitude Mu “D” (which was not at all mu-d like) — but not the first burst of a beam of light from “B”.  Not trusting the seeing to hold, I made a quick swap to a 14mm Radian (109x), and was rewarded with my first glimpse of the 6.2 magnitude secondary — an elongated figure-8  image about ready to snap apart. Skipping the 12mm Radian, I grabbed the 10mm version instead (152x), spun the focus knob quickly until the image began to sharpen, then finessed the fine focus knob a bit, and there it was!

Now up to that point I really wasn’t sure what to expect, but I thought it would probably be a very small point of barely visible light.   Not at all!  What I saw was a rounded little orb of pale yellow-white light about half the diameter of the primary separated by just the slightest slice of silky black night — in all it’s mu-nificence.

” . . . a rounded little orb of pale yellow-white light about half the diameter of the primary separated by just the slightest slice of silky black night — in all it’s mu-nificence.” (East and west reversed to match the view in a refractor — click for a version without this caption)

After all the times that dot of light had avoided me, I was more thrilled than a telescope addict turned loose at NEAF with fifty thousand dollars of cash in his pocket!  It really was just flat out of this world fantastic!

So I had my trusty four-legged side kick hand me the ever dependable 7.5mm Celestron Plössl to see what a 203x view was like — but the seeing was just not quite that good.  The split was wider — not by much — but that ir-rhythmic hopping image kind of killed the esthetic attraction on display in the 10mm view.  So I slipped that eyepiece back into the diagonal and stared until my visual receptors were saturated.

And then I realized there was something strange taking place in the eyepiece.  The color of the primary.  There wasn’t one.  I mean there was, but I didn’t know what it was.  What I mean is — there was more than one.   Color, that is.

At first it was white.  Unquestionably white.  Then it was yellowish.  Without a doubt, yellowish.  But then, after looking a bit longer, it took on a deep yellow hue.  Very yellow, very deep, very definitely.  And every now and then, there was just the slightest flash of orange!  Just a flash, a very fast flash, but a repetitive flash.  And then it was white again.  And then back to a weak yellow.  I started to wonder about my sanity at this point, which wasn’t helped in the least by the euphonic yipping of the chorusing coyotes in the not very distant distance.  Although it didn’t seem to bother my four-legged observing partner in the least.

The 60mm f/15 perched atop the six inch f/10, click for a larger view.

I had just about decided to chalk this colorful chromatic eyepiece experience up to the euphoric effect of having separated “B” from “A”, when it dawned on me that it might be a good idea to take a look in the 60mm f/15 refractor mounted on the six inch scope.  So I stood up, peered into the 20mm TV Plössl (45x) sitting in the diagonal — and saw a sun-yellow primary with a very noticeable orange tint.

So it was back to the books — to see what colors others had seen.  Haas didn’t look at it , but she quotes two observers who did: Heckman, who saw a “nice, whitish pair”, and Webb, who saw “Yellow, [with] tawny or blue or lilac.”

Then I went back in time to 1832 to consult with Admiral William H. Smyth, who saw: “A” white, “B”and “D” blue.  Then I backed up a little further, to John Herschel and James South’s 1823 observation (Sh 332): “A” white, “B” blue.  And finally, even further, to William Herschel’s 1777 observation (H III 15): “A” white, “B” bluish.

Hmmmm — there’s a pattern here, and I’m not in it — nor is Webb.

So I returned to the present to see what the spectral classifications say.

And what they say is the primary is classified as F6, which would be white leaning toward yellow;  the secondary is classed as G2, which is yellowish white, leaning a bit more toward the white than the yellow;  and “D” is F2, which is more white than the primary, and leaning just a slight bit back toward blue.  So I can only conclude that all of us are in the same ball park, but apparently I’m more out in left field than the others are.  With the exception of Webb, of course, who is on the other side of the left field foul line with “lilac.”

Now I didn’t try to split “DE” that first night with the six inch f/10 because the seeing was just not that good.  The following night, with more stable seeing, I had an AT 111 on the observing deck and was rewarded with another very stirring sight of “B” — so good in fact, that I decided to reach for the “DE” pair.  There was quite a bit of haze in the air, so I didn’t expect a lot of luck.  But I kept increasing the magnification, and the image was holding reasonably steady, so I worked up to a 3.2mm TMB Planetary eyepiece (243x).  I couldn’t quite be sure — I thought I saw something — so I tried the 2.5mm version of that eyepiece, which gave me a seldom seen 311x — and saw nothing but a magnified dark haze.

It was then that the melodius choraling of the coyotes coalesced into a brilliant idea in my Star Splitting subconsciousness — why not put the 7.5mm Celestron Plössl in a 2x Barlow, which would produce 207x?  I did.  And it produced!  With averted vision, I was able to catch the barest glimpse of 13.3 magnitude “E”.  I kept looking, and found that occasionally it would become very obvious — still with averted vision, though.  I never did see it with direct vision, but then I didn’t really expect to see it at all, so I certainly can’t complain.

Well, I admit to getting giddy again.  I even yipped in harmony with the coyotes a few times, just to acknowledge their help.  Klaus, my four-legged observing companion, moved to the other side of the deck — I guess to get a better view of his mu-tant master.

Oh, and I’ve neglected to mention 11.5 magnitude “C” — yes, it was easy to see in both large refractors.  In fact, it was visible in the 60mm f/15 even at 82x in an 11mm TV Plössl, again with the aid of averted vision.

So to Neil —  stellar thanks are due for the suggestion.  And, I suppose if I had mu-scled this one into view on the first night, the taste of success wouldn’t have been half as sweet.  But as it turned out, sweet hardly comes close to describing my delight.

Cygnus will still be well positioned for the next couple of months to catch this stellar beauty, so don’t dawdle —  you absolutely MUST see Mu (μ)!

Clear Skies and decent seeing!


9 Responses

  1. Hi John,

    You had me on the edge of my seat reading that account. Yep, Mu Cygni is a little devil alright. Like Pi Aquilae, I find it very seeing dependent . I think the brightness difference between the pairs doesn’t help either (magnitude delta~1.4). It’s a slow moving orbit that will be closing in the coming decades.

    With my 10 x 50 binoculars, the AD pairing makes for a nice sight.

    As for A/B, my notes show that with my 4” f/15 at 250x (6mm orthoscopic) the image varies between elongation and an occasional very thin black space between them, depending on how good the sky conditions are. The greater resolving power of my 5” f/9 refractor does a better job keeping that black space between them on the better nights.

    Colourwise, I noted the pair as yellowish white – much like our own Sun. I’m going to have to take another look at this system with my refractors. I wonder if A/B can be resolved with my 80mm f/11. It ought to be doable…… but only just!

    Best wishes,


  2. I was able to split Mu last night in my four inch f/15 refractor last night, and I needed 250x to get a definite slice of black sky between the primary and secondary — so that seems to be the magic number for that aperture on this star.

    I took another long, deliberate look at the color of the primary, and I still get that wavering between yellow white and deep yellow, with just a bit of orange at times. The “D” star is a very steady pale bluish white, on the other hand.

    Can’t wait to get another look at Mu with a five or six inch refractor — it certainly is more esthetically pleasing at the larger apertures.

  3. Great report John. Back in July I was able to split MU AB with my 90mm F8.8 achromat The seeing was excellent. The components were barely detected at 133x and fully resolved at 160 & 200x. As you mentioned this double needs good seeing for success. According to the WDS the components are closing. As you mentioned Delta is another challenge. Looking at my observing notes I was able to split it in September of ’06 at 236-295X during brief moments of steady air using my 5 inch F9.3 Apogee achromat. At 170x there was a strong suspicion of duplicity. If I remember the event correctly I had to stop breathing to catch brief glimpse’s of the secondary flitting in and out of the diffraction rings.
    Steady Skies,

  4. Thanks for the astounding amount of info. and the pic.s are exquisite. with an 80mm, white and offwwhite are the normal colours. Examined a few stars before i got to mu. To me it seems like the bottom star of the three which are like an equitorial triangle with a close star above (East) and a wide double also close to the north. Got a” result” as football managers say here. So no clear split-but two distinct round stars kissing, and they have may shown a hairline of dark sky momentarily in wishful imaginings but in the witness box i have to say NO. Hope the colder weather is bringing clear and steadier skies your way. regards, rich

  5. More than a “result” tonight- a win! Took 300X to make sure but that got me two well separated round stars. In my ED 80 the primary was off-white and bright compared to the smaller,greyer companion. Just don`t have enough aperture to show the colours you get. Made a careful note of the PA ( well -careful by my standards). At that power both stars were hurtling into the edge. Thanks to your instruction site i now know where exact due west is- or as you also put it-270 degrees. Tried to figure north as a rightangle clockwise from the point where the stars were just about to disappear and the PA seemed almost to split the rightangle, so 315 degrees. Great. BTW i see a black spot on Jupiter`s Northern belt- its to right of centre of the planet and in the top of that belt, Looks very like a shadow transit of a moon but have been seeing this in the same place for more than a week! Just grateful to” see ” tonight after some lousy weather. Regards, rich

  6. Well now I’ve got something to shoot for! With Karl splitting Mu in a 90mm and Rich in an 80mm, I’ll see if I can reduce that even further to a 76mm Tasco! We’ve got some great competition, Neil!

    I won’t hold my breath, though — it’s going to take some pretty darn good seeing to get it — but it does sound like it can be done. I’ll follow Karl’s advice and turn off the breathing apparatus long enough to get between the diffraction rings. 😉

    At any rate, it’s great to see Mu attract this kind of interest. Even when it can’t be split, the little dot of bluish white light from the “D” component makes it a darn nice sight.

    Rich, that black spot on Jupiter is one of a pair black barges that ply the north equatorial belt (NEB). I don’t know how they became designated “barges,” but it sure does fit considering the way they float across the planet.

    If you catch Jupiter at the right time, you’ll see both of them spaced apart about half the radius of the planet’s disc. They look like a pair of Jovian eyes staring back at you. I could swear I saw one wink at me the other night. 😎

  7. Thanks v much John for your swift comment and excellent info. I had never read or heard about the “barges”. Sorry for my slow response- had to do some emegency DIY and first attempt made things worse. Very “iffy” weather here currently but got one night when the seeing was better than “the propellers”. Split mu again with a Baader “genuine Abbe” ortho 7mm in a Shorty Plus barlow -170X. Again the sep. and PA were seen well-but no yellow. Switched to Delta Cyg and threw all i have at it-Zip. Its the bright tip of a much smaller triangle of three stars which seem like an arrowhead pointing approx.NE. It does seem more likely that mu is either one of the stars at top of its triangle particulary if you start at the centre of “the cross” and work your way to the wingtip. Will spend more time “musing” at lower powers to see “D” and more carefully, check the colour(s). Clocked a “barge” again -only one- and the next night , Io`s shadow transit. The physics of the solar system made visible. Literally wonderful. Hope you get clear and steady skies and a split of Mu in the 76mm. Regards, rich.

  8. Hi John, I set out tonight to have another go at splitting 17 Lyra,
    the sky was clear no wind and the seeing was as good as I have
    ever seen it. So on to 17 Lyra using a 6mm ortho giving me 200x
    I thought I saw something where the secondary should be, for brief
    moments a tiny little dot would appear then disappear but I am pretty
    sure it was the B star. Feeling very pleased with myself I deceided
    to have a go at Mu Cygni, I have tried this double before with no luck
    but as the seeing was so good I thought it was wort a try. I started
    with a 10mm ortho giving me 120x now there was something there
    an out of round disk so I dropped in the 6mm and as if by magic
    two stars appeared at first glance the primary looked white and
    the secondary yellow but the longer I looked at them the colours
    seemed to change almost to reverse so the primary looked yellow.
    The secondary was brighter than I thought it would be and easier
    than I thought.
    I went inside for a while for something to eat and when I came out
    about 30 minutes later the seeing was not as good, I could still make
    out the secondary but not as easy or as clear. I looked at a few other doubles in Cygnus then as it was getting close to midnight and
    I was getting cold I called it a night butv it will be a long time before
    I forget the sight of those two little dots in the Zeiss.

    PS I see the latest seperation for Mu is 1.6” must be close tothe
    limit of a 80mm scope.


  9. Great to hear, Pat! And you had the same experience with Mu Cygni’s colors that I described in the post above. Glad it wasn’t just my eyes! Not the first time I’ve suspected a close pair of stars were swapping colors back and forth.

    And it sounds like you almost coaxed 17 Lyrae’s shy secondary to come all the way out of the glow. I’ll face the east and bow to the Zeiss!

    The seeing in your part of the world must have been something special tonight. Just before your comment came in, I got a message from Neil English (in Scotland) that he split 36 Andromedae (magnitudes of 6.1 and 6.5, at 1.0 arc second of separation) with his 127mm Tal refractor, also affectionately known as Tonya, which he describes here (scroll down to 2300 on September 21st).

    Still clouds here, but I sense a change in the next few days — rumor is the sun may even return on one of them. 😎


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