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The Omega (ω) and the Omicron (ο) of Cygnus: ο-1 and ο-2; and ω-1 and ω-2 (S 755 and S 756)

Confused by the title?

Well, it’s no wonder.  Appearances just aren’t what they seem in Cygnus.

The Omega (ω) and Omicron (ο) of Cygnus, tucked into its northwest corner. (Stellarium screen image with labels added, click to enlarge)

There are rumors of some very strange things happening up in that sector of sky.  One Greek letter subdivides in the night, followed by another, and then one of those halfs halves itself again  ………………..

……………….   and the result is rampant confusion on a stellar scale.

But have no fear.  Any Star Splitter worth his eyepieces and focus fingers should be willing to accept the challenge of making order out of this disorder.

So here I am.  😎

But I have to confess, every time I’ve looked at this section of the sky, I’ve been unable to  make heads or tails out of it — not to mention AB, AC, or AD.  What little information I had was very nebulous and no help.  I couldn’t tell Omicron-1 from Omicron-2, and the Omega pair puzzled me at least as much.  Without a scorecard, I was lost in space — and  I suspect Sissy Haas had a similar experience since she leaves both Omicron (ο) and Omega (ω) out of her listings.

So  —- we’ll start with Omicron-1, and for a taste of the astronomical confusion surrounding it, take a look at Jim Kaler’s first paragraph on  it.

You’ll find Omicron-1 and -2 located halfway between Deneb and Delta (δ) Cygni and slightly to the north. Omicron-2 forms a low and wide triangle with Deneb and Delta (δ), which may be a better way to locate it, depending on your sky brightness. I had no trouble seeing both of them with the unaided eye while the moon was ninety percent of the way to being full. There’s a more detailed chart further down the page. (Stellarium screen image, labels added, click to enlarge)

Splitting Tools for Omicron-1 and Omicron-2: The “C” and “D” components of Omicron-1 are a breeze in a 50mm or 60mm scope as well as in hand-held or mounted binoculars, and are stunning sights — see Greg’s comments at the end of this post for some of the details.  Both components of Omicron-2 are also easily seen in 50mm and 60mm scopes —  although visible in a pair of mounted binoculars as well, they lack the visual appeal of Omicron-1.  The first three sketches below were all made with a 50mm scope!

Omicron-1 (ο-1)  (Σ I 50)  (H VI 10 – AC only)       HIP: 99675    SAO: 49337
RA: 20h 13.6m   Dec: +46° 44′
*****    Magnitudes   Separation  Position Angle   WDS Data
AB:         3.9, 13.4            36.0″                327°                2007
AC:         3.9,   7.0          106.7″                174°               2008
AD:         3.9,   4.8          333.8″                325°               2008
Distances:  “A” is at 1354 Light Years; “D” (30 Cygni) is at 717 Light Years
Spectral Classifications: All K2

As the Washington Double Star Catalog (WDS) numbers above would seem to reveal, Omicron-1 is a quadruple star.  But “B” — which at a magnitude of 13.4 you can count yourself fortunate to see — is listed with a separate designation, HJ 1495 (h 1495 in Sir John Herschel‘s catalog).  And “D,” at a distant five and a half arc minutes away — and also designated as 30 Cygni — does not, per Jim Kaler’s comments above, seem to be part of this system.  And that becomes obvious when you take note of the 636 light years separating 30 Cygni from the numerously-named primary.

Ah, but the real question, the one that draws us here in the first place, is what does Omicron-1 look like in a telescope.

It’s stunning.

The stunning gold of o-1 and the white white of 30 Cygni (“D”). (East and west reversed to match the refractor view, click for a view without this caption.)

The primary is a deep gold, with even a touch of reddish-orange flickering to life every now and then.  “B” is invisible — no surprise there — and “C”, despite its dimness relative to the the rest of the system, is distinctly bluish-white.  And “D”, aka 30 Cygni, is a rather pure shade of white.  All this in a 50mm scope, no less.  And in larger apertures, the colors are much richer and vivaciously vibrant.  Did I say stunning?

And keep in mind I was looking at them on a summer evening when the moon was about eighty percent full. On a dark night?  My photonic receptors quiver in anticipation at the thought.

Omicron-2 (ο-2)  (S 743)  (H VI 33)      HIP: 99848    SAO: 49385
RA: 20h 15.3m   Dec: +47° 42.5′
Magnitudes: 4.2, 8.4
Separation: 208.4″   (WDS 2002)
Position Angle: 177°
Distance: 1109 Light Years
Spectral Classification: K3, B3

Now Omicron-2 is a much simpler case, being a regular old dual pair of orbiting suns, and with only one additional designation, S 743, bestowed on it by James South.  If we had started here, though, we would have probably made things worse by working in reverse.  But we better look at this one quickly before someone adds another name to it, or it subdivides itself.

If you ban 30 Cygni from the view, the resemblance between o-1 and o-2 is downright uncanny. (East & west reversed, click to enlarge)

I had to blink a couple of times and shake my head just a bit to clear the moon beams out of my eyes.  That 80% of full moon was peering over my right shoulder when I was looking at the Omicron (ο) twins, but it was the color of the o-2 primary that caused me to blink.  Darned if it wasn’t virtually identical to the Omicron-1 primary.  Nor does the resemblance end there.  I could see the same bluish-white in the 8.4 magnitude secondary that I saw in Omicron-1’s “C” companion  ……  and the the position angles of the two stars — 174 degrees for Omicron-1 “C” and 177 degrees for Omicron-2 “B” — are so close you can’t see any visual difference.  Throw 30 Cygni out of the picture (go away, stray star!) and Omicron-1 and -2 come very close to being identical twins.

BUT —  if you really  want to feel the full impact of their virtual twin-ness, slip a low power, wide angle eyepiece into your scope, and catch both Omicron-1 and -2 in the same field of view together.

The virtual twin-ness of o-1 (at the left) and o-2 (at the right) in the same field of view. (East & west reversed, click for a view without this caption.)

Your star starved eyes will immediately be struck by those matching primary colors, as well as the echoing positions of the two stars to their south.  It really is  very striking.

And now, on to the Omega (ω) brothers — and this time we’ll run in reverse and save the confusion for last.

You’ll find the Omega (ω) twins two degrees to the northeast of Omicron-2, which will show up nicely in a 6×30 or 8×50 finder. If you don’t have a finder, move north one degree and then east two degrees to get there. These are astronomical directions, not compass directions! (Stellarium screen image, labels added, click for a larger view)

(See this post if I confused you with the comment above about astronomical directions).

Omega-2 (ω-2):
S 755  (H IV 23 — AB only)        HIP: 101206    SAO: 49731
RA: 20h 30.5m   Dec: +49° 12.5′
Magnitudes    AB: 6.6, 9.7     AC: 6.6, 13.5
Separation     AB: 59.3″         AC: 20.4″
Position Angles   AB: 278°  (WDS 2002)   AC: 118°   (WDS 2000)
Distance: 463 Light Years
Spectral Classification: A2

S 756  (Ruchba)        HIP: 101243    SAO: 49741
RA: 20h 31.2m   Dec: +49° 13.1′
Magnitudes: 5.6, 10.2
Separation: 60.7″
Position Angle: 327°   (WDS 2010)
Distance: 404 Light Years
Spectral Classification: M2

Omega-2 actually consists of what are apparently two different systems, each designated separately, one of which is a triple, and the other a double.  But at a difference in distance of 59 light years, it would be a slight stretch to call them related.

The South siblings sit at the center of the field while Omega-1 imitates inconspicuousness over in the south corner of the view. (East and west reversed once more to match the refractor view, click for a view without this caption)

On the other hand, there’s nothing difficult about this pair of multiple stars.  They leap right out at you from the eyepiece because they’re so close together.  And their tenth magnitude companions are obvious — at least in four or more inches of aperture.  I needed averted vision to see either of them in the 50mm Zeiss, but the moon had raised its wattage to about ninety percent of full on this particular night — otherwise I suspect they would have been obvious even at that aperture.

In the four inch scope, both primaries seemed to be floating above the black background.  S 755’s primary beamed back at me in pale white, and S 756’s beamed even brighter in a light gold color, flashing an occasional few photons of orange and red.  The 9.7 and 10.2 magnitude secondaries were faint — too faint for detecting color — but obvious with direct vision.  I had tried to pry those colors loose with a 6mm Radian in the AT-111 (130x), and even though that didn’t work, I did find that I could pick out two very faint stars hovering around S 756’s primary.  One was at about 280 degrees and the other was about 70 degrees, which meant they were bracketing the 10.2 magnitude secondary (both are shown in the sketch).  Apparently they’re not companions, since they’re not listed in the WDS database, but they certainly had that appearance.

And, even as I hesitate to add to the mix of characteristic confusion, it must be said, that S 756 is the star that officially carries the ω-2 designation.  S 755 must be there to keep an eye on it.

Now, as for the first Omega (ω)   ……………………..

Omega-1 (ω-1)  (Bu 669) (45 Cygni)       HIP: 101138    SAO: 49712
RA: 20h 30.1m   Dec: +48° 57′
Magnitudes    AB: 5.0, 12.9       AC: 5.0, 9.4
Separations   AB: 17.3″             AC: 55.9″
Position Angles:   AB: 336°      AC: 88°         (WDS 2000)
Distance: 870 Light Years
Spectral Classification: B2.5

……………..    I couldn’t find it.  You can see it in the sketch above, but I didn’t realize that was it for one simple reason:  I couldn’t see the 9.4 magnitude “C” companion!

I looked several times — it wasn’t there.

I went back to the Cambridge Double Star Atlas  more times than I can count, got my bearings relative to 43 Cygni and the South siblings, and looked again and again and again.  I couldn’t see anything that came close to the 5.0 magnitude of the primary — other than what turned out to be the correct star — and I kept ignoring it because of the absence of a visible companion.

Finally I realized it had to be the one, put the 6mm Radian (130x) back in the scope, looked again, and glimpsed the devious little disappearing devil — and even at that magnification, I had to revert to averted vision to pluck it from the darkness.

Now that companion has a magnitude of 9.4, which is brighter than either of the very visible companions of the South pair — and the primary in this case is only six tenths of a magnitude brighter than the primary of S 756 — so why the difficulty in seeing it?  Could it be it’s just shy?  Maybe the moon knows why.

All I can conclude is it must be part of the “things aren’t what they seem” scheme.

However, and anyway, and finally — I have now accomplished something that I’ve been meaning to do for well over a year, which is to make some kind of sense out of this Cygnetic confusion.  I can now distinguish Omicron-1 from Omicron-2, separate 30 Cygni from the scene, linger over the twin view of the Sh siblings, link Omega-2 to the correct star — and even find Omega-1.

I now have the scorecard I so sorely lacked last year.

So if you want to know who’s on first, or what’s on second, or if what’s on third instead — feel free to start here.

And if that fails, the thing to do,
is just admire the wide field view,
of Omicron-1  ……  and Omicron-2.

Clear Skies!


Update and Corrections!!!

Now, as should be obvious from the above, the reason I wrote this post was because these stars had me totally confused as to what designations belonged to which stars.   After reading Sir Jame’s South’s entries in his catalog of 1826, I can see that the confusion has been around since at least 1824.  The “brief” title of that 1826 work, by the way, is Observations of the Apparent Distances and Positions of 458 Double and Triple Stars, Made in the Years 1823, 1824, and 1825; together with a Re-Examination of 36 Stars of the Same Description, the Distances and Positions of Which Were Communicated in a Former Memoir, and it’s available in .pdf format here.

But let’s start with the confusion in The Cambridge Star Atlas, which has labeled the two stars that constitute ω-2 with the wrong prefix, “Sh”.  That prefix belongs to the stars listed in a separate publication, Observations of the Apparent Distances and Positions of 380 Double and Triple Stars, Made in the Years 1821, 1822, and 1823, and Compared with Those of Other Astronomers . . . . , which was a joint effort of James South and John Herschel, published in 1824, and available here  (scroll down to the last entry on the page).

But — the ω-2 pair is cataloged in South’s 1826 publication (the first one mentioned above), and all of those stars are assigned the prefix “S.”  So, the correct identification for the ω-2 pair is  S 755 and S 756 — and the post above has been corrected to reflect that.

But back to Sir James South:  At the conclusion of his remarks on the observations he made in 1824-25 of  S 755,  Sir James comments, “There is some reason to suppose that Sir W. Herschel has erroneously called this star ω-2 Cygni, which it certainly is not; ω-2 as far as my instruments can inform me is single.”  (p. 259 of the 1826 catalog)

The first part of that statment is correct — S 755 was NOT assigned the ω-2 designation — it was assigned instead to S 756.  Who knows why.  Of course it doesn’t help that both S 755 and S 756 show up on most star maps as ω-2.

But if you’ve read the entry above, you’ll know that ω-2 (S 756) is not single, but a double.  The reason for South’s remark becomes a bit more clear at the beginning of his entry for S 756 when you see that he identifies it as ω-3 — and I have no idea where THAT came from.  Again —  that one is actually ω-2.  At any rate, I don’t have the first clue WHAT star Sir James was referring to in the reference to ω-2 as single, and I’m not about to hazard a guess as to why he used ω-3 in connection with S 756.

All I can say is this obviously has proven to be a very confusing part of the sky for a lot of people for a long time.  So if you lose your bearings and begin muttering miscellaneous numbers incoherently, don’t let it bother you.  You’re in some pretty darn good company.  😎


4 Responses

  1. August 15th, 11 PM

    Took another look tonight at Omicron-1 and -2, as well as Omega-1 and -2, using an old Sears f15 80mm scope with a Towa lens. I haven’t used that scope for a while, so the first thing I noticed was how narrow, as in restrictive, the field of view is. It was impossible to get O-1 and O-2 in the same field. On the other hand, it does tend to concentrate your view right where it really counts — on the stars themselves!

    I noticed once again that the 9.4 magnitude companion of Omega-1 was playing hide and seek, even as the 9.7 and 10.2 magnitude companions of S-755 and -756 were easily seen. There’s a star slightly to the northeast of O-1 (shown on the sketch of Omega-1 and -2 above) and just beyond the 9.4 magnitude companion, which has a magnitude of 9.1 according to MegaStar. That star pops into view with averted vision first, and then if I continue to look, I can usually catch a glimpse of the 9.4 companion. But even the 9.1 magnitude star is more difficult to see than either of the fainter companions of S-755 and -756.

    I noticed the same thing last night as well when I was using a six inch Celestron refractor. Despite the additional aperture, those two stars surrounding O-1 were still more difficult to see than those around the two Sh- primaries. Both nights I was competing with a very bright moon, still at about seventy to eighty percent of full even though it was on the wane.

    So it would seem that the 5.0 magnitude of O-1’s primary is just enough brighter than the primaries of S-755 (6.6) and -756 (5.6) to cast a slight glare over the surrounding area. At any rate, trying to pry those two stars out of the glare seems roughly equivalent to looking for stars a full magnitude fainter than what they are.

    I doubt a dark, moonless night will make all that much difference — glare is glare, after all — but I’ll be back to take another look anyway.

    Just in case.

  2. Binocular Perspective – I used Omicron 1 as one of my first test objects for Canon 10X42 Image Stabilized binoculars and found it an interesting challenge. I also used 8X40 Celestrons, 11X56 Garrett Optical, and 15X70 – real cheap – Celestrons.

    The results – the 15X70s handheld showed me the most color and I’m sure if I had mounted them would have given me the best view of all.

    As it as, the 11X56 – when mounted on a parallelogram mount – gave me the best view. The extra light grasp was needed for “C” in my skies.

    The 10X42 IS allowed me to just pick out the “C” component. It was very nice because you got the same steady view as with the 11X56 mounted – but you had the freedom of NOT using a mount.

    The 8×40 handheld showed me just “A” and “D”.

    Bottom line – for the right binocular this makes a real nice binocular triple! I have the Canon 10X30IS ont he way and am looking forward to learning if I can pick out the “C” star with their small objectives.

  3. I believe that Omega-1 is BU 669, which is not recognised by either Smyth or Webb (but see below). Omega-2 is S 755, which is Smyth’s 199 P. XX Cygni. Both Smyth and Webb have a Omega-3 which is no longer recognised, but they differ as it what is. Smyth’s Omega-3 is clearly S 756, but Webb’s appears to be BU 669 as he mentions the fainter companion mentioned by Burnham and has the right data for BU 669.

    • Hi Peter,

      You’re correct — Omega-1 is also Bu 669 and Omega-2 is S 755, both of which are mentioned in the data lines in the post above. You’re also correct, Omega-3 is no longer recognized, which I believe is also mentioned in the addition to the post — that seems to have started with James South’s use of it. And Admiral Smyth was kind enough to include William Herschel’s H IV 23 designation in his description of 199 P.XX Cygni, which is also included in the data line of the post above — so at least we know which star that particular Piazzi label refers to.

      You’re right about the general confusion in this area as to what labels apply to what stars. Too many cooks in the kitchen no doubt, but then considering it’s both a very attractive and interesting area, I suppose it was inevitable.

      Clear Skies!


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