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Sigma (σ) Cassiopeiae: A Smalt Blue Jewel on the Lady’s Shoulder

Sigma (σ) can be seen here to the southwest of Beta (β), just above the back of the Queen's chair.  (Stellarium screen image with labels added, click for a larger view).

Sigma (σ) can be seen here to the southwest of Beta (β), just above the back of the Queen’s chair. (Stellarium screen image with labels added, click for a larger view).

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A beautiful double star on the lady’s left elbow, and one degree south of [NGC 7789], which lies . . . between σ [Sigma] and ρ [Rho].  “A” 6, flushed white; “B” 8, smalt blue; the colours are clear and distinct, though less fine than those of  ε Boötis, of which this is a miniature.”

(Admiral William H. Smyth in The Bedford Catalog, 1986 edition, pp. 541-42)

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The venerable Admiral had a way of describing double stars (and deep sky objects as well) that sometimes leaves you lingering over his words.  In this case, he couldn’t be more correct in his description of Sigma (σ) Cassiopeiae as a “a beautiful double star,” and his colors  —  “flushed white . . . smalt blue”  —  stand the test of time as well.

But I’m not sure what artwork he was gazing at when he placed Sigma (σ) Cass at the Queen’s elbow, or for that matter, on her left side.  Our illustration here favors her right side and shoulder, instead — but that’s not to say the Admiral was in error.  If you look closely, you’ll see north is facing down in the portrayal above — which means if you position north at its usual place at the top of an illustration, you’ve placed the poor Queen on her head.  So in order to uphold the Royal dignity, I rotated the illustration one hundred eighty degrees.  All of which is a long way of saying the Admiral may have been looking at an illustration totally opposite in orientation to the one above.

A lot can happen in the 168 years since he cast his eyes on the Queen.

Different orientation now, with west at the top of the chart and east at the bottom, matching Cassiopeia's position in a late August sky at about one in the morning. An easy way to find your way to Sigma (σ) is to imagine a line at Beta (β) running at a ninety degree angle to the one that connects Alpha (α) and Beta (β). Extending that line four degrees to the southwest will put you right in the middle of a tight little cluster of three stars, the brightest of which is Sigma (σ). Rho (ρ) and Tau (τ) can be seen hovering to its northwest, with the open cluster NGC 7789 midway between Sigma (σ) and Rho (ρ). (Stellarium screen image with labels added, click for a larger view)

Different orientation now, with west at the top of the chart and east at the bottom, matching Cassiopeia’s position in a late August sky at about one in the morning. An easy way to find your way to Sigma (σ) is to imagine a line at Beta (β) running at a ninety degree angle to the one that connects Alpha (α) and Beta (β). Extending that line four degrees to the southwest will put you right in the middle of a tight little cluster of three stars, the brightest of which is Sigma (σ). Rho (ρ) and Tau (τ) can be seen hovering to its northwest, with the open cluster NGC 7789 midway between Sigma (σ) and Rho (ρ). (Stellarium screen image with labels added, click for a larger view)

Sigma (σ) Cassiopeiae (Σ 3049)  (H I V — AB only)     HIP: 118243    SAO: 35947
RA: 23h 59.0m   Dec: +55° 45′
Magnitudes    AB: 5.0, 7.2        AC: 5.0, 10.4
Separation     AB: 3.0″              AC: 106.2″
Position Angle      AB: 326°  (WDS 2011)     AC: 66°  (WDS 2008)
Distance: 1524 Light Years
Spectral Classification   A:  B1    B: B3
Status    AB:  Possible binary (Haas, Kaler)     AC: Physically unrelated (Kaler)

Now I’ve had Sigma (σ) Cass waiting in the wings since August of this past summer, but could never seem to get back to it for a more detailed look, much less spend some time in the surrounding area to see what else was lurking beyond the reach of my two naked eyes.  In fact, I believe it was Neil English who pointed this one out to me.  I can’t find his description of it now, but I recall him mentioning it formed a pair of exquisite suns in his 80mm f/11 refractor — and resisting a description like that is more than my powers of resistance can resist.

I looked at it that first night Neil mentioned it, did my best to capture its exquisite excellence in a sketch, and then, with the best of intentions, started back to it several times in the following weeks and months  — more times than I can count, actually.  But the heavens are bountiful, and I’m always curious, and other constellations called to me (I’m easily diverted when near a telescope), and, well,   ………….   I just hope the Queen will continue with that Royal pension, and the Admiral will maintain his usual calm disposition, and Neil will keep sending me gems like this one.

But to get back to the Admiral’s description  …………………………
Is Sigma Cass actually “beautiful?”
Does a sloop sail silently on a calm sea?

Is it “flushed white ……. smalt blue ?”
I had no problem flushing the white out of the primary  ……..
but that blue   ……….   maybe a trace if you look closely.

What I saw was bright white in the primary and pale white in the secondary — and I looked at it half a dozen times.  Haas describes it as “a bright yellow star almost touching a dim ash white” and The Cambridge Double Star Atlas adds this: “Tight pair with intense bluish & greenish tints.” (p. 14)  Jim Kaler describes them as “nicely blue-white”, although he may have been referring to the spectral class, as opposed to a visual observation.

And Sir William Herschel, who discovered this star on August 31st, 1780, described the primary as “w. [white] a little inclining to r. [red]” and the secondary he called “dark,” which in his usage seems to have meant dusky or red.  And he placed it at “the vertex of a telescopic isosceles triangle turned to the south.” (And it is).

So I guess we can say the verdict on color is  ……………  unanimously colorful.

My first view of Sigma (σ) Cass was through a 102mm Celestron f/10 refractor on August 19th (2011).  I approached it first with a 12mm Radian (83x) perched in the C102’s diagonal, and quickly discovered I had to sharpen my visual apparatus considerably in order to see the very close secondary.  It wasn’t that it was all that tough to split, but it required a disciplined application of Star Splitter scrutiny in order to excavate it out of the primary’s fifth magnitude glow.  A step up to a 10mm Radian (100x) separated it distinctly and very pleasingly, allowing me to relax my scrutinous scowl just a notch or two.

Encouraged by the steady appearance of both stars in my normally seeing challenged skies, I called out to the always-ready 7.5mm Celestron Plössl (133x), which leaped from it’s location in the eyepiece box directly into the diagonal.  After landing with a solid chrome clang, it scrunched its squat little self into its comfortably confined new home and went to work.  A few quick turns of the focus knob and I had Sigma (σ) centered and focused, and still with very minimal vibration.  I should have increased the magnification, but I knew that was the view for me, so I captured if for posterity, as you can see below.

“A” 6, flushed white; “B” 8, smalt blue; the colours are clear and distinct, though less fine than those of ε Boötis, of which this is a miniature.” (East & west reversed to match the refractorized view, click to see it again without this caption).

The 60mm f/15 keeps the 102mm Celestron company in this rare exposure to daylight.  These scopes are really black, but I think they inhaled some of that beautiful blue sky that afternoon.   (Click for a larger view)

Mounted on the C102 (and painted in the same sinister black color) is a 60mm f/15 refractor I built, which has slayed many a close double star in the dark of night since I first allowed light into its lens.  It lived up to its reputation that night, coaxing a very delicate, tear inducing hair split from Sigma (σ) at 82x through an 11mm Televue Plössl.  I remember trying to go deeper with the 7.5m Celestron (120x), but finding it lacked the same magical effect produced in the larger scope, I went back to the 11mm eyepiece and let my eyes soak up those two very tight points of white starlight while trying to restrain the tears.

And the 10.4 magnitude “C” component?  It was faintly obvious in both scopes.  At thirty-four times the distance which separates the primary and secondary, it appears it might be along just for the ride.  Discovered in 1879 (not sure by who, but apparently not S. W. Burnham, as I first suspected), it’s generally considered to be unrelated to either of the other two stars.

Those two others, however, are probably a true binary pair.  Kaler conjectures that “A” and “B” are about 1400 astronomical units apart, resulting in an orbital period of something like 14,000 years.  I looked at data going back to 1850 (p. 698 of this book), when the measurements seem to have become more consistent on this pair of stars, and found there appears to have been very little change, if any, in the separation and position angle of the two stars.  That adds some substance to Kaler’s statement that the probable orbital period is too long to allow computation of a probable orbit.

At any rate — take a look at Sigma (σ) Cass the next time you’re poking around in Cassiopeia.  It tends to get neglected I think, partly because Eta (η) Cass gets a lot of well-deserved attention, and also because it isn’t included in the usual zig-zag “M” or “W” asterism that is normally sketched between the constellation’s stars.  And while you’re there, slide your field of view one degree further north and take in the exquisite open cluster beauty of NGC 7789 (discovered by William’s sister, Caroline Herschel), also described admirably well by the Admiral:

A fine galaxy cluster of minute stars, on a ground of star-dust, on the upper part of Cassiopeia’s chair or throne . . .  it lies in mid-distance between ρ and σ, stars of the 5 1/2 and 6 magnitudes, each of which has a companion of the like brilliance.  It is, indeed, a very glorious assemblage, both in extent and richness, having spangly rays of stars which give it a remote resemblance to a crab, the claws reaching the confines of the space in view, under an eyepiece magnifying 185 times.”   (The Bedford Catalog, p. 539)

Clear Skies!  😎

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UPDATE

I came across the neat little asterism shown in the sketch below a few nights ago — a wide, warped parallelogram with an interestingly shaped and dimmer version near the middle of it.   It’s actually very obvious, but I never really saw it until I backed off for a wide field view and quit riveting my eyes on Sigma (σ).   Details are in the caption and in the first comment which follows the sketch.  If you look very closely, you can see the Sigma (σ) secondary clinging tightly to the edge of the primary.

For those interested, the magnitude of HIP 118077 is 5.55, HIP 118057 is 6.55, and HIP 118250 is 8.35. Starting at the top of the inner parallelogram and going in the same clock-wise direction, the magnitudes are 9.50, 9.25, 9.60, and 8.40. All of which adds up to a very visible little asterism. (Sketch started from a SkySafari screen image, which provided the colors! Click to lose this caption).

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3 Responses

  1. I had a chance to get back to Sigma (σ) Cass a few nights ago and give it the long look it deserves. By chance, I happened to have the same pair of scopes outside that evening I had used in the post above — the 102mm Celestron f/10 with the 60mm f/15 refractor riding on top of it. And I spent a very good hour with Sigma (σ) this time.

    Now how, you may well be wondering, do you spend an hour on a single star?

    Like this:

    I started with an 18mm Radian (56x) in the larger scope and a 15mm TV Plössl (60x) in the smaller one. Neither was enough to catch any sight of the secondary, but then considering the seeing was erratic — verging on the poor at that particular moment — it was no surprise. I swapped out the 18mm and replaced it with a 14mm Radian (71x) and still no luck, although it looked like the secondary might have been trying to bud into view. So on to a 12mm Radian (83x), which was enough to do the trick. The view was splendid, as that little dot of secondarial light clung tightly to the larger diameter of the primary. I had to look closely, though, because both stars were anything but still in those disturbed skies.

    I kept going, though, hoping the slight increase in magnification of a 10mm Radian (100x) wouldn’t put me past what was possible. It was basically borderline, but for brief moments, I had a good view of a very thin slice of black sky between the two stars.

    So I switched over to the 60mm scope to see what I could see. Peering into the 15mm Plössl, I found I was still looking at a single star. So in went the 11mm TV Plössl (82x) that had provided me with such a tear inducing view the last time — and no change. Too much jumping and gyrating for my eyes to fix on the image for even a micro-second.

    Back to the 102mm scope for another look in the 10mm Radian. Just for the heck of it, I went back to the 18mm Radian, and that was when I discovered the asterism shown above. Not sure how I missed that the first time, except that this time, knowing I wasn’t going to see the secondary, I was more interested in what the wide field view looked like. That just goes to show how easy it is to miss something so obvious when your attention is fixed on one thing only, and your eyes are marching in step.

    That asterism was eye-catchingly distinctive in the 102mm scope, though, and there’s just enough variety of subtle color present to dress it up nicely. I could see it easily in the 60mm scope, also, but even with the wider field of view offered in a TV 20mm Plössl (45x), it lacked the snap that was so obviously there in the larger scope — basically a reflection of light gathering ability more than anything else.

    By this time, I had lost track of time. I suspect close to forty-five minutes had passed by, but just as I was about to move on to other things, I noticed just the slightest hint of the secondary in the 18mm Radian/C102 combination. I didn’t quite trust my eyes, so I tried the 14mm Radian once more — and sure enough, the secondary was very distinct. Very small and very close, but very distinct.

    I had to try the 18mm again. So in it went, and I immediately felt myself riveted to my chair. I was transfixed, frozen in place, in another zone, insensible to the entire universe except for those two stars. The seeing had improved, of course, or that view wouldn’t have been possible.

    Even if it hadn’t, the view at the lower magnification would have been more crisp anyway. But as it was, every star in the field of view was just as sharp as it is possible to be. And that pair of Sigma (σ) siblings was stealing the show. Imagine this if you can:

    An intense micro-dot of blue-white light, gleaming steadily, with a mere puff — hardly a hint, even — of a globe of impossibly small light, indescribably close to it — so close that if it was any closer it would disappear without a trace. I almost caught that view in the sketch above, but actually the secondary was even closer than I could put it in that sketch. It was flat out, downright, photon-flattering ……… beautiful!

    Admiral Smyth said that first — and now I know exactly what he meant.

    I sat there for a long, long while — not sure how long, really, but I didn’t give it up until the seeing deteriorated once again and removed that view from my sight.

    And that’s how you spend an hour — probably closer to an hour and a half — on a single star.

  2. I observed this double on November 14th with a Tele Vue 102mm and found it difficult with a 12mm Nagler, but cleanly split with a 10mm Radian, noting that “Sigma forms a right triangle with two other field stars to the west.” In your drawing above, these stars are HIP 118250 and the star direcly above Sigma, toward the WSW. I did not note the parallelogram you mention above in either eyepiece.

  3. Thanks for the comment, Mark.

    Sounds like we’re in the same ballpark as far as magnification on the four inch scopes. The 12mm Radian for the C102 was 83x, and the 10mm Radian for the TV102 would be 88x. I used to have a TV102, and it was a great scope — kind of miss it at times, now.

    It’s strange what will attract a person’s eye. As I mentioned, I missed the parallelogram entirely the first several times I looked at Sigma, and I never did see the right angle asterism you saw until I did the sketch. That fainter star to the WSW at the apex of the right angle triangle is shown by Sky Safari as HD 240467 (and SAO 35962), with a magnitude of 9.4.

    Clear Skies!

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