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).

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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)

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).