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A Radiant Study in Stellar Contrasts: 28 and 31 Aquilae — First Part

Even though those of us who have been involved with astronomy for at least a few years are well aware the heavens aren’t static, we don’t actually expect to see the stars change positions relative to one another during our star-gazing life spans.   After Orion disappears over the western horizon for the last time in the Spring, we know its stars will still be where we expect them to be when the Hunter first hoists his head out of the glow of the eastern sky in mid-August.

But move they do, and when we can detect it (as in the case of Porrima), it sends an electrical jolt from the ends of our focus fingers all the way to the tips of our tensed-up toes.

Now the two stars in the title line of this post just happens to be as opposite as can be in this regard.  One of them, 31 Aquilae, consists of a group of stars that is changing relative positions so rapidly they all qualify for membership in a stellar square dance club.  In fact, as a group, they can’t seem to stay in place long enough to hang onto a position angle for more than a year or two.  The other one, 28 Aquilae, is every bit as remarkable for it’s diametrically opposite behavior, namely a stolid ability to stand its ground — or to be more accurate, maintain its coordinates in the sky.

It was 31 Aquilae that drew me into this area, captured and puzzled as I was by the discovery that its relative position angles have already shifted very noticeably since 2010.  It also happens to be a stunning triple star with an odd fourth star that qualifies it for quadruple status — which makes it deliciously irresistible.

But —— the main puzzle which puzzled me so perplexedly was why 31 Aquilae was roundly ignored until 1852, the year Otto Struve discovered it.  I knew darn well if it had resembled a multiple star in the least little bit at the time the Herschels (William and John) and James South were patrolling the heavens, one of them would have cataloged it — and since none of the three did, I harbored a simmering suspicion it was the rapidly changing positions of each of the three principle stars that was responsible.  Eventually that suspicion simmered its way up to an inevitable boil, and that meant one way or another —- come clear skies or constant clouds —- I had to pull this boiler off the burner and find the puzzle’s final piece.

First, some navigational material:

You’ll find Aquila the Eagle winging its way across the celestial meridian, that invisible line in the sky that divides east from west, at about 10 PM to 11 PM every August. As it wheels and tilts into a forty-five degree angle during its cross-meridianal flight, you can’t miss it, especially with blue-white first magnitude Altair blazing a path straight to Delphinus. (Stellarium screen image with labels added, click for a larger view).

To get where we need to go, the best jumping off point is 0.75 magnitude Altair at the Eagle’s head. A two degree leap northwest will land you on 2.70 magnitude Tarazed, and another full degree hop in the same direction will put you on top of 5.80 magnitude Chi (χ) Aquilae, with 6.3 magnitude 46 Aquilae hanging just northwest of it. (Note how all four of these stars conveniently form an almost straight line). Now — if you look five and six degrees out to the west, you’ll spy the scintillating glows of 31 and 28 Aquilae — but to get there, we better move to the next chart. (Stellarium screen image, labels added, click for a larger version).

As you start to move west from Chi (χ) Aquilae, aim your directional arrow halfway between 6.70 magnitude HIP 96420 and 6.0 magnitude HIP 96481, which is at the point of a distinctive line of three sixth and seventh magnitude stars lying on a northeast-southwest line. That line will lead you straight to 5.2 magnitude 31 Aquilae, which holds down the southwest corner of a skewed rectangle, and another degree to its northwest you’ll spy 5.5 magnitude 28 Aquilae at the northern tip of a triangle it forms with 6.0 magnitude 29 Aquilae and 5.3 magnitude Omega (ω). (Stellarium screen image with labels added, click to lose this caption).

Center 28 Aquilae in your finder first —– because we’re going to look at it first.

28 Aquilae   (H V 34)   (S 717)   (STTA 179)         HIP: 94982   SAO: 104722
RA: 19h 19.7m   Dec: +12° 22′
Magnitudes  AB: 5.6, 9.0   AC: 5.6, 13.0
Separation   AB: 60.7″       AC: 73.9″
Position Angle  AB: 174°  (WDS 2009)     AC: 107°  (WDS 2001)
Distance: 347 Light Years
Spectral Classification: F0, G5  (A and B)

A wide and unequal double star, on the Eagle’s back; it is 8° distant from Altair, on a west-north-west line leading upon ε, in the tail.  “A” 6, pale white; “B” 10, deep blue.  This is 34 H. v., but though it was enrolled in 1781, there is only a random remark that the distance was about 35″.  It was afterwards measured by Sir James South, from whose results, with my own, little, if any, appreciable motion is discernible.”  (The Bedford Catalog, p. 445)

That’s Admiral William Henry Smyth‘s 1831 observation, which included measurements of 59.8″ and 175° 7’.  The James South measurements he referred to were made on July 11th, 1824, and July 20th, 1825, and averaged out to a separation of 59.28″ and a position angle of 175° 6′ — and included this comment: “The small star is decidedly blue and bears a good illumination.”  (p. 231 of South Catalog)

Now there are two distinct features of 28 Aquilae which are already glowingly obvious.  The first, as I mentioned at the beginning of this piece, is how little the separation and position angle of the primary and secondary have changed since Smyth and South’s measurements — these two stars have barely budged in the last 180 or so years.  Solid and dependable things they are, a quality the ancients would have gratefully approved, given their undying admiration for the unchanging perfection of the heavens.

The second distinctive feature is that both Smyth and South were very specific in their description of the secondary’s color, which was something I noticed immediately as well.  It’s more than just a little unusual for a ninth magnitude star to be as distinctive in color as this one is —- in fact, the blue hue it radiates is not at all in agreement with the yellow-white color that is characteristic of its G5 spectral classification.  Nevertheless —- it’s as blue as a fresh autumn sky —- and you sure can’t miss it:

Where the Admiral saw pale white in the primary, I saw a glowing gold-white, but we both saw that distinctive blue emanation from the secondary. (East & west reversed to match the refractor view, click for a closer look).

And that leaves the fleetingly faint photons of the 13th magnitude “C” component —- which were every bit of the battle I expected they would be in my six inch refractor, mainly due to the glow from the two brighter companions.  If I hadn’t been looking for it, chances are I never would have seen it.  But persistence —- paired with the avaricious application of averted vision —- eventually did the trick, resulting in a neat and tight — if somewhat elusive — little triangular asterism.

So now —— because it’s going to lead us to 31 Aquilae, let’s step about 230 years back in time and take a look at Sir William Herschel’s observation of 28 Aquilae, which was mentioned above by Admiral Smyth:

V.
34.    In constellatione Aquilae, A. FL. 28.
July 25, 1781.     Double. It is one of two stars near A. Distance about 35″.
(p. 80 of The William Herschel Double Star Catalogs)

Now it pains me greatly to say it, but this is one rather confused and muddled observation — not that I haven’t made a few myself.   But I better explain.

First —- the separation of 35″ is way off in comparison to Smyth and South’s measurements.  If you remember, Admiral Smyth referred to it as “a random remark” in his comments above —- and . . . . . . . . . .

Second —- the “A” Herschel refers to in the third line (“one of two stars near A.”) is another designation for 28 Aquilae, a designation which he actually includes in his second line (“A. FL. 28.”, where “FL.” refers to John Flamsteed).  In other words, he’s using “A” to refer to itself, as in “A is one of two stars near A.”

If you look at Johann Bayer‘s portrayal of Aquila in this copy of his 1603 Uranometria  and click on the larger view option at the upper right, you’ll see that 28 Aquilae is labeled there as “A” —-  and if you look closer, you’ll also see two nearby stars, “b” and “ω” (Omega), which in all probability are the two stars referred to in that third line of Herschel’s.

Now if I had to guess, I would guess that Sir William meant to say “b” when he said “A” – as in “It is one of two stars near b.” —- or to be very specific – “28 Aquilae is one of two stars near b.”   I suspect he was having one of those confusing and/or frustrating nights at the eyepiece that we’ve all had at one time or another.

Sorry to belabor the point at such length, but I have a very good reason for it:  That star labeled “b” just happens to be 31 Aquilae !!!!!!!!!

IF my attempt at unraveling is on track, it means Sir William was well aware of 31 Aquilae.   And, given that the procedure at that time was to observe a star when it transited the celestial meridian, it was merely a matter of waiting five minutes for 31 Aquilae to follow 28 Aquilae across that invisible line.   So it’s downright difficult to believe he wouldn’t have centered 31 Aquilae in an eyepiece, especially since he was looking at 28 Aquilae anyway.  And —- if he did —- he certainly didn’t see anything that led him to suspect a double star, or he would have cataloged it.

So maybe we better see if we can figure out what he saw in his eyepiece over 230 years ago — which will take us to the Second Part of this intriguing tale.

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