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56 and 57 Aquilae, and the Double That Wasn’t a Double

Well, it looked pretty darn convincing at least.  Two seductive dancing orbs of attractive light, they are, too.

But we’ll circle back around to the source of my circular mystification shortly.  First, a bit of background.

I was pointed to a Yahoo double star site recently by Steve McGee, which was followed by an email from the inspired creator of the site, Chris Thuemen.  The name of the site is “double_star_imaging” and it can be found at this link.  If you look at the introductory paragraph on the home screen, you’ll read that the site sprung into existence in order — to use Chris’s words — “to create an archive of images to reveal the sheer volume, variety and intrinsic beauty of these little gems.”

By little gems, he means double stars — and I’m the last person on this planet who would argue with that description.  In addition to the collection of Chris’s photos in the site’s photo section, there is also quite a variety of helpful material in the files section added by other members, plus a very good double star atlas in the links section.

At any rate — while I was scanning through the photos on the site, I came across an intriguing shot of 57 Aquilae, which was my first acquaintance with this star.  After savoring the view, I couldn’t resist the urge to point a telescope at it, which explains how it wriggled its way into the title of this post.

The first star of the title, 56 Aquilae, is one of those little gems that hides in the background of star charts and atlases, masquerading as a single star.  But it earned double star status way back in 1827 when Sir John Herschel discovered it has a faint companion, so it’s well past time to focus some attention on it once more.  I stumbled across it here, near the bottom of the screen, in the graphed display that shows neighboring double stars.  That particular feature is extremely handy for identifying suspected double stars that materialize unexpectedly in the field of view of an eyepiece.

And the third item in the title, the double that wasn’t a double — well follow me, and you’ll see.

First, I gotta get you where we’re going, though:

Starting with the wide angle view, if you project a line in the sky between 3.20 magnitude Theta (θ) Aquilae and 3.05 magnitude Beta (β) Capricornus, you’ll find 56 and 57 Aquilae lying about six degrees west of its midpoint. (Stellarium screen image, click for a larger view).

(Stellarium screen image with labels added, click to enlarge the view).

Now those two stars are hard to see unless you have dark skies that are reasonably transparent — 56 Aquilae’s magnitude measures out at 5.8, and 57 Aquilae’s magnitude just happens to be 5.7 — so you may need a pair of binoculars to see them.  And notice that I chose Beta (β) Capricorni to triangulate my way to our destination because it provides a much better midpoint than Alpha (α) Capricorni does.

Just as an aside, Beta (β) Cap is brighter than Apha (α) by half a magnitude, which I didn’t notice, mainly because the persistent late spring murk is still clogging up the skies above me a month into summer.  On the other hand, Apha (α) is a rather arresting visual double.

57 Aquilae  (Σ 2594)  (H IV 14)      HIP: 97966   SAO: 143898
RA: 19h 54.6m   Dec: -08° 14′
Magnitudes: 5.7, 6.4
Separation:  35.5″
Position Angle: 170°  (WDS 2011)
Distance: 335 Light Years
Spectral Classification: B7, B8
Status: Both components are spectroscopic binaries

56 Aquilae (HJ 900, or h 900)       HIP: 97928   SAO: 143894
RA: 19h 54.1m   Dec: -08° 34′
Magnitudes: 5.8, 12.3
Separation:  46.4″
Position Angle: 76°  (WDS 2000)
Distance: 394 Light Years
Spectral Classification: K5

The ND (Non-Double)
SAO 143917  (HD 188473)               SAO 143913  (HD 188472)
Magnitude: 8.6                                     9.6
Distance:   6.5 Light Years                Unknown
Spectral classification G1                 A3

OK, since I’m guilty of luring you this far, let’s take a look at the sketch so you can see what I saw.  Before we do that, though, you should know it was drawn with the intention of getting all three of the subject stars in one field of view, so you’ll see 57 Aquilae at the top, 56 Aquilae at the left, and the non-double at the bottom:

Those two paired stars positioned at the bottom of this sketch look pretty convincing — at least they did to me — but all of my research shows they’re unrelated to each other. (East & west reversed to match the refractor view).

I never could find a distance for the fainter of the two at the bottom edge of the sketch, SAO 143913, but the brighter one, SAO 143917, is certainly a close neighbor of ours at a mere distance of 6.5 light years.  If we could come up with a traveling technology that would propel us through the galaxy at something close to the speed of light, we could get there and back in a bit more than 13 years — and there aren’t many stars you can say that about.  Meanwhile, I’ll have to be content to with admiring their almost perfect mirror image imitation of 57 Aquilae — which, if nothing else, adds a distinctive touch to the field.

The star holding down the top of the sketch, 57 Aquilae, is a splendid sight in aperture-challenged scopes.  Haas viewed it with a 60mm at 25x and classified it as a showcase pair:  “A wide pair of bright stars, mildly unequal, that are pure white and scarlet tinted white.  Webb calls them ‘pale yellow, pale lilac” and ‘distinctly contrasted.’ ”  I could only coax a slight yellow tint out of the primary, and nothing more than white from the secondary.  Sir William Herschel, who discovered this pair on August 2nd, 1780, described them as “white”and “white inclining to red”, which is virtually identical with what Haas saw.

Friedrich Georg Wilhelm von Struve, who ended up with his last initial (the Greek equivalent of which is “Σ”) on the catalog designation, Σ 2594 (STF 2594), first viewed the pair in 1821, and from what I can determine, saw both stars as white.  John Herschel and James South took a peek at them two years later — July 24th, 1823 — and reported both stars as “bluish.”  That date by the way, is exactly 189 years prior to the date of my sketch, which may explain why I had a feeling I was being watched when I was drawing it.  😉

And then of course there was Admiral William Henry Smyth‘s 1834 observation, which agreed with Herschel and South’s: “both blue.”

So that brings us to 56 Aquilae, which as I’ve already mentioned, was discovered by Sir John Herschel in 1827, who left this short description: “L. yellow. S. blue.  Is 56 Aquilae.” (p. 210 of his third catalog, which is volume three on this list).  And it’s a tough one to crack.

The 90mm refractor I was using on that murky July 24th evening was able to provide me with a few averted vision glimpses of the elusive 12.3 secondary.   Considering the unrepentant murk and the 6.5 magnitudes of difference between the two stars, which translates into the secondary being about 500 times fainter than the primary, that was actually a rather remarkable performance.  But I was back the next night, when it was even murkier, with my five inch f/15 D&G refractor to take another look.  Even with the extra aperture, the secondary wavered on the border between averted vision and direct vision in that thick sky soup.  Still, it was enough of an improvement that I was able to double check the secondary’s position on my sketch, which was what I came for.

Looking at prior measures of 56 Aquilae, I found that John Herschel had measured a separation of 40″ and a position angle of 75°, which is recorded in his third catalog, published in 1828.  The always eloquent Admiral took a peek in on the same day he viewed 57 Aquilae, and described the pair this way:

A delicate double star between the head of Capricorn and Antinous’s leg, in an absurd boundary hook 17° due south of Altair.  “A” 6, deep yellow; “B” 12, pale blue.” (The Bedford Catalog, p. 462)

His measurements, though, differ considerably from Herschel’s — a separation of 43.0 arc seconds and a position angle of 72.5 degrees — which he described as “a mere estimation.”

The next observation I can find was made by S.W. Burnham, who made two observations, one in 1879 and one in 1880, which average out to a separation of 46.66 arc seconds and a position angle of 77.8 degrees.  Those observations are recorded as No. 637 in his “Micrometrical Measures of 748 Double Stars made at Chicago with the 18 1/2-inch Refractor of the Dearborn Observatory, from November 1878 to December 1880”, which can be found on page 307 of this book.

If you compare his 1879-1880 measurements with those of Herschel in 1827, you would come away with the impression that the secondary had moved away from the primary during the intervening fifty-three years.  On the other hand, when I made my sketch, I carefully noted the separation of 56 Aquilae was just slightly more than that between the two stars of 57 Aquilae, and it took the second observation with the five inch refractor to confirm the position angle I had drawn.  Next stop features four double stars in one field of view — stay tuned!

And Clear Skies!  😎


One Response

  1. John, I got HJ 900 tonight but its not bright enough to measure with my Astrometric EP. I’ll try again the next new moon. Its 12th mag and a bit evasive but I got two sketches I’ll send along to you.

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