If you wandered to this point without reading the First Part, you can switch to it by clicking on this link. You’ll be lost in space without a star-ship if you don’t.
31 Aquilae (STT 588) HIP: 95447 SAO: 104807
RA: 19h 25.0m Dec: +11° 57′
***** Magnitudes Separation Position Angle WDS Data
STT 588 AB: 5.2, 8.7 105.4″ 281° 2010
STT 588 AC: 5.2, 10.3 145.2″ 281° 2009
COM 7 AD: 5.2, 10.3 78.8″ 140° 1914
STT 588 BC: 8.7, 10.3 44.2″ 266° 2010
Distance: 49 Light Years
Spectral Classifications A: G8 B: G0 C: G5
Status: AB and AC are optical pairs, “D” is non-physical to the others.
I first got tugged toward this intriguing enigma when my eyes landed on the Aladin photo at the right.
Now, speaking of eyes, if you let yours wander up to the data line above, you’ll see that as of 2009 and 2010, AB and AC are both listed with the same 281 degree position angle — and if you return your gaze to the photograph, you’ll see “A” and “B” are clearly not lined up at the same position angles.
So I grabbed a telescope, trundled it out under the heavens, aimed it skyward . . . . . . . . . .
and proceeded to produce this sketch:
And that’s when I began to wonder what was going on.
Which led me — eventually — to this graph, which shows you the motions of the primary and secondary. The long red line pointing up and to the left is the primary (“A”), which is moving rapidly to the northeast — in fact, it’s actually moved past the edge of that graph. The red line to its immediate right, pointing down and slightly to the left is the secondary (“B”), which is moving to the south-southeast at what works out to be 4.4% of the primary’s rate of travel.
The third component, “C”, isn’t shown there, but it turns out it’s moving in the same general direction as the primary, but at a mere four-tenths of one percent (.4%) of the primary’s speed. So it’s “A” and “B” that are doing all the relative dancing.
For those interested, the actual rate of movement of each of the stars (from the Washington Double Star Catalog), measured in arc seconds per thousand years, is:
A: 721 East 643 North
B: 032 East 087 South
C: 003 East 001 North
. . . . . . . . . . which just serves to emphasize how much “A” is moving in comparison to the other two stars.
Still in a muddled state of mystification, I yielded to temptation and ordered a Meade astrometric eyepiece in order to try my hand at measuring the current position angles. I made two measurements on successive nights, and came up with a position angle of 284 degrees for AB, and 279 degrees for BC. It took a lot of effort, but helped by the discovery that swapping a Rigel illuminator for the Meade illuminator allowed a much dimmer setting of the brightness level, I even managed to eke out a measurement of 185 degrees for AD’s position angle. The results for AB and AC are considerably changed from the 2010 and 2009 WDS measurements, but they look about right for the present configuration —- which left me scratching my head over the 2010/2009 numbers.
While I was contemplating that, I happened to stumble on some 1991 Tyco measurements, which showed AB with a separation of 96.36″ at a position angle of 295.2 degrees. AC’s measurements weren’t included, but BC was, so I plotted its position (which you can see at the left center of the left circle below) —- and that allowed me to come up with AC’s numbers: a separation of 135″ and a position angle of 285 degrees.
So here are two charts of what 31 Aquilae looked like in 1991 — the one on the left shows the plotting I did, and the one at the right is the same view with all the obscuring hardware removed:
I began ransacking my computer for nineteenth century measurements, and suddenly it dawned on me — the WDS includes the first and last measurements! And since Otto Struve is credited with discovering 31 Aquilae’s multiple personality in 1852 ——- I had my measurements! Which were: AB with a separation of 142.5″ and a position angle of four degrees!!! —— and AC with a separation of 117.1″ and a position angle of 332 degrees.
So again, here are two charts — one with the plotting paraphernalia, and one without:
Now at that point, I decided to step back in time to 1781, which was the year Sir William Herschel recorded his observation of 28 Aquilae — previously described in the First Part of this enormously educational post. 😉
In order to do that, I continued with my plot of plotting the stellar motions in reverse — from 1991 to 1852 to 1751. To make the motion of “A” and “B” in relation to a fixed point and to each other very obvious, I moved “C” to the center of the plot since it’s barely budging from its post in the sky —- and came up with the results shown below on the chart at the left, with the data parked underneath. Even though I was working backward in time, the red arrows below show the stellar motion in a forward direction. And if you compare them with those on the linked graph above, they correspond rather well.
Then I eliminated everything from the chart on the left except for the 1781 positions of the three stars, returned “A” to the center in order to match the 1852 and 1991 views, as well as my own 2012 sketch, and ————— now you’re looking at the same configuration of the three stars that William Herschel would have seen in 1781 if he decided to take a peek at 31 Aquilae!
Then I began to wonder: what magnification does the field of view on the right side of the chart above correspond to? In 1781 Sir William was using a 160mm reflector with a seven foot focal length, or 2130mm, and he typically employed magnifications of 227x, 460x, and 932x — or to use his more descriptive word, diameters. The field of view in that chart on the right is six minutes of arc —- and a reasonably educated guess is the apparent field of view in the eyepieces Herschel used was about forty-five degrees. Now divide the apparent field of view of each of his eyepieces by their magnification —- and by sheer good luck we just happen to get a 5.9 arc minute field of view for Herschel’s 460x.
You can see in the 1781 view above, if he was examining the field at 460x, “B” would have been sitting just outside it. And NOW —- FINALLY –— AT LAST!!!! —- we have a pretty good idea of why Sir William wasn’t impressed enough with 31 Aquilae to catalog it as a multiple star. Heck, it really wasn’t all that interesting in 1852 when Otto Struve decided to catalog it.
Meanwhile —- back at the eyepiece in 2012 —- what about that pale little “D” companion I’ve ignored all this time?
Well first of all, if you go back to the data lines above for 31 Aquilae, you’ll see it hasn’t been measured since 1914. The WDS, which shows the first measurement took place in 1887, reports a total of three measurements were made, and lists an 1887 separation of 82.2″ and position angle of 122 degrees. Looking at the 1914 data, with a separation of 78.8″ and a shift in position angle to 140 degrees, it would seem that “D” moved 3.4″ closer to the primary and shifted southwest in position angle by 18 degrees. Of course, we already know that “A” is moving rapidly relative to it’s companions — and unfortunately the WDS doesn’t list any data with regard to the proper motion of “D” —— so it’s difficult to come to a conclusion with regard to how much of that apparent motion is attributable to either “A” or “D.” But — the WDS does at least include a note that the star’s proper motion indicates it is non-physical in relation to “A”.
So —- after all this work, what we have here is a collection of four stars that are not gravitationally linked in the least to each other. In fact, we actually happen to be looking at them during a brief window in time when, from our earthbound perspective, they’re passing closely enough to each other to appear visually interesting. Two hundred and thirty years ago they didn’t capture Sir William Herschel’s interest for that very reason, and two hundred thirty years into the future, someone will have to look at my sketch to see what they missed. 🙄
One last illuminating jewel of information: the COM 7 that is used as the WDS identifier for “D” refers to George Cary Comstock, a professor who was involved in helping organize the American Astronomical Society, and later became its president in 1925 —- this link will take you to a short summary of his career. And included in the references listed at the bottom of that page is a link to a .pdf document entitled “Biographical Memoir of George Cary Comstock.”
Next stop: Scutum!
Clear Skies! 😎