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

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.

East & west are reversed here, click to enlarge.

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 “B” and “C” aren’t both lined up at 281 degrees here, either.   Hmmmmmm . . . . . . . .  (East & west reversed to match the telescopic view, click for a larger and closer view).

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.

Keep in mind that east and west are reversed in these plots to match the image as seen in a refractor or SCT. In other words, where the Aladin chart referred to earlier shows a northeast vector pointing up and to the left, the reversed view has the red arrow pointing up and to the RIGHT.

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!  😎


14 Responses

  1. John, a great piece of research and a very enjoyable read,
    it just shows what at first glance seems a nice if ordinary
    triple star can have such an interesting history. A couple of weeks
    ago I was in that part of the sky and had a look at 31 Aquila
    the CDSA has it down as a double with a sep. of 100″ which is
    what I saw, it just shows you should always look deeper.
    Keep up the good work
    PS I have been trying to split 17 Lyra recently with no success
    is this a double you have tried. I have only been using the 80mm
    Zeiss maybe I need something bigger.

    • Split 17 Lyra last night with 1270 mm focal length RV6 using 6 mm ep. Art Cloutier, Page, AZ

      • Hi Art,

        Pretty good work with an RV6! I got a very clean split with a 6mm Radian in my 150mm f/10 refractor (200x), so we’re pretty close magnification wise.

        I’ll have a post done in a few days on 17 Lyrae — stay tuned!


  2. Thanks for the kind words. Lots and lots of hours went into those two posts, so it’s very much appreciated!

    I haven’t looked at 17 Lyrae, but I see I’ve been in the area several times — it’s just east of Gamma Lyrae. The WDS lists magnitudes of 5.3 and 9.1, with a separation of 3.7″. It was last measured in 2001, but I see Struve measured it at 2.5″ in 1828 — so it looks like the primary and secondary may be moving apart at a very slow rate.

    I’ll give it a shot as soon as the skies clear. Judging by the magnitude difference and separation, I would say it’s a very tough split for an 80mm refractor — if anything could do the trick, it should certainly be the Zeiss. I’ll take a look with 90mm and 100mm’s and see what happens.

    I also see the WDS has five more companions listed in the 10th to 11th magnitude ranges, and one of those, “C”, has five of its own! Complicated little star!

    Here’s a look at all the data: LINK.

    Clear Skies, Pat!


    • A complicated little star indeed would be interesting to see
      how many companions could be spotted in say a 100mm scope.
      I see oneof “C” companions has not been measured since
      1908, a project there for someone.

      Hope your skies are clearer than mine


      • Hi Pat,

        I did get a look at 17 Lyrae finally with a six inch refractor, but the evil orb has been busy bathing the sky with it’s reflected light, so I couldn’t quite get all of the fainter companions I had hoped to — need to go back on a dark night, moonless night.

        The secondary is tough even in a six inch scope, though. I kind of suspect it’s beyond the reach of an 80mm, even one with the pedigree of the Zeiss. You would need perfectly stable skies, very little moisture in the air, and a moonless night in a dark sky area to even have a chance.

        As soon as I can get a look at it under darker skies, I’ll write up a post on it. Definitely a beauty in a six inch scope!

        As for the focuser on Sir William’s reflector, it really does look like the only way to operate it would be to slide the eyepiece up and down in the tube. If that’s the case, I wonder how he secured it? Sure couldn’t hold it at 480x, that’s for sure!


      • Hi John,
        I had another look at 17 Lyrae last night I tried to get it
        before the moon was up although there still was a glow from
        it. No luck with the secondary I think your right everything would
        have to be perfect to split it with 80mm scope. I did pick up 4
        of theother companions not sure which ones though they were
        very faint in my scope.


  3. Hi John!
    I will echo Pat’s comments, a real fine piece of work. One comment that you made about Herschel needing to only, wait for 5 minutes to have 31 Aql pass through the fov of his telescope, got me to thinking about the processes used to observe the heavens in those early years. Telescopes where typically stationary setups with not a lot of mobility. The time you would be able to observe and collect your thoughts and write your notes on a particular stellar group or faint fuzzy, was the length of time that it took for the object to pass through the field of view. It gives one another reason to marvel at the tenacity of these earlier astronomers to make the observation that they did. Isn’t surprising that the occasional error crept into the notes. What is even more amazing was the data was a detailed and as accurate as it was.

    I have been on a bit of a quest myself with what appears to be a well known double, Howe 13, supposedly located in the winter constellation Canis Major. The records all indicate that Howe 13 is made up of 2 mag 7+ stars with a separation in the order of 12.5 arc seconds. I have imaged the area on a number of occasions and it is quite obvious that no such double exists in that area of the sky. I have corresponded with Brian Mason, one of the keepers of the WDS and he was unable to offer any insight into this. I suspect the records from day one have been mistakenly recorded. I have not been able to find the catalogue where the first observation was recorded. I have a couple of copies of catalogues from the Cincinnati Observatory providing the published observations by Dr. H.A. Howe from 1876. It is difficult to ascertain if the double known as Howe 13 is amongst the items in these catalogues. When you are wondering what that next double star challenge should be, you might concider Howe 13.

    I really appreciate the research that goes into your posts…not to the mention the craft of telling the story. It always brings a smile to my face. As Pat says,,,Keep up the great work!!

    Cheers, Chris.

    • Vielen Dank, Pat!

      I did a bit of looking around at Sir William’s telescopes because I have a hunch he was able to coax some lateral movement from them. I’m not sure on the large ones — like this one — enmeshed in huge timbers, but I think he was able to get a bit of lateral movement from that elaborate setup. If anyone can shed some light on that, please do.

      The telescope used for much of Herschel’s double star work can be seen here — and if you look carefully, I believe there are wheels underneath the two rear supporting legs, which would have provided some lateral mobility. Aside from that, the system of pullies and cranks adds up to an elaborately clever means of moving the scope in declination.

      At any rate, I suspect in many cases Sir William was able to keep an object in view long enough to make some solid observations. At some point, regular equatorial mounts took over, and allowed for the longer observations.

      As for Howe 13 — sounds very intriguing!!! I even have a couple of ideas already — you’ll have to stay tuned to find out what they are! :mrgreen:


      • Hi John!
        In spite of the wheels, I suspect that F14 reflector required some real finess to keep pointed in the same place of the sky. It is unfortunate that more has not been written about the processes that the early astronomer types used to ply there craft. I suspect this museum piece would not be available for us mere mortals to try some hands on observing. I guess we will have to leave it to our imaginations.

        It begs the question, if these were new objects being observed for the first time, would they not have required the telescope set to the local meridian and the appropriate declination to get a proper fix on the object?

        Next life…maybe there will be time to study that entire subject of the early techniques and processes.

        I await with baited breath for your ideas on Howe 13.

        Cheers, Chris.

      • HI John, just looking at Sir Williams double star scope I was struck
        by the focuser, it seems to have no means of moving in or out.
        I was wondering if it slides up and down the tube.


  4. Hi John,

    You the Man!

    Magnificent piece of work; superbly researched!

    Heck, that’ll keep me busy for at least two years LOL. I’m going to have to get Pi eyed again in Aquila sometime soon.

    Best wishes,


  5. John,
    Last night we viewed 31 Aql. for quite a while from here at the summit. We GOT the D companion, although it was too dim to measure with the AM EP. I’m thinking its a bit higher mag. than the 10.3 listed… I did render another sketch which I will scan/invert when I get home & send off to you.
    There is an unrelated star to the north that doesn’t show up in either Stellarium or SkySafari, although the Aladin pic shows it. Colors were tough as the conditions were intermittent with high cirrus clouds and there were a bunch of people taking looks through the EP which reduced my concentration capabilities substantially. JD Armstrong of Faulkes had a good “Wow” look and promised he will put this system on the cue for the 2 meter FTN to image so we can reduce the data with Astrometrica!!!

    • Great news, Steve! Glad to hear that J.D. Armstrong was taken by the view. I may be able to wind up my measurements of the AC pair tonight, so it will be good to compare what the two of us have with the FTN image data.


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