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Close Encounters in the Lair of the Serpent with OΣ 303 and Σ 1988; or How to Take a Double Star for a Walk

Now no doubt you’re wondering how in the heck I managed to end up in the lair of a snake.   (No, I can’t talk about the walk just yet, but we’ll get there shortly).

Well  …………….  it all started with what I thought was a great idea.  And it was, except that it didn’t work where it was intended to work.   If all had gone according to plan, I probably would have stayed north of the border in Corona Borealis, where the worst danger is being blinded by starlight reflected off the crown jewels.

Serpens Caput is seen here to the south of Corona Borealis, wedged between Hercules, Boötes, Ophiuchus, and Virgo.  (Stellarium screen image with labels added, click on the chart for the full view).

But since all Great Plans are subject to the momentary whims of the Great Dislocater, an anonymous and mysterious creature who decides where to draw the boundary lines between constellations, here I am, searching in the serpent’s lair.  Hopefully he’ll take a long nap while we’re here, or at least keep his eyes firmly fixed on Boötes’ right arm.  If any part of that sounds a bit strange, a more mundane description can be found here in the first few paragraphs.

What I actually was looking for in Corona Borealis — an exotic collection of closely separated double stars within resolvable telescopic range — I found tucked away in a small sector of Serpens Caput.  Now that certainly came as something of a surprise — it really was the last place I ever would have looked — and I suppose that’s because snakes just don’t get very good press.  But there’s a genuinely challenging pair of glittering gems here that need to be exposed to the lenses and mirrors of sleuthing Star Splitters, so follow me and I’ll show you how to get to them.  Might want to bring along a snake bite kit, though, in case this sleeping slitherer wakes up.

Our search starts to the south of the Serpent’s head at Gamma (γ) Serpentis. From there, drop one degree south to Phi (φ) Serpentis and you’ll find a collection of stars spaced at almost identical distances below it and and to the sides, in a sort of warped diamond shape. 39 Serpentis holds down the west corner of the diamond, our second target (Σ 1988) holds down the south corner, and our first target (OΣ 303), occupies the eastern corner. So from Phi (φ), slip another degree to the southeast, where you’ll find OΣ 303 parked comfortably close to another star, 7.5 magnitude HIP 78525 / SAO 101883. (Stellarium screen image with labels added, click for a larger view).

OΣ 303                 HIP: 78446   SAO: 101874
RA: 16h 00.9m   Dec: +13° 16′
Magnitudes   AB: 7.7, 8.1     AC: 7.7, ???
Separation    AB: 1.6″            AC: 79.4″   (BU 1536)
Position Angle   AB: 174°  (WDS 2010)      AC: 12°  (WDS 1997)
Distance: 317 Light Years
Spectral Classification: F7

Now from the twin perspectives of motion and history, this is really a very interesting pair of stars — and it even comes with a mysterious third component scintillating somewhere out there in the depths of interstellar space.

Johann Heinrich von Mädler discovered the AB pair in early 1843 from the Dorpat (Tartu) Observatory in Estonia, and Otto Wilhelm von Struve was close behind in 1846, using a fifteen inch Merz and Mahler refractor (its twin can be seen here) in the Pulkowa (Pulkovo) Observatory.   Herr von Struve got credit for the discovery, at least as far as the catalog designation for the two stars goes — the two capital Greek letters, Omicron (O) and Sigma (Σ), supply his initials, as in Otto Struve.

At any rate,  Mädler’s and Struve’s measurements showed very tight separations of .58″ and .65″, the wider one being Struve’s.   Up through 1900 the separation continued to increase — the measurements varying somewhat from one observer to the next — but by 1900, the average measured separation was about .90″.  During the time frame of 1843 to 1900, the position angle changed from 113 degrees to 145 degrees.  (That information comes from p.133 of Micrometrical Observations of the Double Stars Discovered at Pulkowa by J.W. Hussey, published in 1901 as Volume V of the Publications of the Lick Observatory of the University of California).

I lost track of subsequent measurements of OΣ 303 , but obviously the separation has continued to increase, reaching the 1.6″ listed in the Washington Double Star Catalog (WDS) as of 2010, along with a continuing change in position angle.  So if the AB pair is gravitationally linked in any way, it would be in a very slowly changing orbit.

But the main thing that concerns us right now is whether we can split Mädler’s and Struve’s discovery now that’s its widened to 1.6 seconds of arc.

The D&G caught at work under cover of darkness.  Click on the photo for a larger view.

Since I decided a long time ago that my second story wooden observing deck wouldn’t quite support the weight of a fifteen inch refractor and mount, I went with my five inch f/15 D&G version instead.  Once I found my target, which wasn’t all that difficult, I started with a 24mm Brandon (79x) to get the lie of the sky, and was greeted by a slightly elongated white dot with a slight hint of pale yellow decorating it.  But I hadn’t risked sneaking into the serpent’s lair just to look at dueling photons trying to merge into a single star, so I worked my way up through both a 15mm (127x) and an 11mm TV Plössl (173x) before I was sure the star’s dual nature was within reach.  It was beginning to hop around like a peripatetic hornet at that point, but I made a leap of faith and dropped a 20mm TV Plössl into a 2.4x Dakin Barlow, which gave me an extended ocular stack equivalent to an eyepiece of 8.3 millimeters in focal length, or in more meaningful numbers, 230x.  And you can see the result in the sketch below if you look at the inset at the lower right.

” . . . a very interesting pair of stars . . . “, each with a pale yellow tinge present. This and the next sketch are much better when seen with the lights turned off! (East & west reversed to match the refractor view, click to lose this caption).

Most of the time the two stars were touching, but for very brief and very rare micro-seconds of time, the atmosphere above me would calm down just enough to glimpse two distinctly separate dots of pale yellow light.  The irrational phrase “more magnification” kept rolling through my mind, but fortunately I wrestled it into submission and avoided the torture to my eyes that would have resulted.  One thing that did catch my eye, though, was that diagonal line of three faint stars visible in the inset on the north side of the dancing primary/secondary pair.

One of those might well have been the mysterious third component I mentioned at the start of this discussion of OΣ 303.  If you look at the data line above, you’ll see the “C” component is listed with a position angle and separation, but no magnitude.  The middle of the three faint stars lines up closely with the twelve degree position angle listed in the WDS, but I’m not convinced that it’s far enough away from the AB pair to be the correct star.  If it is, it’s about a 14th magnitude star at best.  I came to that conclusion based on MegaStar assigning a magnitude of 14.1 to the star immediately to the right and below the center star.

Now, if you go back to the chart above (you can see it again here, too), you’ll see the subject of our second close encounter, Σ 1998, lurking at the bottom of that diamond-shaped outline of stars.  And, believe it or not, we’re going to take this pair for a walk.  Really.

Σ 1988                  HIP: 78097   SAO: 101829
RA: 15h 56.8m   Dec: +12° 29′
Magnitudes: 7.6, 7.8
Separation:  1.9″
Position Angle: 250°  (WDS 2010)
Distance: 314 Light Years
Stellar Classification: F1

“So how do you take a pair of stars for a walk?” you ask.   Well, hold on, we’ll get there — I gotta wait for the right moment to spring it on you.   😉

I started again with the 24mm Brandon (79x), again found a slight elongation present, and then tried the 15mm TV Plössl (127x), which showed both stars attempting to break free of each other.  I went back to the 24mm Brandon, slipped it into the 2.4x Barlow (191x), and found the hair split pair you see in the sketch below.

“Two very round dots of orange/red light . . .” and “flat out beautiful.” East & west reversed again, click to lose this caption.

When they would hold still, they were flat out beautiful.  Similar in magnitude to the prior pair, but noticeably easier to split due to the additional three-tenths arc seconds of separation, they set themselves apart thanks to their delicious red/orange coloration, subtle though the colors were.

This pair, by the way, was discovered by Otto Struve’s father, Friedrich Georg Wilhelm von Struve, in 1830, using a nine inch refractor (with a focal length of fourteen feet, making it about an f/18.8 instrument!) at Dorpat.  He described both stars as white, and measured a separation of 2.91″ and a position angle of 266.3 degrees.  The data on p. 434 of this book shows a narrowing of the separation from 2.91″ in 1830 to 2.59″ in 1903, and it certainly seems as if the stars have continued to move closer to each other since that time.

Now I tend to favor a non-driven alt-az mount over a driven equatorial mount, simply because —— well, because I do.  Actually, it’s because I feel more connected to the image in the eyepiece when it’s moving.  Of course, it isn’t actually moving, the earth is, which means I am, which, when I think about that while I’m observing, makes the connection between me and the image even more thought provoking.   Sure, it’s more work when trying to capture a won’t-hold-still image on a slightly damp piece of paper in the dark, but there’s certainly no question about which direction is west — just follow the motion in the eyepiece and the other three primary directions click into place like a coin dropped into the slot of a vending machine.

Now when I’m trying to sketch an image under those circumstances, naturally I want to keep the center of attention at or near the center of the eyepiece’s field of view.  But once I’ve finished the sketch, I find I like to retrieve my link with the earth’s motion by positioning the object of my attention at the extreme east edge of the eyepiece — in a refractor on an alt-az mount, that means the right edge — and letting it drift across to the opposite side.

So I slid the Σ 1988 pair over to the east side of the 24mm Brandon/2.4x Barlow combination and watched the two stars hop and jump and leap as they drifted across the field of view.  And then I did it again.  And again. And again.  Over and over and over — until it suddenly occurred to me — it’s almost as if I’m taking these two stars for a walk!   Not much different than my four-legged observing partner, who sometimes hunts back and forth in a zigzag pattern while I’m trying to keep him walking straight ahead.

At the risk of sounding like I was outside in the damp cold for too long (and I probably was), I found it  ………….  well …………. exhilarating!    And as the stars waltzed and spun and twisted across the field of view, they almost always settled down for a couple of brief moments to two very still and very round dots of orange/red light, which was even more exhilarating.   I’m sure I was grinning from ear to ear when that happened.

So that’s what I did for about twenty minutes.  I walked those two dancing stars repeatedly across the eyepiece’s field of view, and then when I was done, I took my four-legged partner for a similar walk on the surface of good old Mother Earth.

You know, you just never know what you might find during a comfortable and successful night under the stars.  Try it — it’s kinda neat.  No leash needed.

Clear Skies! 😎


4 Responses

  1. Hi John,

    Really enjoy the site and all the work that has gone into this. The references material that you put into the articles really add meat to them. Have been a double star observer for close to 10 yrs. My favorite scope to observe is the Tal 4″ f/10 refractor. Pulled this out again because of the articles. By the way are these your drawings. Take care.

  2. Hi Frank,

    Thanks for the compliments — I really appreciate them! You’re the second person to mention they were using a four inch Tal refractor. Kind of sounds like they’re developing a devoted following.

    Also glad to read that you appreciate the reference material. I find it helps me to appreciate a specific star more when I know something of the background and history of it.

    Yes, the sketches are mine — they’re always a work in progress as I find new ways to improve their resolution and color. The charts are from Stellarium, usually with additional labels and lines added.

    Clear Skies!

  3. John,
    I also really enjoy this site and the stories you weave of your adventures under the stars. Like you, I use mostly alt/az and I also enjoy the experience of letting the object I’m viewing traverse across the field of view. I’m fairly convinced that some of that “settling down” you speak of is due the the object travelling across the ideal configuration between the different parts in the optical train, and probably some intermittent steadying of the atmosphere as well. Whatever it is, those few seconds can bring pure satisfaction to the evening.

  4. I’ve often thought the same thing, Mike. I’ve noticed in a couple of different scopes that the image tends to settle down in about the same location every time. In some cases when it’s already fairly stable, the image actually seems to get a bit sharper in those locations. You’re right, though — it makes the evening’s observing a real delight.

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