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The Lost Double Stars of Serpens Caput: 39 and 49 Serpentis; and One Not So Lost Triple Star, Σ 2007

There are more gems buried in Admiral William Henry Smyth‘s 1844 stellar survey, The Bedford Catalogue,  than you’ll ever discover in a jeweler’s warehouse.  About a month ago I was sleuthing around in Serpens Caput and slipped right past 39 Serpentis.  In fact, in the short piece I did on that area of the sky, I included this chart showing its location at the west edge of a warped diamond-shaped configuration of stars.  None of the charts or information sources I was using at the time said anything about 39 Serpentis having a dual personality, so I never bothered to cast a moderately magnified telescopic glance at it.

But the Admiral was well aware of it, and he even gave it a considerable going over, which he was thoughtful enough to record on p. 350 of The Bedford Catalog, where I could stumble across it 168 years later.

He also knew about its slightly higher numbered neighbor to the east, 49 Serpentis.   Now that one isn’t lost in the same sense that its lower numbered relative is.  Unlike 39 Serpentis, it’s clearly labeled as a double star in The Cambridge Double Star Atlas — but you won’t find it where you would expect to find it.  It seems it’s strayed across the border into Hercules.  In fact, that leads to one of those rather rare stellar situations in which you have a star sporting a name from an earlier incarnation in an adjacent constellation.  Apparently it prefers the Hero’s domain over the Serpent’s lair.

So follow me into Serpens Caput once again, where in the best tradition of that Heculean Hero, we’ll rescue these two stars from relative obscurity.  And in the process, we’ll stop and take a look at a terrific triple star, Σ 2007, that managed to wedge itself about halfway between them.

In order to get your bearings, here are two charts showing the location of Serpens Caput in relation to the surrounding constellations – one here, with a little artwork to dress it up; and one here, which is an unadorned version of the same area of sky.

And after you’ve looked at those, the chart below is the one we’ll use to navigate our way to all three of our choice targets:

From Gamma (γ) Serpentis, which marks the southeast corner of the Serpent’s head, move one degree south to Phi (φ), which is at the top of the distorted diamond shape configuration outlined in blue. Move another degree, this time to the southwest, to reach 39 Serpentis at the west corner of the diamond. (Stellarium screen image with labels added, click for a larger view).

39 Serpentis  (OΣ 583)  (H III 25)        HIP: 77801    SAO: 101792
RA: 15h 53.2m    Dec: +13° 12′
Magnitudes: 6.1, 11.8
Separation:  102.4″
Position Angle: 86°  (WDS 1998)
Distance: 57 Light Years
Spectral Classification: G0
Status: Optical pair (WDS notes)

On the evening I stumbled into this star in Admiral Smyth’s Bedford Catalog, I already had a pair of telescopes set up, an Astro Tech 90mm f/6.7 refractor and a 63mm Zeiss f/13.3 refractor.  The Admiral listed the primary and secondary at magnitudes of seven and fifteen, respectively, which I knew was based on a system considerably different than the one in use today.   So I did some quick checking in the Washington Double Star Catalog (WDS) and dug out the numbers shown in the data line above.  Now I had my doubts about prying that faint 11.8 magnitude secondary out of the sky with the 90mm scope, but since that’s what I had ready to go, I gave it a try.  And, much to that chubby little devil’s credit, I was able to see this:

The secondary is just a faint dot of light in a 90mm refractor, with a deep gold primary keeping a close watch over it.  (East & west reversed to match the refractor view, click for a slightly larger version).

That primary was a very pleasant yellow-gold color, an attribute that caught my eye during my earlier trip into this area.  Surprisingly, the secondary wasn’t all that hard to see, even at the 60x I used for the sketch, and the fact that it escaped most of the primary’s glow by floating out there all alone at a relatively remote distance of almost two arc minutes made the chore significantly easier.  Just for the heck of it, I also took a peek in the 63mm Zeiss, and picked the secondary out of the sky with averted vision in a 15mm TV Plössl (56x).   Chances are I wouldn’t have seen it at all in the 63mm scope if I hadn’t spied it first in the larger scope, but a lot of credit goes to the better than average transparency and seeing conditions, both of which made possible what otherwise would have been unlikely.

After learning from The Bedford Catalog that Sir William Herschel had discovered the double star status of 39 Serpentis, I found my usual source of information for his double star discoveries doesn’t include this star at all.   So after some additional searching, I found the original Herschel double star catalogs in a .pdf file at this source , and quickly located his observation of 39 Serpentis on p. 71 of the 1782 catalog (the first of the three catalogs listed below the page one painting of Herschel).   So here, fresh from the musty archives of the Philosophical Transactions of the Royal Society,  is Sir William’s verbatim account:

25. *  In constellatione Serpentarii, FL. 39.
Aug 29, 1780.   Double.  It is the most south and largest of the two in the finder.  Very unequal.   L. w.;  S. inclining to blue.  Distance 10″ 2′”, a little inaccurate.  Position 87° 14′ n. preceding.”

His terms require some translation, so here goes:  The “FL” in FL 39 refers to Flamsteed (as in Flamsteed number); “Very unequal” means that Herschel saw four magnitudes of difference between the two stars; “L.w.” is Large white (Large refers to the primary), and “S” is for small, meaning the secondary.  The position angle referred to in the quote translates to 356° 46′ in today’s usage.

Admiral Smyth contributed his observations in 1835, resulting in slightly different measurements of 12.0″ and 355 degrees.  And after that, the trail begins to fade into the forest and the details begin to diverge.

The separation and position angles in the WDS which are labeled “first” are based on 1862 data, which is apparently when Otto Struve made his observations.  But when I went to my usual source to get the details, I found it wasn’t cooperating either — it skips right past OΣ 583.   Normally that book also includes the observations made by other observers up to within a few years of the book’s 1906 publication date, observations which would have been especially useful in this case since a huge discrepancy emerges starting with the 1862 data.   I have a hunch OΣ 583 is included in Struve’s supplement to the Pulkova Catalog, but I have yet to succeed in tracking that one down.

But let’s look at the discrepancy now.  If you go back to the separations measured by both Herschel and Admiral Smyth, you’ll see they’re reasonably close (10.0″ and 12.0″, and 357 and 355 degrees), especially taking into consideration the fifty-five years between them.   According to the WDS, the 1862 measurements were 106.4″ and 131 degrees, which can be compared to the last WDS measurements in 1998 of 102.4″ and 86 degrees — and from what I saw, those 1998 numbers have changed very little, if at all.

So between 1835 and 1862, a time span of 27 years, the secondary suddenly jumped from 12.0″ and 355 degrees to 106.4″ and 131 degrees — and 136 years later the separation was still reasonably close to the 1862 figure, accompanied by a significant change in position angle — and keep in mind this is an optical pair.  Something is obviously wrong here, and it would be a huge help to have a list of observations made between 1835 and 1862 to see when the separation changed, and hopefully with an explanation of what was discovered that resulted in the change.

On the one hand, it’s difficult to believe that both Herschel and Admiral Smyth erred in their measurements, and yet on the other, it’s obvious the secondary’s separation is still pretty close to Otto Struve’s 1862 measurements.  This star deserves to become “un-lost,” if for no other reason than to determine what took place.

But let’s go on to a trio of stars that didn’t disappear into a black hole.

Going back to the chart above (or open it in a separate window here), from 39 Serpentis move due east about a degree to OΣ 303 at the the opposite side of our warped diamond configuration, and then continue east for almost the same distance and you’ll find Σ 2007 parked next to 7.30 magnitude HIP 78993.  (OΣ 303 and Σ 1988 on that chart are doubles, which were discussed here).

Σ 2007  (Sh 218)         HIP: 78875    SAO: 101922
RA: 16h 06.0m    Dec: +13° 19′
Magnitudes   AB: 6.9,. 8.0        AC: 6.9, 10.8
Separation    AB: 38.2″             AC: 162.9″
Position Angle   AB: 322°  (WDS 2010)     AC: 137°  (WDS 2009)
Distance: 579 Light Years
Spectral Classification: G8
Status: The AB pair is physically related (WDS).

I was using my six inch f/10 refractor the night I came across this one, and it really does shine magnificently at that aperture.  It’s a respectable sight in a 90mm scope as well, but I highly recommend at least a five inch scope to get the full effect.

There’s nothing quite like the straight-lined beauty of a well configured triple star, with the field brought to life by a sprinkling of star dust hovering to the west of it.  (East & west reversed again, click for larger captionless view).

As you can see, it’s a beautiful triple star, with the second and third components arrayed on either side of the primary in almost a straight line.  That fact alone draws your eye to it, but it doesn’t hurt at all that the primary is an enticing shimmering white with a distinctive hint of yellow in it.  I had the impression that the color of the secondary was a pale imitation of that, and the distant “C” component was simply too faint for me to detect any color.  You can see a close pair of 12.5 magnitude stars in the northwest corner of the sketch, but according to MegaStar, they’re not related to each other.

Haas caught this one with a 60mm scope and described it as “A wide pair with pretty colors — an ideal object for low power.  It’s a Sun yellow star with a little red companion.” — but if there was any red in the secondary, I sure didn’t see it.

This system apparently got past Sir William Herschel, but his son John, along with his sidekick James South, pried it from the sky on May 6th, 1823, and cataloged it as Sh 218.  They made a second observation on June 4th of that year, and from the two observations came up with an averaged separation of 31.94″ for the AB pair, with a position angle of 328.7 degrees.  The primary they reported as yellowish white and the secondary as blue. (p. 245 of Herschel/South Catalog, scroll to bottom of the page).

Friedrich Georg Wilhelm von Struve added an observation in 1830, with very similar measurements — 31.97″ and 328.3 degrees — and also recorded the primary as yellowish white, but saw the secondary as white.  By 1898, the separation had increased to 34.27″, with the position angle creeping westerly to 325.1 degrees. (pp. 440-41 of this book).   As the figures in the data line above show, the secondary has continued around the primary in a westerly direction, slowly increasing its distance at the same time, which leads to the conclusion in the WDS that the two stars are physically related in some way.

Now, going back to the chart above, if we continue east for about another degree, we’ll find ourselves crossing the border into Hercules, where 49 Serpentis currently resides.

49 Serpentis  (Σ 2021)  (H I 82)  (Sh 221)           HIP: 79492    SAO: 102018
RA: 16h 13.3m    Dec: +13° 32′
Magnitudes  AB: 7.43, 7.48     AC: 7.43, 11.3
Separation   AB: 4.1″                AC: 206.7″
Position Angles   AB: 356°  (WDS 2012)     AC: 118°  (WDS 1998)
Distance: 79 Light Years
Spectral Classification: G8
Status:  AB gravitationally linked, orbit in WDS; “C” optical

A close double star, which, though pertaining to the Serpent, is absurdly placed on the left arm of Hercules.  “A” 7, pale white; “B” 7 ½, yellowish.  This is 82 H I, and is a fine and tolerably easy object, therefore its measures at the several epochs are entitled to confidence . . . a rough investigation of the whole, gives about 600 years for the orbital revolution of the satellite about its primary, —- or rather, of one sun around the other.   . . .   Meantime he who wishes to fish the object up, may find it at the distance of 10° ½ to the north-east of α Serpentis, on the line leading from thence to Wega.”  (Admiral Wm. H. Smyth in The Bedford Catalogue, p. 355)

“. . . a fine and tolerably easy object . . .”; “two very well defined dots of almost orange light . . .” (East & west reversed, click on the sketch for a closer view).

In the Zeiss I used for the sketch above, as well as in a 60mm f/15 at 45x, this is an alluring and attractive pair of stars.  The primary and secondary are two very well defined dots of almost orange light that are separated by about the width of a hair, and even the 11.3 magnitude “C” component isn’t all that tough to pick out of the dark sky in a 60mm refractor.  The colors lose some of their tilt toward orange in a six inch refractor, and an eight inch Edge SCT shows both stars to be white with a light sprinkling of yellow.

Orbital motion had already been noticed in the AB pair when Admiral Smyth wrote the description above — in fact, Sir William Herschel gets credit for detecting it in 1804.  A graph of that orbit can be found here, based on WDS data, which puts the current estimate of the orbital period at 1354 years, a little more than double the 600 year period the Admiral estimated — but considering the differences in technology between 1835 and today, he didn’t do too badly.

Sir William also gets credit for being the first observer to discover the paired nature of 49 Serpentis.  He observed it on March 7th, 1783 and described it as a “a very minute and beautiful object.”  Both stars appeared pale red to his eyes, which I suspect is an indication of some shade of orange, and he measured the separation and position angle on that date at 2.5″ and 291 degrees.   (p. 180 in the 1784 Catalog)

This star is cataloged as Sh 221 on pages 247-48 of John Herschel and James South’s joint catalog of 1824.  They observed it twice — once on June 9th, 1822, and again on April 11th, 1823 — and described both stars as white and nearly equal in magnitude.   And Friedrich Georg Wilhelm von Struve also saw white in both stars when he added an observation in 1832 (p. 443 here).

As to which constellation 49 Serpentis should call home, Admiral Smyth was rather blunt about where he thought it belonged in his comments above.  The re-location took place in 1930, when the IIAU formally re-drew the boundaries of the constellations in order to provide some consistency to them.   A map of the Serpens area prior to the 1930 change can be seen here in a Johann Bayer chart.   If you click on the “larger view” link at the upper right hand corner of that chart, you’ll be able to see Gamma (γ) and Phi (φ) Serpentis at the base of the Serpent’s head.   Not shown is 49 Serpentis, but a line drawn from Beta (β) through Phi (φ) and extended about one and a half times that distance will put you very close to the location of that star.   As you can see, there are no boundaries at all on the chart, a significant source of confusion which led to some stars or objects being associated by name with more than one constellation.

So there you go — one lost double star re-discovered, only to reveal a substantial secondarial mystery; one dazzling triple to add some spice to a short tour; and a wayward triple star that seems to have found a permanent home.   Coming up next is one last trip through Serpens Caput, and then it’s time to move on and frolic in new fields.

Clear Skies!  😎

2 Responses

  1. I simply love your write ups and the sketches you prepare. Thanks for doing such a great job. My only suggestion would be to include the size of the sketches field-of-view. I sometimes have trouble seeing which star is the secondary when other stars are involved. At any rate the FOV size would make it easier for me.

  2. Thanks for comments, Buddy!

    Somehow I managed to mix up the formula for magnification with the formula for the eyepiece field of view the first time I responded, so scratch everything I said before. I’ve got a spreadsheet I use to keep track of magnification and fields of view for all of my eyepieces and telescopes, so I rarely actually use the formula.

    At any rate, the correct formula for the eyepiece field of view is the apparent FOV of the eyepiece divided by the magnification, so unless you happen to know the apparent FOV of the eyepiece, you wouldn’t be able to determine the FOV in the sketches — in other words, I see your point more clearly now.

    There’s a slightly different formula used by Televue, which is the eyepiece field stop divided by the focal length of the telescope, and then that result is multiplied by 57.3 — that gives you a slightly different number from the first formula. Of course, the field stops are different for almost every one of their eyepieces, so again you’re in the dark without that information.

    If you see any sketches where you’re wondering about the field of view, the Radians I frequently use all have a sixty degree apparent FOV and the TV Plössls have fifty degree fields of view. The 24mm Brandon I use frequently has a fifty-three degree FOV. That should cover most of them.

    At any rate, I’ll look at adding that information in future sketches. Sorry about the mix-up. The side effect of relying on computers to do all our calculations is that we tend to forget the forumlas!

    John

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