South of the Sagittan Arrow: OΣ 396, S 730 & H IV 100, and 15 Sagittae & Σ 2622

The more time I spend delving into the details of double stars, the more convinced I become that it’s a field requiring the talents of a dedicated detective.  With his sleuthing skills, I’ve often though Sherlock Holmes would have made an excellent double star observer.  On the other hand, for all I know he may have tried it and decided crime sleuthing was a less demanding occupation.

Small though it may be, Sagitta is one of those areas of the sky that harbors a few mysteries worthy of Sherlock Holmes’ talent.  One of the better known puzzles, now solved, is M 71.  For a long time it was considered an open cluster, but eventually a few astronomical detectives came to the conclusion it’s actually a globular cluster — although I have to say  it still looks more open and less globular to me.

Of course, there are also double star mysteries lurking in the constrained boundaries of the Sagittan landscape as well, twinkling innocently in hopes of luring an unsuspecting character into their stellar webs of intrigue.

Like me.

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This tour started a couple of months ago with an email from Peter Morris of the UK in which he shared his intriguing observations of 15 Sagittae with me.   After surveying the scene surrounding it, I began perusing the area to see what else I could find. There are a total of five stars in this tour, grouped into a trio, and we’ll ease into it by starting with an uncomplicated, old-fashioned, straight-forward double star with no tricks up its scintillating sleeves:

Although it’s a dim constellation, Sagitta is not all that hard to find. A line drawn from Altair to Albireo will take you directly over the rear of its identifying arrow. (Stellarium screen image with labels added, click to enlarge).

The first star on this tour is located a degree and a half southeast of 3.50 magnitude Gamma (γ) Sagittae. It also forms a triangle with Gamma (γ) and 5.05 magnitude Eta (η). (Stellarium screen image with labels added, click on the chart for a larger view).

OΣ 396           HIP: 98738   SAO: 105608
RA: 20h 03.3m   Dec: +18° 30’
Magnitudes:  6.13, 10.38
Separation:   46.4”
Position Angle: 208°  (WDS 2012)
Distance: 849 Light Years
Spectral Classification: K3

This is pure simplicity itself:

Even though there are five stars surrounding the primary in this sketch, only one of them has been associated with it, which I’ve labeled. I couldn’t avoid lingering over the delicate orange rays of light beaming forth from the sixth magnitude sun at the center of it all. (East & west reversed to match the refractor image, click on the sketch for a better view).

There’s actually a bit more white in the primary than what the sketch shows here, and the surrounding field abounds in interesting patterns that could possibly harbor other stellar pairs.  I checked my usual sources, though, and came up empty-handed.

Click on the image to enlarge it.

What I did find, though, was a solid consistency in this pair of stars.   The first reliable measure of position angle and separation was made in 1866 by Baron Dembowski, which was 205° and 47.71”.  The additional measures shown in the excerpt at the left from W. J. Hussey’s 1901 compilation of Otto Struve’s Pulkovo Catalogue show a very gradual change, which matches up well with the 2012 WDS figures of 208° and 46.4”.  Chances are pretty darn good that this is a pair of physically unrelated stars.

So much for simple.  Now our celestial surroundings take a turn towards the complicated as we slide southwest another degree and a half to a fascinating pair of multiple stars.   Here’s that chart once again, and we’ll star with the northernmost of the two stars, S 730.

S 730         HIP: 98443   SAO: 105523
RA: 20h 00.1m   Dec: 17° 37’
Identifier      Magnitudes        Separation      Position Angle        WDS
S 730       AB: 7.16,    8.45         112.60”                   14°                  2012
S 730       AC: 7.16, 10.21           78.50”                 338°                  2012
S 730       AD: 7.16,   9.90           40.50”                 198°                  2012
HO 638    AE: 7.16, 10.20             2.70”                 283°                  1928
S 730       BC: 8.45, 10.21           68.40”                 238°                  2002
Distance: 1274 Light Years
Spectral Classifications: “A” is K0, “B” is F5, “C” is A2

Instead of cluttering your mind with the numbers in the data above, let’s first take a look at a sketch of what you’ll find in the eyepiece of a five or six inch refractor:

Nothing like a picture to bring a row of numbers to life! Fortunately I picked an almost full moon night for this sketch, or I might have been sitting behind the eyepiece all night trying to capture all the stars in this star-saturated Milky Way field. Even in the moonlight, I was able to capture all but one of the components of S 730 and H IV 100, as well as the subtle colors — orange/white in the case of the S 730 primary and yellow/orange for the “C” component of H IV 100. (East & west reversed once more, click on the sketch to bring it to life).

Click to enlarge the image.

The only component of S 730 I didn’t pry out of the moonlit sky was 10.20 magnitude “E”, which eluded me on first attempt (see update at end of this post).  That one carries a WDS prefix of HO, which belongs to George W. Hough, who we met in our last outing in Delphinus.  His original 1899 observation, included in the thumbnail at left, placed the position angle at 290.8° and the separation at 2.15”, so it would appear the distance between the secondary and primary is widening very slowly.  But I’ll have to qualify that by noting this pair hasn’t been measured since 1928.

Click anywhere on the image for a larger view.

As for the AB pair, Sir James South’s 1825 observation is shown at the right.  But if you look closely, you’ll see the second of his first two measurements (both on July 13^th, 1824) refer to the primary of S 730 and Chi (χ) Sagittae (aka 13 Sge), which lies just north of H IV 100.

And that leads us to Sir William Herschel’s H IV 100, which is situated six arcminutes south of S 730.  As you slide your eyes to the south edge of the field of view, you’ll pass a pair of closely spaced stars which have been designated SLE 675, with magnitudes of 11.4 and 11.6 and separated by 7.4” at a position angle of 310°.

H IV 100/Bu 1477     HIP: 98438   SAO: 105522
RA: 20h 00.1m   Dec: +17° 31’
Identifier       Magnitudes         Separation      Position Angle      WDS
H IV 100    AB: 9.96,  10.12          24.20”                  256°               2012
H IV 100    AC: 9.96,    5.57        113.60”                  296°               2012
Bu 1477    CD: 5.57, 13.40          27.70”                  208°               2010
Bu 1477    CE: 5.57, 12.30          43.00”                  115°                2001
Distance: 746 Light Years
Spectral Classifications:  “A” is M4, “C” is F0; “C” is 13 Sagittae
Notes:  “A” is the variable star VZ Sge

Now if you look carefully at the data above, you’ll find we’re faced with a very unusual situation:  the brightest star in that list is labeled as the “C” component, which has led to more than a minor amount of labeling confusion.  If you use any of the popular sky software programs, chances are they’ll incorporate some aspect of that confusion into their data.   The only one I’ve found that is close to being correct is Sky Safari, which correctly distinguishes the AB pair of H IV 100 from the CD pair of Bu 1477.

The best place to start the sorting out is at the source, and in this case that would be William Herschel’s 1782 observation of the pair he cataloged as H IV 100:

And you can see the reason for the confusion on the title line, where he places H IV 100, χ Sagittae, and Flamsteed 13 at the beginning of his entry.  The Latin on that line translates as “below the middle reed”, which is a reference to the description by Latin authors of the material from which the arrow was constructed – interesting, but not much help to us right now.  However, as you continue, he refers to the closely separated pair first (“The two nearest equal”) and provides a measurement: “Distance 23’’ 2’’’.  Position 10° 12’ s. preceding.”   That last number translates to 260° 12’, so both his position angle and separation compare favorably with the 2012 figures in the WDS.

Click on the image to enlarge it.

At that point, he adds the third star, which is Flamsteed 13, or χ Sagittae, depending on your preference.  And again, the measurements he provides there match up well with the 2012 WDS figures.  (His 10° or 15° north preceding translates to 280° to 285°).

All of which would seem to indicate he was designating the closely separated pair as “A” and “B”, and the brighter of the three as “C”.  Which is the way S.W. Burnham read it also, as you can see in the excerpt at the right from the second volume of his 1906 catalog.  In fact he comments on it twice, once in the top entry for χ (13) Sagittae, and also again in the bottom entry for H IV 100.

Click …. to ….. enlarge.

There’s one additional error, also pointed out by Burnham in that last entry, which has to do with Σ 2608 (STF 2608).  You can see his “rej.” comment in parentheses, which is because Σ 2608 is cross-referenced in Struve’s 1827 catalogue with H IV 64.  That pair is actually located in Perseus, and is correctly cross-referenced in the 1827 catalogue with Σ 292.   Both of Struve‘s catalogue entries are shown at the left.

And in fact, if you search for Σ 2608 in the WDS database using the Stelledoppie site, you’ll find it doesn’t exist.  Nevertheless, the WDS notes file for H IV 100 still includes this comment: “Also known as STF2608.”   I’ve also found that Thomas Lewis skipped over Σ 2608 in his 1906 compilation of Struve’s catalogue.   So even though that error was corrected well over a century ago, it still persists.

Now that we’ve got that settled, let’s move on (here’s our chart again) to another case of stellar confusion, 15 Sagittae, aka STT 592, aka STTA 592, aka BUP 202, etc., etc.:

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You can click anywhere on the data to get a larger view of it.

As you can see, this is a complex system – although as the notes indicate, several of the components are optical, meaning there’s no physical relation between them and the primary.   But instead of dwelling on the data above, let’s get a look at 15 Sagittae and its stellar retinue first:

There’s a lot of white in these stars, although the stellar classifications of “A” and “C” would indicate otherwise. I’ve glimpsed a slight gold tinge in “A”, but haven’t seen the first hint of the orange in “C” that its K class would lead you to expect. (East & west reversed once more, click on the image for a larger view).

I’ve labeled all the components in the inset at the right of the sketch, and also identified Σ 2622, located a bit more than four arcminutes south and slightly east of 15 Sagittae’s primary.  That closely-knit trio stands out clearly in a six inch refractor, including the “C” component, which can be seen with careful use of direct vision.

Now if you look closely, you’ll see I’ve labeled one of those stars surrounding 15 Sagittae’s primary as F/H – which is because there’s only one star of the appropriate magnitude at the point indicated by the position angles and separations of AF and CH.   Below is a plot of the two pairs I did on Vizier which has been flipped to match the orientation of the sketch above:

Click on the photo to enlarge the view.

You can see the position angle and separation (1.601’ = 96”) of CH matches the WDS data of 2001.  And in the case of AF, the separation shown on the chart is almost an exact match (2.385’ = 143.1”), while the position angle, at 310 degrees, is off slightly from the 2010 WDS figure of 314 degrees.  And notice also, there clearly is no other 11.6 or 11.7 magnitude star four degrees south of that star (the small star beneath the arrow and southeast of F/H is a few magnitudes fainter).   Complicating the issue somewhat is that the photograph was taken in 1991, and more significantly, several of the stars in this grouping show considerable proper motion.  The primary actually leads the pack, with a proper motion of .395” per year west and .408” per year south, as shown in the Simbad plot below:

I’ve also flipped this image so east and west match up with the sketch above, which has the added benefit of making it easier to identify the stars labeled on the chart. Click on the image for a larger view.

There are also several other stars shown on that plot that are a bit less than stationary, which I’ve identified, so you can see there’s a lot movement going on among the various components.   At any rate, considering the rapid movement of “A”, it’s not surprising that the position angles in the 1991 photo don’t match up precisely with the 2010 data in the WDS.

Click on the image to enlarge it.

That still leaves us with the puzzle of whether the star at the junction of the arrows is “F” or “H.” The WDS identifier of “BUP” for CH tells us it was discovered and measured by S.W. Burnham, and also that we should be able to find his measurements of it in his 1913 Proper Motion Catalogue – and as you can see from the page at the right, that’s exactly where I found it.

 I’ve added the current identifying labels in red for the stars Burnham referred to as “A and a”, “B and b”, and “C and c.”  The one we’re interested in is “C and c”, which as it turns out corresponds to the WDS label for CH!  I was helped considerably by the historical data in the WDS which shows the first measurement of that pair was made in 1900, resulting in a position angle of 185 degrees and a separation of 94.2”, both of which match up closely with Burnham’s 1908 results.

That takes us back to AF, which the WDS historical data tells us was first measured in 1984 by an anonymous someone at 308 degrees and 142.7”.   So we have a two degree difference in position angle between 1984 (308°) and the 1991 photo (310°), a seven year interval — and a four degree difference between 1991 and 2010 (314°), a 19 year interval.  Kind of looks to me like my original suspicion that “F” and “H” are the same star is correct.  If that’s the case, AF is really AH, and “F” can be dropped as an identifying label.  And since there are no competing stars anywhere near it, I’ll leave it at that.

But before we leave 15 Sagittae, it’s worth noting its role as a stand-in for our own star.   Due to the fact that 15 Sagittae “A” bears a close resemblance to a younger version of our sun, it has come in for a lot of scrutiny in hopes of learning what the sun, as well as the earth, might have been like in the past.  There’s some illuminating (no pun intended) information here, and you’ll find a photo of the brown dwarf, “b”, shown on the very first line of the WDS data on this very interesting site.

Meanwhile, I think I’ll find something less challenging for a few days, possibly a Sherlock Holmes novel.  Maybe Watson will have something enlightening to add on the subject of double stars.

Next trip takes us to the northern reaches of Cygnus – until then, Clear Skies!  😎

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UPDATE:

It took two nights and a combined viewing time of close to two hours, but I was able to finally get a few glimpses of G.W. Hough’s HO 638, which is the 10.20 “E” component of S 730.   The first attempt was with my six inch f/10 refractor — no hint whatever of the faint star — and the next was with a 9.25 inch Celestron Edge SCT.   Seeing both nights was poor, although it started out somewhat better on the second night.

I worked through a whole range of focal lengths — 18mm, 14mm, 12mm, 10mm, 8mm, 7mm, 6mm, and 5mm — because I wasn’t sure at what point the glare of the primary would become impenetrable.   The first glimpse of it was in the 6mm eyepiece (an Astro-Tech Plössl), and I was encouraged enough by that to try a 5mm UO Ortho.   I had a glimpse of “E” in it, but the seeing became so erratic I went back to the 6mm.   What was at first a reasonably sharp image would suddenly balloon into an un-focused blob, and then slowly it would return to a semi-sharp condition as a it vibrated all over the place.   Here’s the sketch from that second night:

You have to look closely, but “E” can be seen at about the ten o’clock position — clicking on the sketch will provide a much better view. You have to imagine this image hopping all over the place with “E” visible only in quick, fleeting glimpses. (East & west reversed to match the SCT view.)

It wasn’t pretty, but it was there at least, and it doesn’t look like it’s changed appreciably in either position angle or separation.