Xi (ξ) Scorpii (Σ 1999) and Σ 1998: A Triple Triple Maybe, a Double Double Definitely

It seems like I’ve spent most of the past spring and early summer with slitherous stellar creatures that most people would hardly pick for pets.  Most of April and May were spent creeping through Serpens Caput, and then I slid south and spent a few weeks in Ophiuchus, aka the Serpent Bearer (don’t know why anyone would want that job), and then I found myself pulled into Scorpius, certainly one of the least likely of creatures for a leash.  But it turned out to be one of the better moves I’ve made this year.

Chances are I never would have slipped quite so low below the celestial equator if it hadn’t been for a comment left by Steve McGee, in which he mentioned working with a group of people studying Σ 1998 and Σ 1999 in northern Scorpius.  I was immediately intrigued by his description of what sounded like a spectacular stellar display, but considering that most of Scorpius is blocked by tall coastal pines from where I peer into the heavens, I could only hold my breath in hopes the two stars were visible.  However, when I looked at their declination and found they were only about ten degrees south of Marfik in Ophiuchus, where I had just been a few days earlier, I knew — to borrow a phrase from William Tecumseh Sherman — I was in high feather.

So the first clear night I had, I broke out my Meade AR-5, a 127mm f/9.3 refractor, and headed south.  I stopped at Marfik, just for the heck of it to see what luck I would have prying apart it’s two 1.44″ companions — none whatever — and then looked to Marfik’s south, spied Delta (δ) and Zeta (ζ) Ophiuchi, and triangulated my way over to Xi (ξ) Scorpii, and spied it glimmering somewhat faintly through the coastal murk at me.  But it was well situated above the coastal pines, so downward went the scope, and forward to eyepiece went I.

And I raptured myself right into my observing chair with a resounding thud that came echoing back to me a few seconds later.

As I hinted above, you can find your way to Xi (ξ) Scorpii/Σ 1998 and Σ 1999 with a triangular approach. Locate 2.7 magnitude Delta (δ) — sometimes known as Yed Prior — and 3.2 magnitude Epsilon (ε) Ophiuchi — lesser known as Yed Posterior — I’m NOT making this stuff up — at the southwest corner of Ophiuchus and look about eight degrees to their southeast to see 2.5 magnitude Zeta (ζ) Ophiuchi. Then draw a line due west about another eight degrees and you’ll spy Xi’s 4.2 magnitudes at the western corner of a triangle it forms with Zeta (ζ) and Delta (δ) Ophiuchi. If you have really good skies, or sharp eyes, you’ll also notice that Xi (ξ) forms a smaller triangle with 5.2 magnitude Chi (χ), two degrees to the east, and 4.9 magnitude Psi (ψ) Scorpii, which provides a rather pleasing view in a finder. (Stellarium screen image with labels added, click for a larger view).

Xi [ξ] Scorpii (Σ 1998)  (AB is H I 33, AC is H II 20)        HIP: 78727    SAO: 159665
RA: 16h 04.4m   Dec: -11° 22′
Magnitudes  AB: 5.2, 4.9     AC: 5.2, 7.3
Separation   AB: 0.99″         AC:7.6″
Position Angle   AB: 359°  (WDS 2010)    AC: 44°  (WDS 2009)
Distance: 79 Light Years
Spectral Classification:  F7
Status:  AB gravitationally linked, orbit in can be seen here

Note: You might see Xi (ξ) Scorpii referred to as Graffias, as is the case with the orbit link above.   At one time it was, but it isn’t any longer — now Graffias is used to refer to Beta (β) Scorpii.  See Jim Kaler’s comments on his Stars site.

Σ 1999  (AB is H II 21)          HIP: 78738 (A), 78739 (B)   SAO: 159668
RA: 16h 04.4m   Dec: – 11° 27′
Magnitudes  AB: 7.5, 8.1     AC: 7.5, 11.0    AD: 7.5, 4.4
Separation   AB: 13.3″         AC: 83.1″          AD: 278.5″
Position Angles   AB: 101° (WDS 2009)  AC: 84° (WDS 1999)  AD: 351° (WDS 2006)
Distance: 82 Light Years
Spectral Classification: K0
Status: AB is physical, AC is optical; “D” is the AB pair of Σ 1998 and is physical

Let’s get to the view right away — you have to see this:

Σ 1998 is the brighter pair (“A” and “C”) just above and to the left of center in this sketch, and the fainter pair below and slightly to the right of center is Σ 1999.  It’s “C” component is flickering faintly just to the east (right).  The 4.9 magnitude “B” component of Σ 1998, at a very unreasonable proximity of 0.99″, isn’t seen in this sketch, which is because it has proven to be beyond my reach so far.  (East & west reversed to match the refractor view, click for a closer look).

Now I was peering through a lot of murky air, made even worse by the fact that my telescope was pointed about thirty degrees above the horizon.  And that situation was aggravated by the additional frustrating fact that the Pacific Ocean was at the bottom of that thirty degrees, feeding its molecular moisture up and into the murk.  But even with all that obscuring air, I was dazzled beyond belief.  Maybe the shimmer of damp photons in that moist air had something to do with it.

When you first look at this pair of stars, your eye is drawn right away and absolutely immediately to the brighter Σ 1998, which radiates a ravishing yellowish glow.  The companion you see instantly is the 7.3 magnitude “C” star, which seemed to me to be absorbing most of the yellowish photons leaking out of the primary — but the seeing was rather poor (about a II on this scale), so my eyes may have been deceived by the back and forth dance of nervous photons between the two stars.

I studied the dazzling primary very closely for several minutes at several magnifications, trying to detect at least a hint of the secondary attempting to elongate its way into view, but my eyes couldn’t lock the yellow ball of light into a motionless state for long enough to come to any kind of conclusion.  That frustrating atmospheric state was well described by Admiral Smyth as “certain teasings from the variable refractions of such a low altitude.”   And he was referring specifically to these two components of Σ 1998 when he muttered that, although, unlike me, he was successful in separating them.

So I shifted my gaze down to the Σ 1999 trio and spent some time scrutinizing the 7.5 and 8.1 magnitude primary and secondary.  They were displaying what could best be described as an ash color, but there were flickering flashes of a weak reddish orange in momentary moments of slightly better transparency.

The 11.0 magnitude “C” sibling was almost an averted vision affair because of the murk soup, but once my eyes adjusted to it’s isolated faint light, I had no problem holding it in view.  In fact, I felt a wave of sympathy for the poor thing, glimmering weakly at such a distance from its brighter and cheerier companions.  But since “C” is an optical companion, it’s really just a line of sight star passing in the depths of interstellar space on a lonely sojourn in search of some other stellar attachment — give it a few million years and it might meet with success.

What I find particularly interesting about these two Struvian delights is their physical relationships.  The too-close AB pairing of Σ 1998 is linked in a very tight and very short period orbit of 49.6 years.  The good news is they’re actually moving farther apart right now, reaching a maximum separation in 2021 — the bad news is that separation will only be 1.128 arc seconds.  In other words, if you can’t split it now, your chances aren’t going to improve a whole lot in the next nine years.

In the case of Σ 1999, the Washington Double Star (WDS) data shows the primary and secondary are physically linked, meaning they’re moving together through interstellar space in a parallel waltz.  Even more intriguing, the cramped AB pairing of Σ 1998 is classified as the “D” component of Σ 1999, which is because the two systems are also physically moving together through the galaxy.

As I alluded to in the title, Σ 1998 and Σ 1999 can be considered a “triple-triple” pairing, provided you can pry the “B” component of Σ 1998 loose, and if you can see the eleventh magnitude “C” component of Σ 1999.  I didn’t have any problem with the latter, but the frustrating former will no doubt remain little more than an elongated glimmer in my imagination unless I happen to catch a very stable night of seeing — and that’s rather unlikely considering the low down ocean hugging location of these stars from my latitude.   So I’ll call it a “double-triple” for now.

Now from the moment I first spied the data on Σ 1998 and saw Sir William Herschel‘s H I 33 catalog number, I was curious about how he fared in splitting that barely separated pair.  I looked up the latitude of his observing location near  Windsor, England, and found it’s a full six degrees further north of my 45.5 degree latitude, which certainly didn’t make his task any easier.   At the time he pried the AB pair apart, he was using a 160mm (6.2 inch) reflector with a focal length of about seven feet, and discovered the separation with magnifications of 460x and 932x, as you’ll read below.

His first look at this area came on May 23rd, 1780, when he described the two systems as a “double-double.”  What follows is his description of what are now cataloged as Σ 1998 (H II 20) and Σ 1999 (H II 21):

II.
20. and 21.   ξ Librae, FL. ultima.
May 23,      Double-double.  The first set very unequal. L. fine w.  With 227,
1780           nearly 2 diameters of L.   By the micrometer 6″ 23′”, but too large a
measure.  Position 1° 23′ n. following.  The other set both small and obscure.  With 227, perhaps 5 or 6 of their diameters asunder.”    (Source:   1782 Catalog, page 66)

When Herschel made these observations, the stars were considered to be in Libra, which explains the “Librae” in his first line.  (Admiral Smyth lists it as 51 Librae on pp. 352-53 of The Bedford Catalog, but argued that it belonged in Scorpius).   To explain Sir William’s abbreviations, “L.” means large and refers to the primary; “S.”, which he uses in the following excerpt means small, referring to the secondary; and “w.” means white.  When he lists his magnifications, such as “227,” he doesn’t include the “x” that we use these days.  His position angle of “1° 23′ n. following” translates into today’s 88° 37′;  and his “very unequal” refers to his perception of differences in magnitude, in this case between the “A” and “C” components of Σ 1998.

He was back two years later to catch the two stars again as they rumbled across the celestial meridian, and this time, using much higher magnifications, he discovered the primary of what was later to become known as Σ 1998 was itself a double, so he changed his description to “trebel” and cataloged that separation as H I 33:

I.
33.   ξ  Librae. FL. 51. Primam chelam Scorpii attingens.
May 12,      Treble.  Without great attention, and a considerable power, it may
1782           be mistaken for a double star; but the largest of them consists of two.  Very little unequal.  Both w.   With 460, 1/4 or at most 1/3 diameter asunder; with 932, full 1/3 diameter of L. or near 1/2 diameter of S.  Position, with 278, 82° 2′ n. following.   For measures of the third star see the 20th of the second class.”    (Source:  1784 Catalog, p. 173)

By my calculations, and using an orbital period of 50 years for the primary and secondary of Σ 1998, it looks like the two stars were separated somewhere in the .095″ to 1.00″ range at the time Herschel made his observation — in other words, about where they are today.  From reading his verbal descriptions of the separations, at a magnification of 460x he was looking at a separating distance of 1/4 to 1/3 of the visual radius of either the primary or secondary, and at 932x, he had a more definite separation equal to 1/3 of the visual radius of the larger star and 1/2 the visual diameter of the smaller star.

He also mentions measuring the position angle with a magnification of 278x, so he must have achieved at least an elongated view of both stars at that magnification.  His position angle of “82° 2′ n. following” translates to 7° 58′.  And I believe the Latin in the first line means something like “prior to the reach of Scorpio’s claws.”  This reproduction of Johann Bayer’s 1603 Uranometria shows it well if you select the larger view.

So, not bad work at all from a latitude of 51.5 degrees north with a six inch reflector!  The mirrors of his day were coated with speculum, which is a combination of tin and copper, and by today’s standards, were considerably inferior in reflective ability.  That, however, appears to have had little effect on what must have been a superbly figured piece of glass, which by the way, was ground and figured by Mr. Herschel himself.

I don’t know about you, but if Sir William could pry apart two fifth magnitude stars within one arc second of each other, located twenty-five to thirty degrees above the horizon, using a late 18th century 160mm reflector, it certainly ought to be possible with today’s five and six inch telescopes.  I’m heading back on the next clear night, and regardless of mire, muck, mud, or poor seeing, I’m planning to attack with at least 278x and see what happens.

Until then, hold your breath — or better yet, go try it — and don’t hesitate to holler if you have any success.

Clear Skies!  😎