There’s something rather eerie about that part of the stellar vault called Serpens Caput. I keep meaning to move on to some other constellation in search of dazzling double star delights, but I find myself captured repeatedly by odd and unexpected discoveries scattered randomly through this serpent’s lair, so much that I’m beginning to think it’s one of the most seductive stretches of the sky in the northern hemisphere. Rather surprising in a constellation not known for much of anything particular, except possibly M5.
A couple of years ago I highlighted what I thought were the two showpieces in this crooked constellation, and then not too long ago I found myself involved in a couple of close encounters while the serpent had his head fixed on the Herdsman’s right arm. Since then I’ve come across another mystery worthy of Sherlock Holmes’ sleuthing abilities, along with some of S. W. Burnham’s handiwork, both of which I’ll save for the next two posts. But as for the present, I want to introduce you to a star that is just — well, a little strange.
First, a look at where we’re going:
And now, over at our right, you’ll find the zoomed in view, in which you can see that this is hardly the brightest section of the sky.
In order to get to our destination, you really need either a dark sky location and a moonless night, or a pair of low power binoculars. If you look carefully, you’ll see that 5 Serpentis forms pretty close to a perfect equilateral triangle with Epsilon (ε) and Mu (μ) Serpentis. What I did was to point my scope at the western corner of that triangle and then peer into the finder in hopes of seeing M5 — and it worked! Look for a very small fuzzy little ball of light — it’ll stand out because the rest of the field is in sharp focus. Once you’ve got M5 in view in a wide angle eyepiece, move it to the northwest corner of the field of view, and then look for the yellow light of 5 Serpentis in the opposite corner, as shown below.
Σ 1930 (5 Serpentis) (AB is H III 106) HIP: 74975 SAO: 120946
RA: 15h 19.3m Dec: +01° 46′
Magnitudes AB: 5.06, 10.11 AC: 5.1, 9.1 AD: 5.1, ?????
Separation AB: 11.3″ AC: 147.3″ AD: 713.6″
Position Angle AB: 35° (WDS 2010) AC: 19° (WDS 2009) AD: 275° (WDS 2009)
Distance: 81 Light Years
Spectral Classification: F8
Now what attracted me to 5 Serpentis/Σ 1930 was it’s location adjacent to M5, which is truly one of the more spectacular globular clusters in the sky. I had a hunch — which proved to be correct — that I would be able to capture 5 Serpentis in the same field with M5. Here’s my sketch, which doesn’t do justice at all to that marvelous sphere of stars:
First, I’ll let you look over my shoulder at my observing notes on this one:
Primary a bold, almost deep yellow; “B” was a very small dot of light, “C” was difficult to see, and “D” was very obvious. M5 steals the show — in the six inch f/10, the outer rays of stars almost look like the spiral arms in a galaxy.”
Haas was also impressed with the view: “325mm, 200x: Fantastic view! A brilliant yellow star with a close little speck beside it, and globular M5 is at the edge of the field.”
But let’s go back and look at my notes again, and you’ll see that I found “C” more difficult to see than “B”, which is rather surprising. Not only is “C” a full magnitude brighter than “B,” it’s also a full thirteen times farther away, which puts it well out of the 5.1 magnitude glare of the primary. Just to check myself, I went back about a week later with an eight inch SCT — actually, one of the new Celestron Edge scopes, which is proving to be an excellent double star telescope — to get another look at the “B” and “C” companions.
To borrow part of a phrase from Snoopy, “It was a dark and murky night.” So murky, in fact, that I wasn’t penetrating much deeper into the dark than I did with the 90mm refractor I had been using the prior night. But even in that murky sky, “B” was very obviously brighter than “C.” That left me wondering if the 9.1 magnitude assigned to “C” in the Washington Double Star Catalog (WDS) could be wrong, or if perhaps it’s variable. But when I looked, the WDS, which normally includes that kind of information, was silent on the subject.
Now if by some wild chance you happened to be familiar with the history of this star, you wouldn’t be surprised in the least. Because it seems that 5 Serpentis has been engaged in some weird variant of celestial sleight of hand since at least 1783.
William Herschel apparently didn’t pay much attention to the existence of “C” — maybe he didn’t see it, since I had to look closely to catch it — but he certainly had problems with the secondary, “B,” on the night he discovered it (May 21st, 1783), as James South also would about forty years later. Here are Sir William’s observing notes: “Double. Excessively unequal. L. rw.; S. db. Too obscure for measures. Of the third class, far.” (Source)
Now a little translation here will help to understand what he was saying.
In Wm. Herschel’s lexicon, “excessively unequal” means there exist six magnitudes of difference (as he saw it) between the secondary and primary, and “third class” is a category he used for separations of between five and fifteen seconds of arc, a slot the secondary slips into rather easily. Then, “L.” refers to the primary (Large), but “rw” has me perplexed — normally he used “r” for either red or ruddy and “w” for white, but ruddy white doesn’t quite make sense here, and red doesn’t work at all. Next, “S” refers to the secondary (Small), and “db” means dusky blue I believe, or maybe dark blue.
But it’s the next to last sentence that lingers most — “Too obscure for measures.” And in fact, Sir William left no specific measurements whatever of the distance between the primary and secondary, just that phrase, “Of the third class, far.” — which has a rather distant ring to it.
Which brings us to Sir James South — who, it seems, had one heck of a time with the secondarial light of the “B” companion.
His first observation was on June 9th, 1825 (it’s his catalog number S 670), and it was very similar to William Herschel’s: “Observed when on the meridian with a power of 92; with 181 and 157 I could not see the small star, which bears so very feeble an illumination, that the accuracy of the results is perhaps a little questionable.” He measured the separation that night at 10.7″ and the PA at 39.0 degrees.
He was back on June 14th, 1825: “Excessively difficult. The small star will not bear the slightest illumination. No measures of distance can be procured, and these of position are little else than approximations.”
And again on June 17th, 1825: “Observed on the meridian. Night fine; but the small star will bear only the most feeble illumination.” And he concludes his account with this —– “Sir W. Herschel has no measure of this star.” —– as if to say, “At least I’m not the only one!” (Source for the South quotes)
So considering the trials and tribulations of Sir William and Sir James, I suspect the catalog designation of Σ 1930 is a tribute to Friedrich Georg Wilhelm von Struve (the Greek capital letter for “S” is “Σ”), a way of recognizing his success in measuring this bewitching little devil in 1831. He came up with a separation of 10.07″ and a position angle of 41.0 degrees, and described the primary as yellow. If it gave him any photonic fits, he apparently kept it to himself. However, in addition to his success with the secondary, he latched on to a haunting and hitherto unobserved aspect of this stellar enigma, namely an elongation in the primary — meaning “A” might well be a double in and of itself. (p. 411 of this book).
Which takes us now to the Venerable Admiral Wm. H. Smyth, who will get a chance to speak at some length:
A delicate double star, among the marginal stragglers of the preceding object, No. 5 Messier [which the Admiral describes as ‘. . . a noble mass, refreshing to the senses after searching for faint objects; with outliers in all directions, and a bright central blaze . . .’]: it is 9° to the south-west of α Serpentis, and 24° to the south-east of Arcturus. “A” 5½, pale yellow; “B” 10½, light grey. This fine object is 106 H III, and was discovered in May, 1783; but no angle or distance are given in the original entry, Sir William merely noting that its position was 30° or 40° nf [north following], and that it was excessively unequal, too obscure for measures. Those of Sir James South are, therefore, the first I met with: Position 39° 03′ Distance 10.70″ Epoch 1825.45
My results being as nearly coincident as could be expected in so delicate an object [10.5″ and 40.7 degrees], I considered this star to be ‘done with;’ but finding, on the arrival of the Dorpat Catalog in 1837, that Professor Struve considers “A” to be oblong, and consequently double, I took the first opportunity of apparition to scrutinize closely, and catch another set of measurements. The night was beautiful, the instrument in its best action, and the stars distinct and clear; but no effort could elongate the primary.” (from p. 339 of The Bedford Catalogue)
That instrument “in its best action,” by the way, was a 5.9 inch refractor with a very unusual mount, at least by today’s standards, and can be seen here.
At any rate, by 1903, the measurements of the primary and secondary components of 5 Serpentis had changed very little, culminating as of that year at 10.9″ and 39 degrees — and it’s changed little in the one hundred plus year since. A list of those earlier measurements can be found on p. 411 of the book referred to above (prior to Admiral Smyth’s extended quote), including an 1852 measurement of “D” by Otto Struve, which is considerably at variance with the most recent measurement. Someone added the mysterious “C” component in 1887, but in keeping with the bewitching nature of the 5 Serpentis family, that person remains unidentified.
As I said at the beginning, there’s just something a little strange about this star. Maybe it’s caused by clashing fives — M5, 5 Serpentis. Could be a little bit of judicious renumbering would return things to normal.
In the meantime, go have a look ——– but be wary of something slightly scary and not at all ordinary, lingering under the long shadow cast by M5. And don’t let those ever-present rustling sounds that inhabit the dark of night become magnified by a suddenly over-active imagination. 😉