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Snaking our Way into Hercules from the Serpent: Beta and Delta Serpentis, and Kappa Herculis

Although I normally prefer to stay as far away from snakes as possible, I decided tonight to take a closer look at that area of the sky where a serpent lies poised to strike at Hercules’ left elbow.  To be precise, we’re starting in Serpens Caput, the serpent’s head  —-  which is not to be confused with the serpent’s body, Serpens Cauda, which stretches south and east of Ophiuchus, the serpent bearer  —- and yes, that means Ophiuchus is holding the serpent.  It’s not as creepy a part of the sky as it may sound, but I’ll watch for quick movements.   If I get into trouble, I guarantee you’ll hear me.

What we’re going to do is track down two multiple stars near the Serpent’s head, and then simply because I caught sight of it and found it totally irresistible, we’ll creep across the serpentine boundary into Hercules and cap off our short tour with Kappa (κ) Herculis.  First, some directions would probably be a big help:

You’ll find Serpens Caput is wedged in a tight little section of sky surrounded by Hercules and Ophiuchus on its east side, Boötes and Virgo to its west, and directly to the south of Corona Borealis. Beta (β) Serpentis lies at the southwest corner of the triangle that outlines the snake’s head, Delta (δ) lies to the south of it, and Kappa (κ) Herculis can be see east of Beta (β). (Stellarium screen image with labels added, click for a larger view)

Delta (δ) Serpentis    (Σ 1954)   (AB is H I 42)     HIP: 76276   SAO: 101624
RA: 15h 34.8m   Dec: +10° 32′
Magnitudes   AB: 4.2, 5.2    AC: 4.2, 13.9
Separation    AB: 4.0″           AC: 67.7″
Position Angle   AB: 172°   (WDS 2010)     AC: 17°  (WDS 2000)
Distance:  210 Light Years
Spectral Type: F0, F0

We’ll start with Delta (δ) Serpentis, which Haas describes as a “showpiece” in a 60mm scope — and that it is.  The 4.2 magnitude primary is a bright white dot with a slight yellow tinge to it, and the secondary is a fainter version of the same color nestled up tightly against it.  I put an old 26mm Celestron Plössl (35x) in my 60mm f/15 refractor, pointed the scope up toward the snake’s neck, and quickly found myself staring at a spectacular pair of stars unequal in size that were almost touching one another.  Another old Celestron Plössl, a 17mm version (53x), put some distance between them, and a 12.5mm UO Ortho (72x) improved on that view considerably.  Since the snake seemed content to lie still, I spent some time admiring the view, switching back and forth between 35x and 72x.

The primary is “a bright white dot with a slight yellow tinge . . . and the secondary is a fainter version of the same color . . . “.   East and west reversed to match the view in a refractor, click to lose this caption.   And one tip — all of these sketches do much better when the lights are turned out!

These two stars were much closer together when Sir William Herschel discovered they were a pair on September 3rd, 1782, and cataloged them as H I 42.  He measured their separation back then at 2.5″, and recorded a position angle of 227 degrees.  Delta “B” forms a classic oval orbit around Delta “A,” with a lengthy period of 1038 years.  A diagram of that oval circuit can be found here (scroll part way down the screen).

While you can do just fine on this pair of stars with a 60mm refractor, they really come alive with a bit more aperture.  I spent two nights on Delta (δ) recently, one with a four inch refractor, and the other with an eight inch SCT, both of which really bring the subtle yellow hue out of both stars.  I had high hopes of spotting the 13.9 magnitude “C” companion in the eight inch SCT, but couldn’t quite penetrate the inky blackness far enough to catch a glimpse of it — should be possible, though, under conditions of better transparency.

The 210 light year distance of this pair of stars leaves an observer without any true sense of their brightness in comparison to our sun.  The 4.2 magnitude primary is seventy-one times brighter than the sun, while the 5.2 magnitude secondary is twenty-six times brighter.  According to Jim Kaler’s information, both are about eight hundred million years old and are on the verge of ballooning into giant stars.  So keep an eye on them – they might change tomorrow.  But then again, it could take a few more thousand years, too.

Beta (β) Serpentis     (Σ 1970)   (AB is H IV 36)      HIP: 77233   SAO: 101725
RA: 15h 46.2m   Dec: +15° 25′
Magnitudes    AB: 3.7, 10.0       AC: 3.7, 10.7      AD: 3.7, 8.2  (SHY 281)
Separation     AB: 31.1″              AC: 200″             AD: 999.9″
Position Angle: AB: 264° (WDS 2009)   AC: 212° (WDS 2009)    AD: 254° (WDS 1998)
Distance: 153 Light Years
Spectral Type   A: A2      B:K3      C:K      D:G0

Snaking our way up to the base of the head, which is the next bright star up from Delta (δ), we reach Beta (β) Serpentis.  Using the 60mm scope, and having forgotten the magnitudes of this pair, all I could see was a single star.  Since I had the 60mm scope attached to a 105mm f/15 refractor, I put an 18mm Radian (83x) in its diagonal, and after looking closely, found the faint tenth magnitude secondary winking away off to the west of the primary.  Increasing the magnification slightly with a 16mm Astro Tech Flat Field eyepiece (94x) coaxed the color out just a bit more from the primary –- a gratifying white sight with a slight inclination toward a yellow hue.  But the color of the tenth magnitude companion was out of the reach of both the eyepiece and my eyes.

” . . . a gratifying white sight with a slight inclination toward a yellow hue.” (East & west reversed again, click to lose the caption)

Now that I knew where to look, I gave the 60mm another try with the 16mm (56x) eyepiece in it.  Looking directly at the location to the west of the primary didn’t reveal anything, but I found by averting my vision to the right and just below it, the faint companion would pop into view –  but just barely.  It seemed to be teasing me by suddenly emerging into view and then disappearing again.  Every time it popped into view, I had to fight the urge to shift my vision directly to it – which of course only made it disappear once more.  Increasing the magnification only made the glare worse, so I spent about ten minutes working on my averted vision technique –- I’ll give myself a B, for barely perceptible.

When I had been peering through the larger scope, I also saw another star of a similar magnitude farther away from the primary and a bit south of the secondary.  It tends to catch your eye (once you see it), because it forms a rather extended isosceles triangle with the other two stars.  My curiosity aroused, I took a stroll through the Washington Double Star Catalog and found what I was looking for:  a 10.7 magnitude “C” component at a distance of 200 arcseconds with a position angle of 212 degree, which matched up well with what I saw.  Again, it was just a bit too faint to detect any color.

While I was looking at the WDS database, I also discovered that the “D” component, SHY 281, is cataloged separately as ROE 75, with magnitudes of 8.2 and 10.7, a separation of 6.2″, and a position angle of 327 degrees (as of 2006).  I didn’t see the secondary, but then I really wasn’t looking all that hard at that star since it was over at the southwest edge of the eyepiece.  It’s certainly worth a second look with a five or six inch scope.

Sir William was here also, on August 13th, 1781, about a year earlier than when he viewed Delta (δ).  He cataloged the AB pair (apparently he didn’t notice “C”) as H IV 36, and again, found it was closer than it is now, measuring the separation at 24″ and the position angle at 267 degrees — and both numbers are little different from the current figure.

Kappa (κ) Herculis (Marsic)    (Σ 2010)  (AB is H V 8)     HIP: 79043   SAO: 101951
RA: 16h 08.1m   Dec: +17° 03′
Magnitudes   AB:  5.1, 6.2    AC: 5.1, 13.4
Separation    AB:  27″            AC: 62.8″
Position Angle    AB: 13°   (WDS 2010)    AC: 212°   (WDS 1998)
Distance:  A – 388 Light Years   B – 470 ?  (See below)
Spectral Type: G7, K1, K

Immediately east and north of the triangle which marks the serpents head is Kappa (κ) Herculis, also known by it’s Arabic name, Marsic, meaning elbow.   Which just happens to fit, since Hercules is actually doing a head stand up there in the sky — head pointed south, feet waving around up north, left elbow resting in the west.  That fact aside, you’ll find this is an easy pair, especially after the disappearing act we just experienced with the previous pair.

Stellarium screen image with labels added, click to enlarge the view.

Now before you even turn a telescope on Kappa (κ), take a look in your finder at this area.  As the chart at the right shows, Kappa (κ) is surrounded by several stars similar in brightness to it.  You’ll see its shadowed by 8 Herculis, a 6.1 magnitude star which at one time was considered a possible component of Kappa “A” due to the fact that it’s at a similar distance.  Current figures place 8 Herculis and Kappa “A” 1.3 light years apart, though, a considerable distance for a gravitational effect of any strength to exist, and measurements show their angular movement across the sky to be considerably different as well.  At any rate, they make a good test of your vision under dark skies, with a visual separation of two tenths of a degree — easy enough for brighter stars, but rather difficult for a pair this dim.  Southeast of these two is another 6.1 magnitude star, q Herculis.  And flanking it on the northwest side is 5.1 magnitude 5 Herculis.

” . . . another eye-pleaser that is well suited to a 60mm scope . . . grapefruit-orange and white-scarlet . . . “, click on the image to lose this caption. (East & west reversed again!).

This is another eye-pleaser that is well suited to a 60mm scope, and also described by Haas as a “showpiece.”  She describes the colors as “grapefruit-orange” and “white-scarlet,” which is actually a very good description of what I saw.  These two take low power very well because of the wide distance between them.  I found the view with the old 26mm Celestron Plössl (35x) was perfect, and spent a good fifteen minutes or so admiring it and listening to the Pacific ocean purring away in the background, half a mile away, lulling the snake to sleep and saving Hercules’ elbow from a snake bite night.

With a bit more aperture, such as an eight inch SCT, that elusive little 13.4 magnitude “C” companion can be pried out of the darkness rather easily.  It’s shadowed closely by another pair of faint stars out beyond it, as shown in the inset at the bottom right of the sketch.  I could see both of those in the 60mm refractor, and at first I wondered if the fainter of the two was the one I was after.  But an alarm went off in my head about that time, shattering the silence, and I realized a 13.4 magnitude star in a 60mm refractor just wasn’t going to happen — especially under my sea level skies, and certainly not at 35x.  I suspected the real “C” was hiding nearby, though, and sure enough, the 140 millimeters of additional aperture provided by the 200mm SCT brought it out of it’s hiding place — but just barely.  I had to avert my vision to find it, and then I succeeded in catching occasional glimpses of it with direct vision.  I returned later with a five inch f/15 refractor and was able to see “C” with averted vision in a 20mm TV Plössl at 95x.

There is some question as to whether Kappa “A” and “B” are gravitationally linked doubles or just happen to look like a matched pair by virtue of their position in the sky relative to us.  Kappa (κ) Herc has been observed for the past three hundred years, and during that time, the separation between the two stars has diminished from fifty-seven arc seconds to the present twenty-seven, which would be remarkable for a pair of gravitationally linked stars.  Then there is the large apparent difference in distances of these two stars — 388 light years for “A” and 470 for “B” — an indication if there ever was one that they’re optical, not gravitational, doubles.   But as Kaler points out, the distance to the fainter component is not absolutely certain.  Surprisingly, “C” does appear to be linked gravitationally to the primary, although Kaler puts the orbital period at 340,000 years, and describes “C” as being about as bright as Saturn from Kappa “A.”

And if you read through Kaler’s comments on Kappa Herc, you’ll also find that there’s a lot of confusion about that Arabic name, Marsic.  There are two stars named Marfak (a name also used in reference to Marsic) in Cassiopeia and then there’s Marfik in Ophiuchus.   On top of all that, the WDS database refers to Marsic as Marfik, which I do believe is an error.

Hmmmm  —–  more stellar puzzles to puzzle over in your spare stellar time.

And then,  just as I was about to become lost in stellar reflections on light years and questions of gravitational interaction ………………………

I suddenly found myself being pulled back to terrestrial reality by a sudden tug at my foot.  Fortunately it wasn’t Serpens the snake, it was Herr Klaus the dog, my trusted four-legged observing buddy and part-time snake lookout, reminding me that I still had a dog bone in my pocket and it was time to set it free.

And I know as he was savoring every last morsel of it, he was wishing Clear Skies to all of us human types.


One Response

  1. I hadn’t been back to the three stars in this post since I originally wrote it two years ago, so I decided about a month ago to return and spend more time here.

    And what I found was that all three of them are truly spectacular — they really deserve more attention than they get since they do as well in a 60mm refractor as in a large telescope.

    One of the highlights came a couple of nights ago when I was looking at Delta (δ) Serpentis through my five inch f/15 D&G refractor. I was using a 24mm Brandon, which gave me 79x, and the stars were simply stunning — gleaming white globes with bare hints of yellow in them — and the whole field of view looked as if it was on fire because of all the light.

    I also had a 60mm f/16.7 refractor mounted on top of the D&G, so I took a peek in the 20mm TV Plössl (50x), and saw two glowing white dots of light barely separated by a black slice of sky about the width of the edge of a piece of paper.

    The contrast between those two views was just absolutely stunning — a wealth of white photons illuminating the eyepiece in the five inch refractor, and two pin hole sharp dots of light sitting very calmly at the center of the field in the 60mm scope. Truly one of those sights that stays with you the next morning.

    At any rate, I’ve totally re-written the entire post, and added sketches for each of the stars. Take a look — or better yet — grab a telescope and go see for yourself. You won’t be one bit disappointed!

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