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Rasalgethi (Alpha [α] Herculi) and Σ I 33, a Unique Quadruple System

Rasalgethi  is truly one of the gems of the skies.  It’s spectacular in a 60mm scope, it’s stupendous in an 80mm scope, it’s marvelous in a 90mm scope, it’s eye-poppingly fabulous in a four or five inch scope, it’s downright techni-color-ingly dazzling in a six inch scope  . . . .  and  . . . .  well, I suspect you get the point.

Rasalgethi   (Σ 2140)  (AB is H II 2)             HIP: 84345    SAO: 102681
RA: 17h 14.6m    Dec: +14° 23′
Magnitudes   AB:  3.5, 5.4    AC: 3.5, 15.5    AD: 3.5, 11.1
Separation    AB:  4.6″           AC: 19.9″          AD: 79.2″
Position Angle   AB: 103° (WDS 2013)    AC: 297°  (WDS 1995)    AD: 39°  (WDS 2002)
Distance:   382.4 Light Years
Spectral Classification:  Primary is M5;  the secondary is actually two stars, G5 and F2

I’ve looked at it more times than I can count, and I’ll look at it as long as it’s there:

Rasalgethi "A" and "B" are shown at the center of the sketch as seen in a 60mm scope with a .965 inch eyepiece. The elusive "D" component is included in the inset at the lower right. East and west reversed to match the refractor views. Turn out the lights for the full effect! (Click to enlarge the sketch)

Rasalgethi “A” and “B” are shown at the center of the sketch as seen in a 60mm scope with a .965 inch eyepiece. The elusive “D” component is included in the inset at the lower right. East and west reversed to match the refractor views. Turn out the lights for the full effect! (Click to enlarge the sketch)

The primary is a very pleasant orange color, sometimes with a tint of red to it, depending on atmospheric conditions, but the secondary is a bit more difficult to pin down.  I’ve seen it as green and blue, and Hass decribes it as bluish-turquoise.  But regardless of what colors you see, it’s truly remarkable.  I was using a 60mm scope a week or so ago and I could just see the beginning of a split at 42x and had a clean split at 56x.  So it isn’t difficult to split, and it’s at its best when you hold the magnification at the point at which they just become separated.  The secondary has it’s own companion as well at  a distance of four tenths of an Astronomical Unit (.4 of the distance between the sun and earth), just a slight bit too close to split with any scope in my inventory.

Less known are the two faint “C” and “D” companions.  Good luck prying the 15.5 magnitude “C” component out of the glare, but if you look very closely, “D” is easy to see in a four or five inch refractor.  Actually, I had no idea it existed until almost a year after I wrote this post.  You can read the delightful details of my discovery in the comment attached below.

Rasalgethi with Rasalhague to its southeast. Click on the image for a larger view. (Screen image from Stellarium with labels added)

Rasalgethi with Rasalhague to its southeast. Click on the image for a larger view. (Screen image from Stellarium with labels added)

Now if you haven’t hunted this one down, it’s easy to make the mistake of pointing your scope at the wrong star.  If you look at the chart above, you’ll see a star labeled as Rasalhague, also known as Alpha (α) Ophiuchi.  Whoever named these two stars must have known it would confuse people forever.    In Arabic, Rasalhague means “the head of the serpent collector,” which is a reference to the nearby constellation of Serpens.  Rasalgethi refers to the kneeler’s head, which comes from the classic depiction of  Hercules as shown here.  Note that poor old Hercules is depicted in an upside down kneeling position (ouch!), and his right eye, Rasalgethi, is staring at Rasalhague.  Maybe that image will help to wrestle these two stars into their proper places.

And there’s one more significant quirk.  Alpha (α), the first letter of the Greek alphabet, is normally used to designate the brightest star in a constellation.  But in this case, Rasalgethi is the fifth brightest star in Hercules.  Why or how that took place, I don’t know, but designating it as Alpha is appropriate to its size.  It is a huge star by the standards of our sun.  With a radius of 1.9 Astronomical Units, if it was at the center of our solar system it would extend past Mars into the first part of the asteroid belt.  It also changes in brightness by as much as a magnitude over a varying period of time, as short as ninety days and as long as a year.  That means when at its brightest, it becomes a bit more difficult to split.

Σ I 33 (STFA 33)                     HIP: 83478    SAO: 102564
RA: 17h 03.7m   Dec: +13° 36′
*****             Magnitudes       Separation      Position Angle      WDS
STFA 33    AB:   5.9,   6.2           306.1″                  117°                2012
ARN  15    AC:   5.9,   8.5           231.1″                    11°                2012
SFTA 33    AD:   5.9, 10.4           177.8″                 138°                2012
SFTA 33    BD:   6.2, 10.4           154.1″                 272°                2003
SFTA 33    DE: 10.4, 10.95          98.2″                  208°               2010
Distances for “A” and “B”: 263.5 and 337.6 Light Years
Spectral Classification   A: A1    B: K1    C: G0

Three degrees to the west and slightly south is the fascinating multiple star Σ I 33.  The strange name refers to the first supplement to Struve’s 1827 Dorpat Catalog of double stars, which carries a prefix of STFA in the Washington Double Star Catalog (WDS) …… except that I have another name for this area, which we’ll get to shortly.

Stellarium image with labels added, click for a larger view.

This one is easy to find.  Most finders mounted on telescopes have an optical field of view of at least five degrees, so position Rasalgethi in the center of your finder, then move your scope two degrees to the west and a bit more than a degree  to the south, and it will come into view as a pair of stars between two brighter stars, very similar to what is shown in the chart above.

STScI photo with labels added. East and west are reversed here to match the refractor view - click once for a larger view.

STScI photo with labels added. East and west are reversed here to match the refractor view – click once for a larger view.

I first came across this a few years ago by accident as I was scanning the area with a 102mm refractor.  What caught my eye was the triangular shaped asterism formed by the three brightest stars shown in the photo at the left.  When I increased the magnification, the smaller and fainter triangle shown in the photo (the stars labeled “D” and “E” and the one of similar magnitude just below “A”) gradually became obvious.

I repeated that experience with a 60mm refractor on the same night I was looking at Rasalgethi.  The three stars of the larger triangle were easy to see at 26x, and I could almost pick out all three stars of the inner triangle at 32x.  They were much more defined at 42x, and I worked my way up through 49x, 56x, and 76x – and then the clouds put me out of business, something they’ve been doing all summer.

This really springs to life, though, with more aperture.  In a five or six inch refractor, the “triangle-within-a-triangle” asterism really jumps out at you.  It’s particularly eye-catching in a larger scope, such as an 8, 9.25 or 11 inch SCT.

The night I came across this system, I looked through several star atlases to see if it had been named by anyone.  Other than Struve’s double star designation, I came up with nothing.  My favorite four-legged observing compatriot was lying calmly beside the tripod, and in his own humble way, he suggested I name it after him.  So, “Klaus’s Asterism” it is.

Picking out the individual components of this system is not too difficult.  They’re widely separated, and the faintest, “E,” is not that tough to pull out of the glare of the other stars when using a 60mm scope.  When I first posted information on this star in 2010, a couple of the stars were labeled as “Bb” and “bc” with most recent statistics of 1923 and 1919.  Since that time, the stars have been re-labeled from “A” to “E” with new measurements added.  So if by some chance you were using the old designations and statistics, it’s time to update them!

Meantime, hope your skies are clearer than the ones I’m under here.   😎


One Response

  1. It’s a brisk fifty degrees at 2AM on the morning of July 4th (2011), and all the firecracker fiends have long since lit their last fuse — for tonight at least. The only sound keeping me and my telescopes company is the rhythmic surging of the ocean half a mile in the distance.

    I’ve just pointed my f9.3 127mm AR-5/f13.3 60mm combination at Rasalgethi with intentions of making a quick sketch to add to the post above. But first, I intend to spend a good twenty or thirty minutes looking closely at it with different magnifications in both scopes.

    In the 60mm, using a .965 inch diagonal and eyepieces, it’s a pleasure to run through the various magnifications. I start with an H 20mm (40x), move up to an HM 12.5mm (64x), then to an HM 9mm (89x), make another leap upward to an HM 6mm (133x), and finally finish with an H 5mm (160x). As it gets dimmer with each step upward in magnification, the separation widens just a bit in compensation.

    Now, over to the AR-5. Five inches of aperture collects many times more light than a 60mm lens can capture, so even a casual glance into the eyepiece is stunning.

    The reddish-orange primary is vivaciously vibrant in all its technicolor glory. But the secondary is wearing a different color tonight — it seems to be stealing some of those reddish-orange photons. I’ve seen it as both greenish and bluish, but certainly not now. Even in the 60mm, the secondary had the same tint I see now in the AR-5. There’s a lot of dampness in the air, plus Rasalgethi is perched over the ocean now as I look at it. That puts a bit more moisture between it and my scopes, possibly causing enough refraction to affect the color.

    In an 18mm Radian (66x), the secondary is just barely distinct from the primary. I add some distance with a 14mm Radian (84x), then a 12mm Radian (98x) — and then I see a never-before-seen faint glimmer of light at the north edge of the primary. It comes and goes in the primary’s glare with averted vision, so I swap a 10mm Radian (118x) into the diagonal and find it’s at the extreme limit of direct vision.

    Confirmation time! I grab my secret weapon, the 7.5mm Celestron Super Plössl, and take a look to see what it looks like at 157x in a five inch refractor. It’s just barely there on the other side of the glare, but nevertheless, it’s distinct. It’s like looking at a dimly lit object on the other side of a campfire — look close and you can see it, look into the fire and then look up, and it’s gone.

    I have a hunch that I’m looking at a third Rasalgethi-an component, so I add it to the sketch and make a mental note to check the Washington Double Star Catalog in the morning. And, sure enough, it is. But actually, it’s the “D,” as in fourth, component. The “C” component, which I didn’t see, is four times closer to the primary than “D,” and it glimmers in the gloaming at a meager magnitude of 15.5. Well shucks, no wonder I missed it.

    Now as I wrote at the beginning of the post above, I’ve given up counting the number of times I’ve wrestled Rasalgethi into an eyepiece – it’s at least fifty. And NEVER did I see that 11.1 magnitude point of light trying to shine through the glare of the primary. I’m sure it was calling out to me all that time, but I sure didn’t hear it, much less see it.

    And that, Star Splitting fans, is one of the many aspects of this obsession that makes it so satisfying. No matter how many times you look at a multiple star, you’ll never see all there is to see.

    Added comment to the comment:

    I discovered that I can see the faint 11.1 “D” component in a TV85 at 86x (7mm UO Ortho) and 100x (Celestron Ortho). It takes some persistent peering to get it, but after I got it with some careful use of averted vision, it was actually possible to see it with direct vision. I lost it at 67x in a 9mm UO Ortho, though, so seven seems to be the minimum number of millimeters needed.

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