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:
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.
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.
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.
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. 😎