Struve 2398

     For the first time in our by now several-months-long tour of the Solar Neighborhood, we come across an object that, as well as being a close neighbor to the Sun, is a thing of great beauty. Finally, we can dispense with the bewildered shakes of the head by our companions at a star party, wondering why we are wasting precious dark sky time by “looking for dots”. For Struve 2398 is beautiful!

     I have long been a huge fan of double stars. Indeed, the very first deep sky object that I recall ever seeing through a telescope was the venerable double star Albireo in Cygnus. My friend Craig Covault (of Aviation Week and Space Technology fame) had just been given one of the cheaper varieties of “department store” telescopes (a 3-inch refractor, if I remember correctly) as a birthday present by his wife Nancy. Her heart was in the right place, not knowing any better. And as a matter of fact, neither did we. In blissful ignorance of the instrument’s grave shortcomings, we set it up as soon as it got dark in the front yard of their Vienna, Virginia, home. (This was in 1981, so the light pollution there was nowhere near as bad as today.) Craig knew enough to look for Albireo (I had never heard of it), and focused in on the star. When it came my turn to look through the eyepiece, I quite literally gasped. I was totally unprepared – not only for the wonder of the sight, but for the sheer beauty of it. The electric blue and warm gold of the star’s two components were like nothing I had ever seen; I could scarcely accept their reality. I was hooked for life.

     To this day, I make a point of including a fair amount of double star viewing nearly every time I go out to observe. A night at the eyepiece just doesn’t seem complete without them. I have far too many favorites to list, but they include Albireo (still!), Gamma Andromedae, Iota Cancri, Rigel, the Double-Double in Lyra (of course), and many, many others… and Struve 2398.

     If fact, the first time I laid eyes on the star, I wasn’t thinking about the Solar Neighborhood at all, but rather looking for new doubles to add to my observing list. After some minimally challenging star hopping, when I finally espied the object of my search, there was no mistaking it – a gorgeous pair of easily-splittable ruby red points of light, separated by slightly less than one magnitude of brightness, in a brilliant setting of surrounding stars of purest white. Their very dimness only added to their charm. It was an “Albireo Moment” all over again.

 

Pulkovo Observatory, as it looked in the year of its founding

     Struve 2398 is of course the 2398th entry in the magisterial catalog of 2714 double stars compiled by Danish/German/Russian astronomer Friedrich Georg Wilhelm von Struve (Vasily Yakovlevich Struve) and published in 1837 under the title Stellarum Duplicum et Multiplicum. Believe it or not, this was his second catalog of such objects, having put out ten years previously his Catalogus Novus Stellarum Duplicium. Friedrich (or Vasily, if you prefer) was prolific in other ways as well. A glance at his family tree makes one either jealous or concerned about one’s own progeny. Included among his brothers and immediate descendants are no less than seven astronomers, two mathematicians, six prominent political figures, and one each chemist, anatomist, philologist, historian, and poet! He proposed the building of a great observatory at Pulkovo, outside Saint Petersburg, Russia, and in 1839 was chosen by Tsar Nicholas I to be its first director. Friedrich was succeeded by his son, Otto Wilhelm von Struve, in 1862, who continued his father’s obsession with double stars, adding more than 500 to the Struve catalog (as well as discovering two satellites about Uranus, one about Neptune, and the innermost ring of Saturn. Struve’s great-grandson (also named Otto) emigrated to the United States, and served as director of the Yerkes, McDonald, Leuschner, and Green Bank Observatories (one at a time, of course). Asteroid 768 Struveana is named in honor of the entire family.

     One intriguing feature of Struve 2398, invisible to the terrestrial observer, is the manner in which the two components orbit each other. The two stars A and B are so close to each other in terms of mass (0.36 and 0.30 solar masses, respectively) that neither one orbits the other, in the sense that one is a satellite of the other. Rather, they mutually move about a common center of gravity (see above Figure) in highly eccentric elliptical orbits. The two stars can approach each other as closely as 26 astronomical Units (AU), or be as far apart as 86 AU. Interestingly, there is no consensus amongst astronomers on the most basic elements of their mutual orbits. In 1958, Wilhelm F. Rabe estimated their orbital period at 346 years, with an average separation of 42 AU and an eccentricity of 0.55. But Wulff Dieter Heintz determined the period to be 408 years, with an average separation of 56 AU. Other period estimates range from 295 to 453 years. 1n 1946, French amateur astronomer Paul Baize determined the orbital eccentricity to be 0.70 (a figure often cited to this day). The exact figures remain unknown.

Other than their intrinsic beauty and intriguing orbital arrangement, the two components of Struve 2398 are otherwise nondescript red dwarfs, similar in every respect to the other six we have thus far observed on our tour. With respect to the solar system, Struve 2398 is practically standing still, slowing moving away from us at a leisurely half mile per second.

     Oddly enough, my preferred path to Struve 2398 is to start all the way from faraway Vega in the constellation Lyra. This superbright near-neighbor of the Sun (only 25 ly distance) just happens to be almost due south of our goal. So by following an imaginary line from Vega to the North Star, you will pass not far to the right of Struve 2398, at about the halfway point. To further get one’s bearings, it is useful to take note of the “Head of the Dragon”, formed by Beta, Gamma, Nu, and Xi Draconis. I then use the brighter stars of that grouping, Beta and Gamma, to lead me to a second pair of somewhat dimmer stars, 45 and 39 Draconis (magnitude 4.77 and 4.99, respectively), above and to the left of them. Continuing northwest from this pair, it should take little effort to identify the 4.62 magnitude star Omicron Draconis. This giant sun, despite its dim appearance in our skies, shines with a luminosity equal to 269 suns! It owes its relative faintness to a distance of 323 ly. (Omicron Draconis could serve as the North Star to any wouldbe observer on Mercury, as it sits squarely on the axis of that planet’s rotation.)

     But this is where we reach the limit of what we can do by naked eye alone. From here on out, we’re at the mercy of our telescopes. Omicron (a.k.a. 47) Draconis is conveniently near to Struve 2398, just off to its west. Figure 24 illustrates the general approach to the final search area, to the immediate north of a quadrilateral formed by 39,  45, 48, and 47 (Omicron) Draconis. Halfway between 47 Draconis and our target we find an attractive grouping of five otherwise undistinguished stars ranging from magnitude 7.2 to 9.6, forming a rather lopsided V. Struve 2398 itself is the brightest star to be found between this asterism and a 7th magnitude further to the east. The two components’ magnitudes of 8.9 and 9.69 combine to make it an easy find in my 5-inch refractor using a 10mm Ethos eyepiece. Unlike some of our previous targets, there is no mistaking this one.

      Now, once you've found this gem, don't rush off to the next item on your list! Take the time to appreciate the sheer beauty of this sight, and think about just what it is you are looking at.