The short answer lies somewhere between “Why Not?” and “Because they’re there!” But the long answer is much more intriguing.
My first reasonably serious attempt at double stars was with an eleven inch SCT on a full moon night. Since deep sky objects were washed out by the bright moonlight – and I had frequently read that double stars are a good object for those nights – I decided to hunt down a few doubles and see what I could see. I particularly remember looking at Izar – nice reddish-orange color, but only one star! I looked at several others that night, but eventually came to the conclusion that anything less than two arc seconds apart was not going to split into two stars – at least not that night. I tried for Epsilon Lyrae, the “Double-Double,” numerous times with that scope and never could split either of the two primary stars.
Eventually I found myself with a four inch refractor, and much to my surprise, I was able to split Epsilon Lyrae with no problem. And also Izar, as well as numerous others that I had attempted and failed while using the eleven inch SCT. I also discovered the beauty of refractor images – pinpoint stars sharply in focus against a very black sky.
Now, after much more experience, I can look back and see that my numerous failures were partly due to poor seeing, partly a result of collimation that was off a bit, and partly a result of the larger, fuzzier image in the eleven inch scope – and a large portion of inexperience. But the best was yet to come.
A few years after that, out of curiousity about what it could do, I bought an old 60mm Tasco refractor with a 1000mm focal length, and shortly after that, an older model 80mm refractor with a 1200 mm focal length. I believe the first double I spent any time on with the 60mm scope was Castor. And I found it to be a thing of beauty – two white globes that almost touch at low magnifications and are clearly separate and surrounded by diffraction rings at higher magnifications. Next I “discovered” Algieba – also known as Gamma Leonis – a bright orange star with a very small companion hugging it – magnificent in both scopes. And then one night as I was looking at one of my favorite clusters of stars located in the top of Orion, Collinder 69, I realized that the brightest star in the cluster, Meissa – also known as Lambda Orionis – was a double, very similar in appearance to Algieba. I had looked at that cluster many times, usually with the four inch refractor, but never had I noticed that Meissa was two stars – and that night I saw it with the 80mm refractor. Experience was accumulating and paying off!
Now there are two things about doubles that will eventually hook you if you keep at it. One is the sheer challenge of splitting the more difficult stars – try Iota Leonis or Porrima (Gamma Virginis). And the other – the thing that keeps me coming back – is the sheer aesthetic beauty of these things. I have literally sat staring at some doubles for as long as thirty minutes.
The ones I find most appealing are those like Algieba, Izar, and Meissa, which consist of a large, bright primary with a much smaller secondary nestled up against it. That, plus the orange, red, yellow, and green colors they display, have a quality that I find to be totally unlike anything else in the sky. Next are the doubles which feature two stars of similar size that are right up against each other with just a bit of black sky between them. Castor stands out in this category, as does Porrima – once you’re able to split it. There are other attractions – triple stars (next time you’re attempting Porrima, wander over to Theta, halfway to Spica to the east), impossible stars (Iota Leonis comes very close to falling into this caterory), unique pairs (try Nu Draconis, “The Eyes”), doubles known for their bright colors (Albireo of course), doubles that you can separate with the naked eye (Mizar is well known, but try Theta Tauri sometime, just to the west of Aldebaran), and then of course there’s Epsilon Lyrae, which is in a class by itself (although there is another “Double-Double” in Lyra, Σ 2470 and Σ 2474 on the east side of the constellation).
But there are two tools that really enhance the aesthetic quality of double stars – and that is the right scope, and the right magnification. I’m restricting the “right scope” to a refractor, not because doubles can’t be seen in a reflector or an SCT (I still find that Polaris is much more satisfying in a large SCT than a 100mm refractor), but because the unobstructed view of a refractor provides a different quality of view – stars that snap to focus and are sharp as a pin, which is not only essential for splitting the close ones, but adds considerably to the beauty of the image.
The “right scope” also includes varying sizes of scopes. I’ve found that in many cases, a small long focus refractor – 60 to 80mm – provides a much more pleasing view of a double star than what is seen in a larger instrument. That is especially the case for Algieba. Izar, on the other hand, while it can be split in a 60mm scope on a night of good seeing, is much better in a 90mm to 150mm refractor.
Using the “right” magnification also makes a huge difference. I find that I prefer to restrict the magnification to the point where I can just get a clear separation of a pair of stars with a very slight bit of black sky between them, especially on the more colorful stars. On others, such as Castor, with it’s similar sizes and identical colors, I prefer to increase the magnification until they are well separated and surrounded by those beautiful, shimmering diffraction rings. Castor, by the way, is a triple star – the faint tenth magnitude companion lies 71″ off to one side, but the two bright components are what grabs your attention first.
So double stars are not just two stars close to each other in the night sky. Like anything else, if you persist in searching them out, and spend time on them, you learn more about them; you learn what works best, what looks best, and most of all, you learn that many of them radiate a beauty that is quite unlike anything else in the night sky — not better than a galaxy or an open cluster or globular cluster or a planetary nebula — but different. It’s a subtle thing, not immediately recognizable at the beginning, but something that grows slowly until you find yourself hooked for life.
If you have never spent some serious time with double stars, but are intrigued, my recommendation is to start with a medium sized — say an 80 or 90mm — achromatic refractor with a focal length of a least 900mm. Orion, Meade, and Celstron all sell scopes of this size. You don’t need to spend a fortune on an APO because you’re not going to be looking at first and second magnitude stars, although Rigel is definitely worth spending some time with. In fact, some of the best views I’ve had of both double stars and planets have been in a 90mm Orion scope that I paid ninety dollars for. And use a decent quality eyepiece — I recommend Plössls, which are relatively inexpensive and provide very decent views, although there’s certainly nothing wrong with the more expensive ones if you can work it into your budget. As you get more experience on double stars, though, many people gravitate toward the more narrow fields of view.
But the important thing is, if you don’t find it particularly appealing, let it rest for a few months, and then try again. If you keep at it, and you enjoy the night sky, you’ll eventually find you can’t resist!
I recently came across the following quote by S. W. Burnham, who cataloged several thousand double stars in the late 1800’s and into the early 1900’s. It does nothing to explain the mystery of why some people are attracted to double stars, and others aren’t. In fact, it’s worded in such a way that it only adds to that mystery. But it’s a great quote from one of the greatest double star observers of all time.
As background, he started observing with a 3.75″ refractor, but found both the mount and the aperture didn’t quite meet his needs. In 1869, he met Alvan Clark in Chicago, and after a conversation with him, ordered a six inch refractor on an equatorial mount. I’ll let the honarable Mr. Burnham take it from there:
I told them what I wanted, and what I wanted it for. Every detail was left entirely to their judgement, stipulating only that its definition should be as perfect as they could make it, and that it should do on double stars all that it was possible for any instrument of that aperture to do. In due course of time this instrument was delivered, and was set up in an observatory prepared for it in the meantime. My attention for some reason or other, which I am unable to explain, had been almost exclusively directed to double stars previous to this while using the smaller telescope referred to. This preference was not in any sense a matter of judgment as to the most desirable or profitable department of astronomical work, or the result of any special deliberation upon the subject. It came about naturally, without any effort or direction upon my part.”
A General Catalog of 1290 Double Stars Discovered from 1871 to 1899 by S.W. Burnham, University of Chicago Press, 1900: Chicago, p. viii
As I said, there really is no explaining the attraction, and Burnham seemed to realize that in the last half of his statement. At any rate, he put the six inch to work quickly, and during the many years he used it, he discovered over 1450 double stars. You can view a photograph of the six inch refractor on the un-numbered page following p. xiv of the book in the link above.
Here’s to Clear Skies wherever you are! 😎