Caoimhín's Content

This video is again more on a topic, but it does refer to the state of the sky and positions of objects as they are at the time of the video, late November. Again however, we won’t be looking much at what’s in the sky generally here, we’ll be focused on pairs of stars. Some stars orbit around each other, these are binary stars or binary star systems, while other stars appear to be next to each other in the sky without necessarily being bound together by gravity, these are double stars.

The difference is reasonably simple, but it can be more complicated in practice. Some binary stars are visual binaries, they appear as two stars, usually through a telescope. Some double stars require a telescope to be seen separately, even if they are really quite far apart in space. Add to this the star systems with more than two stars and things can get even more complicated. Some binary stars are only known to be binary thanks to changes in their light spectrum, these are spectroscopic binaries. In the end, some pairs of stars can look like one and turn out to be several under closer inspection, while some stars that appear to be together are not.

We begin with Algol, the eye of the Gorgon Medusa, which featured in a recent video and post on this website. Algol is a binary star, and eclipsing binary. Due to the two stars moving around each other, Algol appears to change brightness every few days, giving it the effect of winking. This may be what earned it its name, the Demon’s Eye or Demon Eye. Unfortunately, even as we zoom in, it still look like one star. Algol is a spectroscopic binary, we don’t usually get to see the two stars separately with our eyes. In fact, it’s more than just a binary system, there seems to be three stars involved here, making this a ternary system instead.

A more visual example is Albireo in the middle of the Summer Triangle. Such a prominent position makes Albireo easy to find, as long as that part of the Triangle is in the sky. It certainly only looks like one star to the naked eye, but as soon as you take a closer look it will be transformed into two. A telescope is recommended but if you have a powerful enough set of binoculars they might do the trick. Albireo is a particularly great example due to the size and colour difference of the stars, with a little blue companion next to a much larger yellow-orange star. Notice that I said “next to” rather than orbiting. It looks like Albireo isn’t an actual binary pair, just a double star.

A wonderful group of stars lie in the handle of the Big Dipper or the Plough, about where the handle bends after leaving the “body” or “pot”. Mizar and Alcor are one of the most famous pairs of stars, also know as the Horse and the Jockey. There is a very good chance that you’ll be able to see these stars as a separate but close pair with just your eyes. This pair seem to be right on the edge of resolution by the human eye, some people will see both, others will see them blurred together as a single blob. It has been recorded that these stars were used as an eyesight test in the past. Some records in Arabic reference these stars being used to gauge vision. In some cases it seems that the star was used to test in a person had eyesight good enough to be a soldier or archer, in other cases, potentially Japanese, it was used to see if you eyes were beginning to fail with age.

However, Mizar and Alcor are both pretty easy to see, it doesn’t seem like a very hard test. What is hard is spotting the third star. Almost between Mizar and Alcor but a little to the side is a much fainter, more difficult to see star, known as Ludwig’s Star. This star is usually visible to the naked eye in good conditions, but is much trickier to see. Some theories state that it was this star that the ancient eyesight tests were based on, however in some cases Alcor is mentioned by name. It is a good extra challenge if you are out looking towards the north at night time. However, it is not gravitationally bound to Mizar and Alcor, it’s actually much further from the Earth, far beyond the brighter pair of stars.

Mizar and Alcor do seem to be orbiting each other, though it must be very slowly and they must be quite distant. You are definitely looking at binary stars though. Alcor itself is actually a pair of stars, while Mizar is four stars coorbiting. This could be a sextuple star system, with a pair of pairs being orbited by another pair! Getting Mizar to look like two stars isn’t too hard with a telescope, even with Ludwig’s Star and Alcor visible in the same field of view, which is a great opportunity for photographs.

There are a number of other famous stars which are binary. The brightest star in the sky, Sirius in Canis Major, is a binary system, composed of a massive bright blue star and the first white dwarf ever discovered. The brightest star in Canis Minor, Procyon, is also a binary system. Castor, one of the twins in Gemini and also one of the brighter stars, is not only a binary star, it’s another sextuple star system, though most of the combinations are only known through spectroscopy, a good telescope should resolve it as binary at least.

A lot of these binary stars or multiple star systems require a telescope or at least binoculars to resolve into two stars, but the ones covered here are all visible to the naked eye and reasonably bright, so you can at least point them out. Mizar and Alcor however, and maybe even Ludwig’s Star if your eyesight is good enough, are certainly visible to the naked eye, and trying to spot all three is a great little quick activity on a clear night. I do hope you get a chance to at least see the stars with your eyes, if not through a telescope. Join me next time for an over view of what’s coming up in December.