Everyone knows how to split the famous double star Mizar (mag 2.4) and Alcor (mag 4.0) in the handle of the Big Dipper. With a separation of 11.8 arcmins this is “easy.” Most everyone I have ever pointed the pair out at a star party can “split” this famous pair with naked eye observing (except the most visually challenged). Years ago I happened to look up at Alpha Capricornus, Algedi and notice this star could be split with only your keen eyesight and a night of generally clear skies. This pair consists of 3.6 and 4.2 mag stars with a separation of 6 arcmins (376”). Another pair that will test your observing skills is up at zenith this time of year. Omicron 1 Cygni (mag 3.8) and 30 Cygni (mag 5) are separated by 5.5 arcmins (338”). You will need to look with determination to split this pair, but it can be done on a night of steady seeing. The faint 5th mag star will pop in and out of view as you look at this pair. All three pairs are well placed for fall viewing. Do try these naked eye doubles at your next observing session and let me know how you did. I would like to know.
In the last section, I listed three naked eye double stars for your observing challenge well placed for autumn viewing. One pair Omicron 1 Cygni (mag 3.8) and 30 Cygni (mag 5) were separated by 338 arcmins. Which means if you can split this pair using only your keen eyesight then you have got 5.5 arcmins resolution. Not too shabby. This season we have three new doubles to test your observing skills. All three pairs are easy to find and, for the record, are not true gravitationally bound binary stars but close visual doubles. You should be able to split these from moderately light polluted suburban skies. So get yourself outside, if only for a few moments of winter observing, and don’t forget to bring your dark-adapted eyesight.
Let’s start with the most challenging pair first. This pair forms the stubby handle of the Pleiades (the seven sisters, M45) open star cluster and is just over 400 light years away, in the constellation Taurus. Many people can count six of the seven sisters in this famous star formation, but it takes some effort to find the seventh. Knowing were to look helps. Begin by forming a tiny dipper asterism of this cluster. The open portion of the bowl points generally north. The stubby handle is oriented east and the easier of the two stars, 27 Tauri, or Atlas at mag 3.6 is seen. Look north of this star, or in the same direction of the open bowl for a 5th mag star known as 28 Tauri aka Pleione. Burnham’s list the separation of these stars at 5 arcmins apart. Guide confirms Burnham’s separation. Position angle (PA) is 3 - 5 degrees or basically north-south (Hey this is a mini set of pointer stars!). These stars, like the rest of the Pleiades, have a large common proper motion which is a fancy way of say they are all moving in the some direction within the galaxy at about 5.5 arcsecs per century. So when you split this pair you can contemplate how far the Pleiades will move in the year 7454 (on July 17th at 3 in the afternoon, it will sunny followed by a brief period of darkness, I think.).
Moving east but still in the constellation of Taurus (and that’s no Bull) we come to the Hyades open star cluster which forms the recognizable “V” of the bull’s head/horns. The Hyades is very similar to the Pleiades in that the stars are both open clusters with a large common proper motion. The Hyades are a third of the Pleiades distance, or only about 130 light years away from us making them the second closest cluster to the Earth. Only the Ursa Major moving cluster is closer at 75 light years away. The Hyades are moving in the direction of Betelgeuse at 11 arcsecs per year. So these stars are cruising past our position space. The double star of our envy is Theta 2 (78 Tauri) at mag 3.3 and Theta 1 (77 Tauri) at mag 4 and are found half way down the east side of the “V” or the same side as the bright first mag star Aldebaran (which is not a Hyades member and approximately 65 light years away). Theta 2 & 1 are separated by 337 seconds of arc or 5.5 arcmins (PA = 347 degrees). These two stars (spectral types A7 & K0) are easy to split owing to their equal brightness. Not so easy as the previous pair which are farther apart in magnitude (1.5 mag vrs 0.5 mag for this pair). Something one learns when splitting binary stars in a telescope is the bigger the magnitude difference for equal separations of arc, the more difficult it is to “split em.”
Moving to our last pair we look at the bottom star in Orion’s sword. I’ll bet you never thought to look at this star in that it being so close to the famous Orion nebula and all the attention directed towards it. I didn’t either. In a desperation effort to find a third naked eye double for this report I happened upon this pair accidentally. One night while searching the winter sky for a third pair, I kept telling myself there must be another pair here when I looked up and there it was. Iota Orionis, Na’ir al Saif, “Bright one of the Sword”, shining at mag 2.8. Just southwest (PA = 225 degrees) is a 4.7 mag little star at 8 arcmins away. At a magnitude difference of almost two this pair is almost as difficult to split as the Pleiades pair. Iota itself is a triple star system for small telescopes with a 7th mag star at 11 seconds of arc away and an 11th mag star at 50 arcsecs. Burnham’s list the colors as white, pale blue, and grape red. I gotta check this one out with the scope.
All these pairs are a beautiful sight to behold and I hope you take the time to hunt these stars down and enjoy the view.
See also Chris Sarnecki. August, 2000. “Amateur Astronomer Makes Record Double Star Split.”