Star Points for July 2008; by Curtis Roelle Double Stars There's a dramatic scene in George Lucas' original Star Wars movie (Episode IV) in which the Luke Skywalker character is standing in the desert on his home planet Tatooine staring off into the distance at a vision unfamiliar to earthlings: the planet's twin suns are setting. Although the idea of binary suns shining in the sky is alien to us, astronomers with telescopes have long been aware that multiple star systems are not only common, they are actually the norm. Astronomers believe that the majority of stars in our galaxy have one or more stellar companions. That's really not so surprising, given that stars are believed to form not alone but in clusters before dispersing. Double stars are described by the brightness (magnitude) of their components and their orientation in the sky. The brighter star in the system is called the primary and the fainter star is the secondary. The orientation, called "position angle" (PA), is measured in degrees between the two stars with respect to north. If the secondary is straight north of the primary, its PA is zero degrees. If the secondary is due east, its PA is 90 degrees, due south is 180 degrees, and due west is 270 degrees. Some stars that appear double in the sky are optical illusions. These "apparent binaries" are stars separated by a great distance that just happen to lie along our line of sight in the sky. "True binaries," on the other hand, are stars that are close together and bound by their mutual gravitational pull. You don't need a telescope to visually separate, or "split", the components in a binary star. Mizar is the star in the bend of the handle of the "Big Dipper" asterism. A sharp-eyed observer will notice that there's another, fainter star close by, named Alcor. Mizar and Alcor are several light years apart, so they are probably just apparent binaries. They are both about 80 light years from us. There is a Native American tale in which a bear, represented by the four stars in the bowl of the Dipper, is being hunted by a number of other characters represented by the stars in the dipper's handle, as well as by several other hungry stars nearby. In this story Mizar represents "Chickadee," and Alcor is the frying pan he's toting along hoping to fry the bear in. Some binary stars are striking for their colors. The best known example in the sky is the star Albireo (or, in Arabic, Al Min?ar al Dajajah, the hen's beak) located in the constellation Cygnus the Swan, which is flying high in July's night sky. Albireo's components are usually described as a yellow and blue pair of stars. You'll need a telescope to appreciate them and to see their majestic colors, and even a small telescope will work. I've noticed that the larger the telescope the more brilliant the colors become. In my 0.5-meter (20 inch) instrument they are golden and sapphire blue. Finding Albireo requires a little more work than do Mizar and Alcor. Deneb, the tail of the swan, is one corner of the "Summer Triangle" of stars. The other corners are Vega, in Lyra the Harp, and Altair, in Aquila the Eagle. The swan's body is a line of stars reaching from Deneb through the center of the triangle, and ending at Albireo, marking the swan's head. A finder chart has been posted on the StarPoints web site (starpoints.org). Scroll down to "Star Points Illustrations" and click on the link for the Summer Triangle finder chart. The named stars, including Albireo, are marked with labels on the chart. Some multiple stars have more than two members. Go back and have another look at Mizar, but this time use a telescope. Mizar has a member much closer and fainter than Alcor. In reality, Mizar has even more companions than that. According to sources that vary, Mizar has either 3 or four companions making it either a quadruple or quintuple star, not even counting Alcor. If you'd like a chance to see some of these and other multiple stars in a telescope, come out to Bear Branch Nature Center on Saturday, July 12 for a planetarium program followed by free telescopic observing provided by the Westminster Astronomical Society (WASI). The planetarium programs are scheduled for 7:30 and 8:30 p.m. The nature center collects a nominal fee ($2) for the planetarium program and recommends that guests register by calling 410-848-9040. The star party begins when the sky darkens, and there is no fee. If you have a passion for double stars you might be interested in the Double Star Club, an observing program of 100 double stars organized by the Astronomical League (astroleague.org). League members completing the program receive a nice pin and a certificate suitable for framing. League membership is a benefit of membership in WASI.