Epsilon Pegasi: The Pendulum Star
September 2009 :
This month, we’re going to pay a visit to epsilon Pegasi (Enif), the “Pendulum Star.” It’s an optical double star comprised of magnitude 2.5 and 8.7 component stars separated by 144 seconds of arc. Pairs this wide usually don’t merit much consideration, but wait! Epsilon Pegasi has a surprise for us.
After centering this pair in the eyepiece field (60-100x is the recommended magnification), mentally trace a line between them. While keeping your eye at the eyepiece, gently jiggle the telescope back and forth so that the two stars move at right angles to the imaginary line. While the golden yellow primary (a K-type star) travels serenely back and forth, the little companion seems to swing wildly to and fro, like a clock pendulum. It’s one of the most amazing telescopic optical illusions you’ll ever witness.
What’s happening? According to Sir John Herschel, who was among the first to describe the “Pendulum Star,” the oscillations are due to the longer time it takes light from the faint star to affect the retina. We detect the motion of the primary a split second earlier, so the companion seems to lag behind. Rapid back and forth movement of the telescope generates the illusion of pendulum-like motion.
The Pendulum Star received plenty of recognition in astronomy guidebooks written in the late 19th and early 20th century – a time when double stars enjoyed tremendous popularity. Nowadays, with attention directed towards nebulae, clusters, and galaxies, epsilon Pegasi receives scant notice.
The finder chart shows the location of epsilon Pegasi. If you hunt down deep-sky objects by the star-hop method, you may recognize it as a pointer (with nearby theta Pegasi) to the globular cluster M15. Next time you plan to visit M15, take a moment to check out epsilon Pegasi. This star will put on a show that’s sure to dazzle!
Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.
