Izar (ε Boötis)
June 2010 :
Most of us are familiar with the novel Moby Dick, whose protagonist Captain Ahab relentlessly hunts a great white whale. I can sympathize with the obsessive Captain. For several years back in the late 1970s, I pursued an astronomical white whale- the double star epsilon (?) Bootis. Instead of the Pequod, my vessel of pursuit was a 3-inch f/10 reflector.
Trying to capture Izar with a 3-inch reflector is like attempting to harpoon a whale from a rowboat. The difficulty lies in the magnitude difference between the components (2.6 and 4.8) and their closeness (2.9 arc-seconds). On numerous evenings I tried to resolve Izar’s component stars without success. Notching this stellar duo became an overpowering obsession. On the evening when I at last split Izar, skies were remarkably steady and I used the highest practical magnification (120X) my little reflector could handle. Even then, the companion played hide-and-seek in the diffraction ring of the primary.
A larger telescope and magnifying power of 200X will readily split Izar and reveal a striking color contrast between the golden yellow primary and its bluish companion. The Russian astronomer Wilhelm Struve, who conducted a double star survey in the late 1820s and early 1830s (Izar became ?1877 in his double star catalog), nick-named it “Pulcherrima” (The Most Beautiful).
But Izar is more than just a close pair of stellar specks. The main component is a K0 spectral class giant 30 times as large as the sun. Its A2-type companion is twice the sun’s size – a virtual twin to Sirius. Separated by 180 Astronomical Units, the two undergo a slow gravitational dance, their orbital cycle encompassing perhaps a thousand years.
Imagine that Izar were moved from its current location 250 light-years away to a distance equal to that separating us from Sirius. The star would be a dazzling sight, rivaling Venus in brilliance. Viewed with even the smallest telescopes, the magnitude -3.6 and -1.4 components, separated by 85 arc-seconds, would be an absolutely magnificent sight.
Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.
