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POLARIS

(Alpha Ursae Minoris). Not seasonal, always there in the nighttime sky, Polaris, the North Star, marks the unchanging North Celestial Pole, for most of us about halfway up the sky to the north, the elevation above the horizon equal to the observer's latitude. Actually, Polaris is slightly off the pole and has a tiny circle around it about 1.5 degrees across. The pole itself, about which Polaris goes, marks true north, the fundamental direction for us in the northern hemisphere that defines the others, east, west, and south. Because of a 26,000 year wobble in the Earth's axis, the pole of the sky is slowly moving closer to Polaris, and then, around the year 2100, will start to pull away. Thousands of years from now, Polaris will be well off the pole, other stars someday taking its place. Polaris also marks the end of the handle of the Little Dipper, the prominent figure of Ursa Minor, the Smaller Bear. Much fainter than its "Big" counterpart, the Big Dipper, the Little Dipper is hard to find in a bright sky. Polaris has the common reputation of being the brightest star in the sky, whereas near dead-on second magnitude (2.02) it comes in at about number 40. Its lower rank, however, is largely determined by its great distance of 430 light years. The star is actually an evolved class F yellow supergiant 2200 times more luminous than our Sun. Hydrogen fusion has stopped in the star's core, and it is now passing through a phase of instability wherein it pulsates over a period of about four days, almost invisibly changing its brightness as the brightest "Cepheid" variable star in the sky. The prototype of this kind of star, Delta Cephei, though fainter, is a much more obvious variable, its changes easily seen with the naked eye. Cepheids are paramount distance indicators in astronomy, as their true brightnesses are revealed by their periods of oscillation. Polaris is particularly interesting as the pulsations have nearly, but not quite, ceased. Just as a violin string has a "fundamental" tone that gives its pitch, it also vibrates in higher-frequency overtones. Comparison with other Cepheids shows that Polaris is pulsating not with its natural fundamental period, but in its first overtone. The star may be in the process of evolving into its fundamental period of 5.7 days to become a more-normal Cepheid with a greater variation.