Thanks to James B. Kaler. These contents are the property of the author and are reproduced from original without the author's express consent because of fair use and valid educational purposes.
Unless you live south of the Tropic of Cancer, you will have no trouble finding Pherkad any time of the year, as it glows at mid-third magnitude in the bowl of the Little Dipper and is circumpolar from anywhere north of 20 degrees north latitude along with its mate Kochab. Third brightest within the Little Dipper, the major figure of Ursa Minor, the Smaller Bear, Pherkad received Bayer's Gamma designation, its superiors being second magnitude Kochab (Beta) and famed Polaris (Alpha), which lies just short of the north celestial pole. (The other stars in the Dipper are fourth and fifth magnitude and hard to see with any kind of lighting). The name derives from the Arabic for "the two calves," which originally referred to both Kochab and Pherkad. Together, the two stars are also called "the Guardians of the Pole," as they nightly draw a close circle around one of the sky's most significant stars, Polaris. Like its bowl-mate Kochab, Pherkad is a giant star, but one considerably hotter, at the warm side of class A with a temperature of 8600 Kelvin. From its distance of 480 light years, we calculate a high luminosity 1100 times that of the Sun, double that of Kochab, yielding a radius 15 times solar. As a warm giant, and a bright one at that, the star is evolving, probably with a for-now quiet helium core surrounded by a ring of fusing hydrogen, its current temperature and luminosity suggesting a mass of around five times solar. If that is the case, it left the "main sequence," where it once (like the Sun) fused core hydrogen, only about 100 million years ago, and will, by odd coincidence, before long turn into a star much like Kochab is today. Many class A stars have odd chemical compositions resulting from selective settling and lofting of atoms in quiet atmospheres, Though evolved, Pherkad is still spinning rapidly, over 170 kilometers per second at the equator, 85 times solar, which keeps things stirred up and the composition "normal." The star nevertheless exudes mystery. It is of interest for its subtle and confusing variability, changing over less than a tenth of a magnitude with a period of only a couple hours. No one seems to know where to classify it. It is too hot and bright to be a well- understood pulsator of the main sequence. It was once thought to be a member of the "Maia" class (after a star in the Pleiades), but the whole class has since disappeared, showing the difficulty of understanding stellar stability. It was also classified as "shell star," one with a surrounding shroud of dust, but that seems to have disappeared as well. If nothing else, the star is easy to study, and maybe the mysteries will be solved before long.