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.
ALPHEKKA or GEMMA
One of the very few stars with two commonly used names, Alphekka is the dominant (mid-second magnitude) Alpha star of the delightful constellation Corona Borealis, the Northern Crown, a semicircle of stars to the east of Arcturus that truly reminds the viewer of a heavenly crown, the constellation representing the crown of Ariadne. The name Alphekka in fact comes from an Arabic root that refers to the semicircle, or crown, itself. The alternative name Gemma rather obviously derives from the star's central placement in the semicircle, it being the "jewel in the crown." As naked eye stars go it seems relatively ordinary, a white "main sequence" (hydrogen fusing) star much like Vega or Sirius with a temperature around 9500 K. It is, however, somewhat more luminous than either of these two more famous counterparts, around 60 times the brightness of the Sun, its second magnitude apparent brightness due to its distance of 75 light years. Alphekka's chief distinguishing characteristic is that it is double, the bright naked-eye star having a much fainter orbiting companion that is more like the Sun, which would not be visible to the naked eye at that distance. Moreover, the faint star produces a barely discernable eclipse of the brighter star every orbital period of 17.4 days. Alphekka is also a part of the so-called "Ursa Major stream," a loose cluster of stars through which the Sun is passing that includes the middle five stars of the Big Dipper, Sirius, and some others.