At magnitude 2.23, Alphecca is the dominant Alpha star of Corona Borealis, the Northern Crown - a semicircle of stars that represents the crown of Ariadne. It is one of the very few stars with two commonly used names. Alphecca, sometimes spelled Alphekka, comes from an Arabic root "al-fakkah" that means "the "broken", referring to the ring of stars that makes the crown. The alternative name Gemma is Latin for "jewel", and derives from the star's central placement in the semicircle.
Components and Eclipses
α CrB is a white class A0 V main-sequence (hydrogen fusing) star much like Vega or Sirius, with a surface temperature of 10,000 K. At 75 light years away, it is much further than either, and so appears fainter. Alphecca has a luminosity of 67 suns, and a radius 2.7 times solar.
Alphecca's chief distinguishing characteristic is that it is an eclipsing binary, with a much fainter class G5 V companion, much like the Sun. The faint companion produces a barely discernible eclipse of the brighter primary, varying from magnitude +2.21 to +2.32 every 17.3599 days. The secondary eclipse, when the faint star passes behind the bright one, is also visible.
Though the components' average separation is 0.20 AU, a modest eccentricity changes it between 0.13 and 0.27 AU. From all available data, Alphecca A contains 2.7 solar masses, while the companion contains 0.92. Bright Alphecca A has a rotation speed of 133 km/sec, giving it a rotation period of 10 days, so its rotation is not synchronized with the orbit. The G type companion is the source of powerful X-rays, revealing a high degree of sunlike magnetic activity, and implying a young age of a few hundred million years.
Dust Disc and Motion
A large disc of dust and debris has been shown to exist around Alphecca, prompting speculation of a similar planetary/proto-planetary system to that currently assumed around Vega. How the binary-star dynamic would affect such a system is the subject of debate.
Alphecca is 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.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]