ALPHECCA or GEMMA (Alpha Coronae Borealis). One of the very few
stars with two commonly used names, Alphecca is the dominant (mid-
second magnitude, 2.23) 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 Alphecca in fact comes from an Arabic root that
alludes to the "broken" nature of the circle, the semicircle that
makes the crown. 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 class A (A0) "main sequence" (hydrogen
fusing) star much like Vega or Sirius (though at 75 light years, much
farther) with a temperature of 10,000 Kelvin, a luminosity 67 times
that of the Sun, and a radius of 2.7 solar.
Alphecca's chief distinguishing characteristic is that it is binary, the bright naked-eye
star having a much fainter orbiting class G5 companion that is more
like the Sun. Most interesting is that the faint star produces a
barely discernable eclipse
(by a tenth of a magnitude) of the brighter star every orbital
period of 17.3599 days (the secondary eclipse, when the faint star
hides behind the bright one, also visible). From all available
data, Alphecca proper has a mass of 2.7 solar masses (very close to
that found from luminosity and temperature), while the companion
comes in at but 0.92 solar. Though the average separation is 0.20
Astronomical Units (half that of Mercury from the Sun), a modest
eccentricity changes the separation between 0.27 and 0.13 AU. A
rotation speed of bright Alphecca-A of 133 kilometers per second
gives a rotation period of 10 days, so the rotation is not
synchronized with the orbit. While the G-type companion is the
source of powerful X-rays, revealing a high degree of solar-like
magnetic activity (and a young age of a few hundred million years),
the primary star is surrounded by a "Vega-like" dusty disk.
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.