Alpha Coronae Borealis
Gemma ( α Coronae Borealis, short α CrB ) is about 75 light-years distant from Earth binary star system and the brightest star in the constellation Corona Borealis circular ( Northern Crown ). There is a covering variable type system Algol. This star is among other things also as Gemma (Latin: gemstone ) denotes Alphekka or Alphecca ( from Arabic ).
The main component is a main sequence star of spectral type A0 ( surface temperature of about 9500 Kelvin) with about 60 times the luminosity of the sun, a mass of 2.6 solar masses and a radius of about 3 solar radii. The companion is a G- main sequence star of about 0.9 solar masses.
The brightness of the system varies with a period of 17.36 days like between about 2.2 and 2.3 mag ( with the naked eye may not be noticeable ).
The proper motion suggests that α Coronae Borealis the Ursa Major stream (Bear Power ) belongs to a far-flung star clusters in the sky, of the main stars in the Big Dipper it is 30-40 ° away.
α Coronae Borealis is an exact equilateral triangle with the bright stars Arcturus and Seginus ( α and γ in the neighboring constellation Bootes ). It lies in the extension of the tiller of the Big Dipper ( Alkaid, pictured top right).