At magnitude 2.81, Alpha Canum Venaticorum is the brightest star in the modern constellation of Canes Venatici, the Hunting Dogs. Cor Caroli, named "Charles' Heart" in honor of England's King Charles II, is easy to find. Look on the line perpendicular to the Big Dipper's handle just toward the south; Cor Caroli is the brighter of a pair of stars - the Hunting Dogs - extending parallel to the Dipper's handle.
Components and Properties
The telescope quickly reveals that α CVn is double, with two component stars 19" apart. At the system's distance of 110 light years, the projected separation between the two components is 650 AU, producing a long orbital period of at least 7900 years. The western component is α1 CVn; the eastern one is α2 CVn.
The system is dominated by α2 CVn, a peculiar white class A0 dwarf with a temperature of 10,300 K, shining at magnitude 2.90. It far outshines α1 CVn, a magnitude 5.60 main sequence star of class F0. α2 CVn shines with the luminosity of 83 Suns, which makes its radius 2.6 times solar, and implies a mass of 2.8 suns. α1 CVn is cooler, estimated at around 6500 K, anomalously cool for its spectral class. It is also far fainter, with only 5 solar luminosities, and a mass just 1.6 times the Sun's. α1 CVn seems to be an iron-rich metallic line star.
Of greater interest, α2 CVn is a "magnetic star," possessing one of the strongest known magnetic fields among hydrogen-fusing main-sequence stars. The Sun has an overall magnetic field that is only a few times stronger than Earth's; Cor Caroli's, on the other hand, is more than 1500 times stronger than Earth's.
The star also has a weird chemical composition in which elements such as silicon, mercury, and europium are enormously enhanced. The spectral strengths of these elements vary with time as the star rotates over a period of 5.5 days, revealing some kind of spottedness. The magnetic field is probably responsible for helping to redistribute the elements in the star's atmosphere, elevating some to the surface, while depleting others. The origins of the magnetic fields of such stars are not understood.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]