COR CAROLI (Alpha Canum Venaticorum). Third magnitude (2.81) and
easy to find, the star is important all out of proportion to its
brightness. Cor Caroli, which means "Charles' Heart" in honor of
England's King Charles II, is the luminary of the modern
constellation Canes Venatici, the Hunting
Dogs, invented in the 17th century by the astronomer Hevelius to
help fill in the blanks left over by the ancients. If you look on
the line perpendicular to the Big
Dipper's handle just toward the south, you find a pair of stars
extending parallel to the handle; the brighter is Cor Caroli, the
Alpha star. The telescope quickly reveals a binary, two stars 19 seconds of
arc apart. In keeping with tradition, the western of the two is
Alpha-1, the eastern Alpha-2. The system is dominated by Alpha-2,
a peculiar white class A (A0) dwarf with a temperature of 10,300
that at magnitude 2.90 far outshines Alpha-1, a sixth magnitude
(5.60) class F (F0) dwarf. From a distance of 110 light years,
Alpha-2 shines with the light of 83 Suns,
which leads to a radius 2.6 times solar and a mass of 2.8 solar.
Cooler (estimated at around 6500 Kelvin, anomalously cool for the
class) and far fainter, Alpha-1 glows at only 5 solar luminosities,
its mass just 1.6 times that of our Sun. Of greatest interest,
Alpha-2 is a "magnetic star," possessing one of the strongest known
magnetic fields among otherwise normal hydrogen-fusing dwarf 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, has one
more than 1500 times stronger than does our planet. The star also
has a weird chemical composition in which elements such as silicon,
mercury, and rarer elements such as europium are locally enormously
enhanced. The strengths vary with time as the star rotates with a
period of 5.5 days (which is more or less consistent with the
observed rotation velocity), revealing some kind of spottedness.
The magnetic field is probably responsible for helping to
redistribute the elements in the star's atmosphere, apparently
elevating some to the surface, while depleting others. The origins
of the fields of such stars are not understood. Alpha-1 seems to
be an iron-rich metallic line star. The projected separation
between the two of 650 Astronomical Units leads to a long orbital
period of at least 7900 years.
Updated by Jim Kaler 5/04/07. Return to STARS.