COR CAROLI (Alpha Canum Venaticorum). Third magnitude (2.81, nearly second) and easy to find, the star's importance is consistent with its brightness. Cor Caroli, which means "Charles' Heart" (named by Edmund Halley in honor of England's King Charles II), is the luminary of the modern constellation of Canes Venatici, the Hunting Dogs, invented in the 17th century by the astronomer Hevelius to help fill in the "unformed" left by the ancients. If you look on the line perpendicular to the Big Dipper's handle just toward the south, you'll find a pair of stars extending parallel to the handle; the brighter is Cor Caroli, the Alpha star, while the fainter is Chara, Beta CVn, a star rather similar to our Sun. The telescope quickly reveals that Cor Caroli is double, with two stars 19 seconds of arc apart. In keeping with tradition, the western (and fainter) of the two is Alpha-1, the eastern Alpha-2. The system is dominated by Alpha-2, a peculiar white class A (A0) dwarf (not to be confused with a "white dwarf," which is a wholly different beast) with a temperature of 11,450 Kelvin that at magnitude 2.90 far outshines Alpha-1, a sixth magnitude (5.60) class F (F0) dwarf. Over the past 236 years they have tracked each other well, the separation changing by only 3 seconds of arc (the first measure made by William Herschel), so they are a real pair. In the nineteenth century, Smythe and Chambers referred to them as "flushed white" and "pale lilac," the juxtaposition of stars of different brightness fooling the eye. The distance measured for the fainter has a large uncertainty, so we'll adopt the distance of 115 light years (give or take 4 in the second Hipparcos reduction) measured for the brighter. Allowing for ultraviolet radiation from the hot surface, Alpha-2 then shines with the light of 113 Suns, which leads to a radius 2.7 times solar and a mass of 3.0 Suns. Cooler, 6785 Kelvin, dimmer Alpha-1 glows at only 5.6 solar luminosities, its radius 1.7 times that of the Sun, the mass 1.5 Suns. Alpha-2 is well along its dwarf lifetime of 350 million years, while lower mass Alpha-1 lags behind, chugging through its dwarf lifetime of 2.7 billion years. It will remain a dwarf long after its mate turns into a giant star.

Of greatest interest, Alpha-2 is the prototype "magnetic star" that defines a class of "Alpha-2 Canum Venaticorum stars" or simply "Alpha CV stars." The Sun has an overall magnetic field strength only a few times stronger than Earth's; Cor Caroli's, on the other hand, has one more than 2000 times stronger than our planet's. The star also has a weird chemical composition in which elements such as silicon, mercury, and rare ones like europium, are enormously enhanced by radiative lofting, while others are depleted by gravitational settling in relatively quiet surface gases. Such is the case in "metallic stars," while in the peculiar magnetic stars like Alpha-2 CVn, the enhanced elements (and magnetic fields) are concentrated into huge long-lasting "starspots" that are akin to smaller ephemeral sunspots. The field axis of a magnetic star is tilted against the rotation axis, so that as the star rotates, the spots swing into and out of view, causing the star to vary in apparent brightness, Alpha-2 by a few hundredths of a magnitude. From the period of variation, Alpha-2 rotates with a period of 5.46939 days. A projected equatorial rotation velocity of 24 kilometers per second gives a rotation period of under 7.4 days, so the rotation axis must be tilted to the line of sight by 47 degrees. The origins of the fields of such stars are not understood, but may be primordial, captured by the stars from their birth clouds. Not to be completely outdone, Alpha-1 seems to be an iron-rich metallic star. The projected separation between the two of at least 675 Astronomical Units leads to a long orbital period of at least 8300 years. If the separation is in fact correct, and were anybody there (which seems highly unlikely), from Alpha-2, Alpha-1 would seem to shine with the light of six of our full Moons, while from Alpha-1, Alpha-2 would glow with 77 of them.

Written byJim Kaler 5/22/98; revised 5/04/07, 5/23/14. Return to STARS.