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.