Star Tour – Cor Caroli and “La Superba”

An artist's rendering of the carbon star La Superba in the constellation Canes Venatici. Credit: Wikipedia Commons.

An artist’s rendering of the carbon star La Superba in the constellation Canes Venatici. Credit: Wikipedia Commons.

Turn your gaze to the sky under the handle of the Big Dipper and you’ll see only two reasonably bright stars. These are the 3rd-magnitude stars Cor Caroli and 4th-magnitude Chara. Cor Caroli (the “Heart of Charles”) was named by Edmund Halley after the martyred English King Charles I. It is a pretty double star, easily split in a small telescope even at 30-40x. The blue-white primary shines at magnitude 2.9; the fainter yellow companion is magnitude 5.6 some 19 arc-seconds away from the primary. The pair is about 110 light years away.

The brighter of the two components of Cor Caroli is formally called α (alpha) Canes Venaticorum. It’s an unusual type of variable star with peculiar concentrations of silicon, mercury, and europium in its atmosphere. It also has an intense magnetic field, more than 5,000x stronger than Earth’s, that creates enormous “starspots”, similar to our sun’s sunspots. The starspots move in and out of view as the star rotates, causing variability in overall brightness.

A computer models shows enormous star spots in the photosphere of a star. Credit: University of Copenhagen.

A computer models shows enormous star spots in the photosphere of a star. Credit: University of Copenhagen.

Some 6º northeast of Cor Caroli, you’ll find the unmistakable dull-red glow of the cool red giant star Y Canes Venaticorum. You’ll need a pair of binoculars or a finderscope to see it: it’s just beyond the reach of the unaided eye. Also called “La Superba”, this cool variable star has an atmosphere rich in carbon compounds that block and scatter all but red and infrared light from shining through. This makes the star far redder than nearly any other star in the sky. It’s one of the most famous examples in the sky of a carbon star.

Any telescope will show the deep red color of La Superba. Try defocusing your telescope slightly to stimulate the color-detecting cone cells in your eye.

The stars Cor Caroli and Y Canes Venaticorum (La Superba" in the constellation Canes Venatici.

The stars Cor Caroli and Y Canes Venaticorum (La Superba) in the constellation Canes Venatici.

The carbon in the atmosphere of La Superba was likely produced by nuclear fusion of helium in the star’s core. Deep zones of convection dredged up the carbon into the atmosphere. The star is only about three times the mass of the Sun, but it has swollen to an enormous size. La Superba has begun to eject its outer layers and appears ready to form a new planetary nebula in the next several thousand years.

The stars lies at a distance of about 1,000 light years.

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