LA SUPERBA (Y Canum Venaticorum). Small packages can hold remarkable treasures. The constellation Canes Venatici (the Hunting Dogs) is best known for its luminary, Cor Caroli (Alpha-2 Canum Venaticorum), a class A magnetic star with a magnetic field 1500 times the strength of the Sun's. Such stars are not all that rare. Much rarer is La Superba, or Y Canum Venaticorum. Its Roman letter name gives it away as a variable star. Of a kind called "semi- regular" (technically SRb), it peaks at mid-fifth (about 4.8) and varies to nearly seventh (6.3 or so) over a 160 day period (which makes it seem more regular than its class implies). Other periods, one of 2000 days, are suspected. Most "red" stars are not really so red. Not so "Y," which is one of the reddest stars in the sky, as it is among the brightest of the " carbon stars," the star classified variously as a C7 supergiant, sometimes as a CN5 supergiant. (In evolutionary terms the star is most likely really a red giant with a carbon-oxygen core, as its mass is nowhere near that of a true supergiant -- it is just highly developed and overly bright.) Carbon stars were originally classed as warmer "R" and cooler "N," and are now combined into class "C." Most red giant and supergiants are richer in oxygen than carbon: carbon stars reverse the ratio. As giants, they are dying, and are in a mass range in which the by-products of nuclear fusion, here carbon (from the nuclear "burning" of helium), are lofted to the surface before escaping into space. Huge absorptions by carbon molecules (carbon monoxide, cyanogen or CN, carbon-2, and carbon-3) are present, giving the star a remarkable spectrum, a combination of them cutting out blue and violet light and making the star quite red. The beauty of the spectrum caused the great 19th century classifier, Father Angelo Secchi, to gave the star its name "La Superba". It was described by Agnes Clerke (1905) in terms of the "extraordinary vivacity of its prismatic rays, separated into dazzling zones of red, yellow, and green, by broad spaces of profound obscurity," the "spaces" the dark carbon absorptions. La Superba, 710 light years away, is one of the coolest of naked eye stars, its temperature but 2200 Kelvin (though one authority puts it at 2800). After a large correction for infrared radiation, the star is seen to shine with a luminosity 4400 times that of the Sun, giving a radius of about 2 astronomical units, notably larger than the orbit of Mars. La Superba is most likely in the process of becoming a luminous giant for the "second time," brightening with a dead carbon-oxygen core, its mass not well defined, but initially probably around (or greater than) three times that of the Sun. Typical of the breed, it is losing mass, La Superba at a rate of about a tenth of a millionth of a solar mass per year (a million times that of the solar wind), with a flow velocity of about 10 kilometers per second. The star is surrounded by a huge detached shell of matter of its own making with a diameter of around 2.5 light years (from Earth an astounding 11 minutes of arc, 0.2 degree!), implying that the mass loss rate was 50 times higher in the past. La Superba seems poised to eject its outer envelope in the process of becoming a white dwarf. But that is not all. La Superba is the sky's brightest "J star." A very rare set of carbon stars has a huge elevation of the heavy isotope of carbon, carbon- 13 (7 neutrons in the nucleus rather than 6), and are classed carbon-J. Though carbon-13 is readily made in the nuclear reactions that help generate stellar energy, no one quite knows how J-stars are actually made. Go look for this red denizen of the northern skies, its red color obvious in binoculars, and gaze in wonder at one of the sky's rarer sights. Thanks to Bob Arr and Mark Seidel, who suggested this star.
Written by Jim Kaler. Return to STARS.