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CANOPUS
As northerners drive south on winter vacations, they encounter something of a surprise. Just below the brightest star in the sky, Sirius, is the SECOND brightest star, Canopus, 30 degrees and almost exactly south of Mirzam, Sirius's announcing star. Nearly 53 degrees south of the celestial equator, and the great luminary of Carina, the Keel, Canopus is not visible from latitudes above 37 degrees north, which excludes all of Europe and Canada and most of the continental United States (though from the Gulf Coast to southern Arizona the two make a grand winter sight, as they do in all the summertime southern hemisphere). Unlike most stars, the name refers to a person, though its origin is unknown. Canopus was originally the Alpha star of the ancient constellation Argo, the ship on which Jason sailed to find the golden fleece. In more modern times, huge Argo was broken into three parts, Carina (the Keel), Puppis (the Stern), and Vela (the Sails). Canopus fell into Carina, and is therefore now Alpha Carinae. Shining at the minus-first magnitude (-0.72), Canopus appears about half as bright as its apparent celestial neighbor, Sirius. Physically, the two have nothing to do with each other. Canopus, much the grander star, is much farther away and is a rare class "F" yellow-white (7400 Kelvin) supergiant. From its apparent brightness and distance of 313 light years, we calculate a luminosity 15,000 times that of the Sun. With a diameter 65 times solar, Canopus is large enough to stretch three-fourths of the way across Mercury's orbit. Canopus possesses an extremely hot magnetically heated "corona." The Sun's corona, a thin two-million Kelvin gas that extends far beyond the bright solar surface, is seen only during eclipse. Canopus's corona is some 10 times hotter and produces both observable X-rays and radio waves. As a bright giant, Canopus has ceased hydrogen fusion in its core, and is in the process of dying, its luminosity suggesting a birth mass 8 or 9 times solar. It may once have been a red giant like Betelgeuse, or it may become one yet, its exact status unknown. Not quite massive enough to explode, Canopus will eventually die as a massive white dwarf like Sirius-B. Most white dwarfs, the leavings of thermonuclear fusion, are made of carbon and oxygen. Canopus is massive enough that fusion reactions may proceed farther to produce a much rarer neon-oxygen white dwarf.