CANOPUS (Alpha Carinae). As northerners drive south on winter vacations, if they are familiar with the sky they encounter something of a surprise. Just below the sky's brightest star, Canis Major's Sirius, is the SECOND brightest star, Canopus, 30 degrees and almost exactly south of Mirzam, one of 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 Canada, almost all of Europe, and half the continental (non-Hawaiian) United States, though from the deep southern US, the two make a grand winter sight, as they do in all the summertime southern hemisphere. Unlike the case most stars, Alpha Carinae's proper 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 rather than Alpha Argo. 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, the much grander star, is vastly farther away and is a rather rare class "F" yellow- white (7280 Kelvin) bright giant. From its apparent brightness and distance of 309 light years (second Hipparcos reduction, with a five percent uncertainty), we calculate a luminosity 13,300 times that of the Sun, from which the radius comes in at 73 times solar, direct measure of angular diameter giving a very satisfying value of 71 (about 90 percent the size of Mercury's orbit). Though lying just off the plane of the Milky Way, there is little if any dimming of starlight by interstellar dust. 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 from Earth only during solar eclipse. Canopus's corona is some 10 times hotter and produces both observable X-rays and radio waves. The star's exact mass depends on our guess as to its evolutionary status, but in any case falls between 8 and 9 times solar. When stars from around 6 to 12 solar masses run out of core hydrogen fuel, they first expand to become red giants, start fusing their helium into carbon and oxygen, and then undergo a dramatic shrinkage and surface heating as they become bluer, moving back into the range of class F and A stars. That is most likely where Canopus is now, quietly fusing its helium, its mass around 8 solar. (If it's still becoming a red giant, then the higher mass is the more likely). One possible scenario is that the fusion chain may continue to carbon-burning but stop before the iron core required for collapse and the resulting supernova, Canopus's fate then to become a relatively rare neon-oxygen white dwarf rather than one of the common kind that is made mostly of carbon and oxygen.
Written by Jim Kaler 12/18/98. Last updated 6/26/09. Return to STARS.