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.