CANOPUS (Alpha Carinae). 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 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 (7800
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 supergiant, 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.