ARNEB (Alpha Leporis). Orion is so magnificent a constellation that, between the Great Hunter and his ensemble of bright northern winter constellations, we hardly ever look at the rest of his neighborhood. Few, for example, point out his representative prey, little Lepus, the Hare, lying beneath him. And no wonder. Of the few stars that make a figure that looks a little like a smashed box kite, the brightest is only third (albeit bright third, 2.58) magnitude. Yet that brightest one, Arneb (the Alpha star), is well worth a look, its Arabic name meaning and epitomizing "the Hare." The star is easy to find, as it is makes the southern apex of a triangle with the two bottom stars of Orion, Rigel and Saiph (Beta and Kappa Orionis). Would you like to admire great Canopus of Carina but cannot see him because he is too far to the south? Then look at Arneb, a more distant version, a class F (F0) supergiant. (Though Canopus is sometimes classed as a "bright giant," the two are quite similar). From its distance of 1300 light years (four times farther than Canopus), as measured from its small parallax, we find the star to have a luminosity 13,000 times that of the Sun, which coupled with its 7000 Kelvin temperature gives it a diameter 75 times solar, enough to make it almost the size of Mercury's orbit. Arneb seems to be single, an apparent dim companion half a minute of arc away probably just lying in the line of sight. The star is clearly dying, having long ago ceased fusing hydrogen into helium in its core and has cooled at its surface and expanded to its present proportions. It may be on its way to becoming a larger red supergiant star (where it will fuse helium into carbon and oxygen), or as many astronomers believe, has already been a red supergiant and is now in the process of heating and shrinking a little. In either case, stars like Arneb and Canopus are quite rare because of the speed with which they change their surface conditions as they age. The ageing process has left Arneb with a somewhat odd chemical composition, its nitrogen content five times higher than that of the Sun's, the result of the fusion of hydrogen into helium through the "carbon cycle," in which carbon is used as a nuclear catalyst, nitrogen produced as a by-product and brought to the surface. It also has double the solar sodium (as does Canopus) as a result of a similar nuclear reaction cycle that also involves neon. With a birth mass 8 to 10 times solar, the star will probable die as a tiny, dense white dwarf about the size of Earth, though perhaps an odd one made of neon and oxygen.
Written by Jim Kaler. Return to STARS.