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.