NUNKI (Sigma Sagatarii). With an unusual name of Babylonian origin
(a proper name of unknown significance), at mid-second magnitude
(2.02) Nunki is barely the second brightest star in Sagittarius, the Archer, the constellation of the Zodiac that holds the Winter Solstice. The stars of the
constellation have Greek letter names that
are rather well out of the usual order of brightness, Nunki the
Sigma star, the brightest being the Epsilon star, Kaus Australis. Nunki is the northernmost
of the stars that make the bowl of the exquisite asterism, the
"Little Milk Dipper," an upside-down (for northerners) five star
dipper with its handle (Kaus Borealis) stuck into the Milky Way. A class B (B2.5) hydrogen-
fusing dwarf, it is a truly magnificent, brilliant star, dimmed to
us some by its distance of 225 light years. Placed next to the Sun it would, to the eye, shine 630 times
brighter. With a surface temperature of 20,000 degrees Kelvin,
bluish Nunki is one of the hotter of the bright stars, and radiates
much of its light in the invisible ultraviolet. When we take that
into account, the star is actually 3300 times more luminous than
our Sun. To receive the same heat, an "Earth" in the system would
have to orbit at a distance half again the size of the orbit of our
Pluto. To be so bright it must also be fairly large, and has a
diameter about five times that of the Sun. Typical of its breed,
Nunki is a fast spinner, rotating with a speed of over 200
kilometers per second, 100 times that of our Sun. Like the Sun,
however, it is a "normal" unevolved star, and shines by the fusion
of hydrogen into helium in its deep core. But its mass is so high,
seven or so times that of the Sun, and the internal fusion rates so
fast, that it cannot live very long, 50 million years at most. How
far along its lifeline it is, we do not know, but it cannot have
too much left to it. The star will ultimately turn into a white
dwarf roughly as massive as the Sun. There is some indication that
it has a close companion.