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.
Written by Jim Kaler. Return to STARS.