KULLAT NUNU (Eta Piscium). If some star names are more obscure than others, this one is surely near the bottom of the list. Unlike most star names, which are from Greek, Latin, or especially Arabic, it seems to have derived from Babylonian and really more refers to the cord that connect the Fishes of Pisces rather than to the star itself. Though Alrescha, the Alpha star, is perhaps best known in this dim constellation (as a result of its placement at the central "kink" in the cord), it is rather well topped by Bayer's Eta star, which at bright fourth magnitude (3.69) is Pisces' brightest, and somehow deserves a name (so "Kullat Nunu," while hardly official, will have to do). Best to think of it more as just plain old Eta Piscium. The star itself is a bit unusual, a bright class G (G7) giant, at 4930 Kelvin a bit cooler than Capella-A. It is one of the few of its class to have had its angular diameter measured. From that and the distance of 294 light years, Eta is 26 times larger than the Sun and would extend 30 percent of the way from the Sun to Mercury. The star's calculated luminosity of 316 Suns and its temperature conspire to give almost the same answer, showing that the measured properties are consistent and correct. In turn, these data indicate a mass between 3.5 and 4 times that of the Sun and that the star is most likely dying and in a state of internal helium fusion. Only about 250 million years ago it was a hot, blue Class B star, and in far less time than that, it will turn into a massive white dwarf rather like Sirius-B. Spinning with an equatorial velocity of at least 8 kilometers per second, the giant may take almost half a year to make a full turn. All this activity has been witnessed by a mysterious, relatively dim companion about which nothing is known (and given the uncertainties about it, once might say less than nothing). The "little one" is only a second of arc away from its brighter companion, making observation difficult. It has been reported to be as bright as 8th magnitude and as faint as 11th (the result of error in observation, not of variability). If at 8th, it is a yellow-white class F dwarf, if at 11th, an orange class K dwarf. The companion illustrates a major problem in astronomy, that we cannot see well in three dimensions. If at precisely the same distance of the bright star, the two are separated by 70 or so astronomical units (1.75 Pluto's distance from the Sun), and would have an orbital period around each other of about 270 years. However, no orbital motion has been seen; that the two are related is shown only by the fact that they are moving through space together. Consequently, they must be farther apart, the fainter star lying somewhat in the foreground or background. There is simply no way to tell.
Written by Jim Kaler. Return to STARS.