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.