Star Facts: Eta Piscium

Star Facts: Eta Piscium

Eta Piscium, also known as KULLAT NUNU, is a white to yellow giant star located 294 light years from Earth in the zodiacal constellation of Pisces. At magnitude +3.26, it is the brightest star in Pisces, although there is nothing about Eta Piscium that it in any way distinguishes it from any of the other stars in this rather dim constellation, apart from the fact that it is one of the few stars of its class that has had its angular diameter measured.

Quick Facts

• Constellation: Pisces
• Coordinates: RA 01h 31m 29.0s |Dec. +15° 20′ 45″
• Distance: 294 light years
• Star Type: G7IIIa
• Mass: 3.5 to 4 (solar masses)
• Radius: 26 sol
• Apparent Magnitude: +3.26
• Luminosity: 316 sol
• Surface Temperature: 4,930K
• Rotational Velocity: 8km/sec
• Age: Undetermined
• Other Designations: 99 Piscium, BD+14° 231, CCDM J01315+1521AB, FK5 50, GC 1839, HD 9270, HIP 7097, HR 437, IDS 01261+1450, PPM 117418, SAO 92484


Even under the most favourable seeing conditions Pisces is not a particularly conspicuous constellation, and inexperienced observers might experience some difficulty in spotting it, as well as its brightest star. Nonetheless, Pisces can be seen from latitudes of between +90° and -65°, where its easternmost fish is situated directly beneath Andromeda, and its westernmost fish is seen below the Great Square of Pegasus. Northern hemisphere observers can best see Pisces at about 9 Pm (Local Time) during the first half of November. Refer to the chart at  the top of the post to find Eta Piscium within the constellation.

Physical Properties

There are few noteworthy aspects of Eta Piscium, with the possible exception that given its mass of between 3.5 and 4 times that of the Sun, and its temperature of 4,930K, it is likely that the star’s bulk derives from the fact that it has exhausted its hydrogen fuel, and is now fusing helium in its core.

Although Eta Piscium now has a G7IIIa classification, making it a white to yellow giant, it is almost certain that it had started its life as a hot, blue B-class star, which is slightly cooler than the star Capella-A in Auriga. Studies show that the transition into the giant phase started about 250 million years ago, and that in the next few tens of millions of years, it will evolve further into a massive white dwarf, somewhat similar to Sirius-B in Canis Major.

Observations have revealed a rather mysterious “companion” of Eta Piscium, but nothing is known about this star, apart from the fact that it is separated from Eta Piscium by one second of arc, which makes it difficult to observe clearly. For instance, some observations show this star at magnitude +8, which would make it a F-type dwarf star, while other observations show it as dim as magnitude +11, which would make it an orange, K-type dwarf. What is certain, however, is that the different magnitudes are the result of errors in observation and not of variability of the “companion”. Moreover, since no orbital motion of the companion star has been detected, it is likely that the second star is nothing more than a “line-of-sight” companion, although the two stars appear to share a common proper motion.


Unlike most other stars that have proper names of Greek, Roman, or Arabic origin, this stars’ name derives from ancient Babylonia. To the Babylonians, the star was known as “Kullat Nunu”, where “Kullat” means “cord or bucket”, and “Nunu” means “fish”, hence the English translation, “The cord that ties the fish together”.

In China, Eta Piscium form part of an asterism together with Rho Piscium, Pi Piscium, Omicron Piscium and 104 Piscium that is known as “[the] Official in Charge of the Pasturing”. Due to its position in this asterism, Eta Piscium is known as “Yòu Gèng èr”, which translates into “[the] Second Star of Official in Charge of the Pasturing”.

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