Piscis Austrinus (“the southern fish”) was known to the ancients and is said to represent the parent of the pair of fish associated with the zodiacal constellation of Pisces. The brightest star in this southern constellation is Fomalhaut (“mouth of the fish”), which has a visual magnitude of +1.16, making it the 18th brightest star in the entire night sky.
Piscis Austrinus is the 60th largest constellation, taking up an area of 245 square degrees of the southern celestial hemisphere. It can be seen by observers located between +55° and -90 of latitude, although best seen from July to September. Piscis Austrinus is found south of the ecliptic path the Sun takes across the sky, and is imagined as a fish with its mouth open, drinking water from the water jar of Aquarius to its north. Other constellations bordering Piscis Austrinus includes Sculptor to its east, Microscopium to its southwest, Capricornus to its northwest, and Grus to its south.
Heavenly Waters Family
Piscis Austrinus is part of the Heavenly Waters family of constellations, together with Carina, Columba, Delphinus, Equuleus, Eridanus, Puppis, Pyxis, and Vela.
The Babylonian knew the constellation as MUL.KU, the Fish, while the Assyrians associated it with the fish god Dagon, which John Milton described in his epic poem Paradise Lost (1667) as “sea-monster, upward man, and downward fish.” In ancient Egypt, the goddess Isis placed the fish in the sky in gratitude for saving her life, while in Greek mythology the constellation depicted the Great Fish who was said to have conceived the two fish associated with Pisces. Piscis Austrinus was also one of the 48 constellations originally included by Greek astronomer Ptolemy in his star treatise called Almagest (150 AD), but by around 1598 AD Petrus Plancius removed a few of its stars to form the new constellation of Grus.
– Fomalhaut (Alpha Piscis Austrini), the constellation’s brightest star, is also the night sky’s 18th brightest star with a visual magnitude of 1.16. It is a white main sequence star (A3 V) situated 25.13 light years away that is estimated to be 440 million years old out of a total life span of around 1 billion years. Fomalhaut, meaning “the mouth of the fish” in Arabic, is 1.84 times bigger than the Sun, with 1.92 times its mass, and 16.6 times its luminosity. In 2008, the planet Fomalhaut b was discovered orbitting the star, making it the first extrasolar planet visible to the human eye through photographic images.
– Epsilon Piscis Austrini, the constellation’s second brightst star, is a blue-white dwarf (B8V) found 744 light years from our solar system with a visual magnitude of 4.18. It has around 7.59 times the Sun’s radius.
– Delta Piscis Austrini, the constellation’s third brightest star, is a multiple star system found 170 light years from Earth of magnitude 4.20. Its primary component is a yellow giant (G8III) that is almost 10 times bigger than the Sun.
Other stars of interest in Piscis Austrinus includes the multiple star system Beta Piscis Austrini, Iota Piscis Austrini, Gamma Piscis Austrini, Theta Piscis Austrini, and Eta Piscis Austrini; the spectroscopic binary star Pi Piscis Austrini; the variable stars S Piscis Austrini and V Piscis Austrini; the white main sequence star Mu Piscis Austrini; the yellow-white dwarf Tau Piscis Austrini; the orange dwarf stars TW Piscis Austrini and HD 216770; and the red dwarf Lacaille 9352, located just 10.68 light years from Earth.
Notable Deep-Sky Objects
There are no Messier objects in Piscis Austrinus, although it does contain a number of interesting deep-sky objects, most noticeably the three interacting galaxies of NGC 7173, NGC 7174 and NGC 7176. The elliptical galaxy NGC 7173 is 114.8 million light years away and has a visual magnitude 11.9; while NGC 7176 is an elliptical galaxy found 115.5 million light years distant of magnitude 11.5; and NGC 7174 is a spiral galaxy with a visual magnitude of 12.5.