Delphinus, meaning “the dolphin” in Latin, was one of the 48 constellations catalogued by the Greek astronomer Ptolemy in his 2nd century treatise called Almagest. Unlike many other constellations, Delphinus actually resembles the sea animal it depicts, making it easily recognizable in the night sky. Its brightest star is Rotanev (Beta Delphini), a blue-white subgiant of magnitude 3.6 located around 110 light-years from our solar system.
Delphinus is the 69th largest constellation, and can be seen by observers located between +90° and -70° of latitude, although best seen in September. A good way to find Delphinus is to first locate the Summer Triangle, and then look below and to the left of the star Altair. The constellations bordering Delphinus are Sagitta, Aquila, Aquarius, Equuleus, Pegasus and Vulpecula.
Heavenly Waters Family
Delphinus is amember of the Heavenly Waters family of constellations, together with Carina, Columba, Equuleus, Eridanus, Piscis Austrinus, Puppis, Pyxis, and Vela.
Two myths are generally associated with the constellation Delphinus. The first states that after the mermaid Amphitrite escaped from Poseidon’s amorous advances, the sea-god persisted in his pursuit and sent his messenger, a dolphin, to retrieve her. The dolphin subsequently managed to persuade Amphitrite to accept Poseidon’s affections, and after they later married, Poseidon honored the dolphin by placing his image among the stars.
The other legend says that Apollo placed the dolphin in the celestial heaven as a tribute for saving the life of Arion, a poet and musician. As the story goes, Arion was sailing back to Greece after performing a concert tour in Italy when two sailors on the ship tried to kill him and steal his money. As a final request, Arian asked them to allow him to sing a final song, and after they agreed, Arion’s voice drew many dolphins towards the ship, enabling him to jump overboard and be carried to safety all the way back to Greece by the friendly porpoises.
– Beta Delphini (Rotanev), the brightest star in the constellation, was discovered to be a binary system in 1873. It is around 1.8 billion years old and consist of a subgiant and a giant located around 101 light years away that orbit each other once every 26.66 years.
– Alpha Delphini (Sualocin), the second brightest star in Delphinus, is actually a multiple system consisting of seven stars with a combined magnitude of 3.77. Stars A and G are a physical binary found 241 light years from Earth, while B, C, D, E, and F are all optical binaries.
– Gamma Delphini is a binary star consisting of a yellow-white dwarf and an orange subgiant situated 101 light years from our solar system. It has an apparent visual magnitude of 3.87.
– Epsilon Delphini (Deneb Dulfim) is a blue-white giant that has a visual magnitude of 4.03. It is situated 358 light years from the Sun, and is a variable star whose magnitude is able to reach 3.95. The classic name of the star, Deneb Dulfim, comes from an Arabic phrase meaning “the dolphin’s tail,” and was later translated into Latin as Cauda Delphini.
Notable Deep Sky Objects
Delphinus lies within a rich star field of the Milky Way, and the constellation therefore contains a number of deep-sky objects. Notable globular star clusters includes NGC 6934 (Caldwell 47) found 50,000 light years away with a visual magnitude of 8.83; and NGC 7006 (Caldwell 42) situated 137,000 light years away with a visual magnitude of 10.6. The small planetary nebulae of NGC 6891 and NGC 6905 (Blue Flash Nebula) can also be found in the constellation of Delphinus.