Delphinus is a constellation in the northern sky, close to the celestial equator. Its name is Latin for dolphin. Delphinus was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains among the 88 modern constellations recognized by the International Astronomical Union. It is one of the smaller constellations, ranked 69th in size.
Delphinus’ brightest stars form a distinctive asterism that can easily be recognized. It is bordered (clockwise from north) by Vulpecula the fox, Sagitta the arrow, Aquila the eagle, Aquarius the water-carrier, Equuleus the foal and Pegasus the flying horse.
Delphinus does not have any bright stars; its brightest star is of magnitude 3.8. The main asterism in Delphinus is Job’s Coffin, formed from the four brightest stars: Alpha, Beta, Gamma, and Delta Delphini. Alpha and Beta Delphini are named Sualocin and Rotanev, respectively. When read backwards, they read as Nicolaus Venator, the Latinized name of Palermo Observatory‘s former director, Niccolò Cacciatore. However, Delphinus is in a rich Milky Way star field.
Alpha Delphini, called Sualocin, is a blue-white hued main sequence star of magnitude 3.8, 241 light-years from Earth. Beta Delphini, called Rotanev, is a close binary star and the brightest in Delphinus, divisible in only large amateur telescopes. To the unaided eye, it appears to be a white star of magnitude 3.6. It has a period of 27 years and is 97 light-years from Earth. Gamma Delphini is a celebrated binary star among amateur astronomers. The primary is a gold-colored star of magnitude 4.3 and the secondary is a yellow-tinged star of magnitude 5.1. 102 light-years away, the components of Gamma Delphini are divisible in a small amateur telescope. The secondary, also described as green, is 10 arcseconds from the primary. Struve 2725, called the “Ghost Double”, is a pair that appears similar to a dimmer Gamma Delphini. Its components of magnitudes 7.6 and 8.4 are separated by 6 arcseconds and are 15 arcminutes from Gamma Delphini itself.
There are several dimmer stars in Delphinus. Delta Delphini is a type A7 IIIp star of magnitude 4.43. Epsilon Delphini, called Deneb Dulfim, meaning “tail of the Dolphin”, is a star of spectral class B6 III and magnitude 4.
Delphinus is also home to several variable stars. R Delphini is a Mira-type variable star with a period of 285.5 days. Its magnitude ranges between a maximum of 7.6 and a minimum of 13.8.
Rho Aquilae moved across the border into Delphinus in 1992.
HR Delphini was a nova that brightened to magnitude 3.5 in December 1967. On 14 August 2013, a possible nova was discovered by amateur astronomer Koichi Itagaki, initially labelled PNV J20233073+2046041, now labelled Nova Delphini 2013.