The Delphinus Constellation

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 recognised by the International Astronomical Union. It is one of the smaller constellations, ranked 69th in size. 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 lacks stars above fourth (apparent) magnitude; its brightest star is of magnitude 3.8. The main asterism in Delphinus is Job’s Coffin, nearly a 45°-apex lozenge diamond of the four brightest stars: Alpha, Beta, Gamma, and Delta Delphini. Delphinus is in a rich Milky Way star field. Alpha and Beta Delphini have 19th-century names Sualocin and Rotanev, read backwards: Nicolaus Venator, the Latinized name of a Palermo Observatory director, Niccolò Cacciatore (d. 1841). Alpha Delphini is a blue-white hued main sequence star of magnitude 3.8, 241 light-years from Earth. Beta Delphini, called Rotanev. 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. The primary is orange-gold of magnitude 4.3; the secondary is a light yellow star of magnitude 5.1. The pair forms a true binary with an estimated orbital period of over 3,000 years and 125 light-years away. The secondary, also described as green, is 10 arcseconds from the primary. Struve 2725, called the “Ghost Double,” is a pair that appears similar but dimmer. Its components of magnitudes 7.6 and 8.4 are separated by 6 arcseconds and are 15 arcminutes from Gamma Delphini itself. Delta Delphini is a type A7 IIIp star of magnitude 4.43. Epsilon Delphini, Deneb Dulfim (lit. “tail [of the] Dolphin”), or Aldulfin, is a star of stellar class B6 III and magnitude 4, at 330 ly.

In Delphinus, in extremes of distance, Gliese 795 is the closest known star at 54.95 ly and rapidly moves east over a period of centuries (863±3 arcseconds per year); whereas the giant of blue colour, W Delphini is at 2203.81 ly at 9.76 magnitude. Its brightness ranges from a magnitude of 12.3 to a magnitude of 9.7 over its variable period as it is a Beta Persei star-type semi-detached system. Other variable stars include R Delphini, a Mira-type variable star with a period of 285.5 days. Its magnitude ranges between a maximum 7.6 and a minimum 13.8.

Its rich Milky Way star field means many modestly deep-sky objects. NGC 6891 is a planetary nebula of magnitude 10.5; another is NGC 6905 or the Blue Flash nebula. NGC 6934 is a globular cluster of magnitude 9.75. At a distance of about 185,000 light-years, the globular cluster NGC 7006 is at the outer reaches of the galaxy. It is also fairly dim at magnitude 11.5. Credit: Wikipedia.