Delphinis the Constellation of the Dolphin is a small constellation of 189 sq degrees currently in our north-western evening sky located between Pegasus to east and Aquila to the west. In antiquity the dolphins were considered the messengers of Poseidon the sea god. The dolphin was credited with persuading the mermaid Amphitrite to return and marry Poseidon and become Queen in his golden palace at the bottom of the sea. In another Greek myth the dolphins rescue the musician and poet Arion of the 7th century BC from threat of death by sailors. While on ship from a concert in Sicily he attracted the dolphins by playing his Lyre and they transported him to safety on their backs. Apollo god of music placed the dolphin along with Arion’s Lyre – Lyra in the sky.
The shape of Delphinis is distinctive with the four brightest stars forming a diamond shape also known “Job’s Coffin” . The brightest star is Beta Delphini (Rotanev) a mag. 3.6 binary with components too close to split in most amateur scopes. The period is 26.6 years and widest separation was in 2004 when 20 cm could just separate them. Can anyone separate this one? Alpha Delphini (Sualocin) mag 3.8 is only slightly dimmer than Beta. The names of these stars read backwards spell the name Nicolaus Venator, Latin name of the assistant, and successor to Italian Astronomer ,director of Palermo Observatory Giuseppe Piazzi who discovered the asteroid Ceres.
Gamma Delphini a binary star marking the nose of the dolphin with components of mag. 4.3 and 5.1 and 10” separation. The stars are yellowish in colour the fainter of the pair has been described as having a green tinge. This double is easily separated in a small telescope. In the same field of view is Σ 2725 another fainter double of magnitude 7.5 & 8.3, 25 ly more distant than Gamma Del.
Delta Delphini mag 4.43 marks the belly of the dolphin. The tail of the dolphin is marked by Epsilon Del. Mag 4.03.
R Delphini is a Mira variable from 7.6 to 13.7 magnitude over 284 days.
Most of the deep-sky objects in Delphinis are quite dim.
NGC 6934 a small condensed irregularly round globular cluster mag. 9, diameter 3.5’ , 50, 000 ly away is the brightest of these. With small apertures it appears as a hazy round patch found SW of Epsilon, the Dolphin’s tail.
NGC 7006 is another small globular cluster mag. 10.5, diameter 1’ , very distant at 100,000 ly. It is found SE of Gamma Del, the nose of the dolphin. In a 5 inch scope it appears as a fuzzy spot. With 30 cm it is still not resolved.
NGC 6891 mag 12 planetary nebula 12” across.
Described as bright bluish with single elliptical prism image and central star streak with 10.5 cm by Hartung. A challenge for everyone.
NGC 6905 12 mag planetary nebula 45” across. Described by Hartung as somewhat elliptical and faint bluish, a dim nebula with 10.5 cm and no central star visible.
Delphinis lies near a rich area of the Milky Way and is a hunting ground for novae. In 1967 Nova Delphini (HR) (RA 20h 40.1 m, Dec +18° 59’), an unusual slow developing nova went from magnitude 12 to mag. 3.5 and remained at maximum for more than a year. It did not return to magnitude 11.5 until 1975. So great things can be discovered in small constellations!
Hartung’s Astronomical Objects for Southern Telescopes, 2nd Ed 1995;
The Night Sky Observers Guide, Vol.2 G.R. Kepple & G.W Sanner, 2000;
Burnham’s Celestial Handbook, Volume 2, 1966/1978; Collins Stars and Planets, Ridpath & Tirion 3rdEd 2000;
Star Atlas Pro Version 6.2;
Photos from Digital Sky Survey www.ngcic.org/dss.
NASA picture of the day 2010 October 9 http://apod.nasa.gov/apod/ap101009.html