Delphinus constellation is located in the northern sky. It is one of the smallest constellations. Its name means “the dolphin” in Latin.
The constellation represents the dolphin sent by the sea god Poseidon to find Amphitrite, the Nereid he wanted to marry.
Delphinus was first catalogued by the Greek astronomer Ptolemy in the 2nd century. It is home to several interesting deep sky objects: the globular clusters NGC 6934 and NGC 7006 and the planetary nebulae NGC 6891 and NGC 6905 (Blue Flash Nebula).
LOCATION, FACTS & MAP
Delphinus is the 69th constellation in size, occupying an area of 189 square degrees. It lies in the fourth quadrant of the northern hemisphere (NQ4) and can be seen at latitudes between +90° and -70°. The neighboring constellations are Aquarius, Aquila, Equuleus, Pegasus, Sagitta, and Vulpecula.
Delphinus has five stars with known planets and contains no Messier objects. The brightest star in the constellation is Rotanev, Beta Delphini. There are no meteor showers associated with the constellation.
There are two myths associated with the constellation Delphinus. In one, the dolphin constellation represents Poseidon’s messenger. When the sea god courted the nymph Amphitrite, one of the Nereids, she resisted his advances and took refuge among her sisters. Poseidon sent messengers to find her and bring her to him, among them a dolphin. The dolphin found the nymph, soothed her and brought her back to the god. The two were later married. Poseidon decided to honour the dolphin and placed his image among the stars.
In the other myth, it was Apollo, the god of poetry and music, who placed the dolphin among the constellations for saving the life of Arion, a poet and musician born on the island of Lesbos whose skill with the lyre made him famous in the 7th century BC.
Arion was sailing back to Greece after a concert tour of southern Italy when the sailors who were also on the ship started plotting to kill him and take the money he had earned.
Surrounded, Arion asked them to let him sing one last song. The sailors allowed this, and Arion’s music drew several dolphins to the ship. As he played, the dolphins swam alongside the ship and Arion decided to take a leap of faith and he jumped overboard.
One of the dolphins carried him all the way back to Greece. Later, Arion confronted the sailors and had them sentenced to death. In this version of the myth, Apollo placed the dolphin next to the constellation Lyra in the sky, and Lyra represents Arion’s lyre.
The constellation was also sometimes referred to as Job’s Coffin because of its long, box-like shape. Mostly, the name was restricted to the four bright stars in the constellation – Alpha, Beta, Gamma and Delta Delphini.
MAJOR STARS IN DELPHINUS
α Delphini (Alpha Delphini)
Alpha Delphini is the brightest star in the constellation. It has a combined apparent magnitude of 3.77. It is a multiple star, consisting of seven components: A and G, a physical pair, and B, C, D, E, and F, which are optical binaries.
Alpha Delphini A and G are approximately 241 light years distant and belong to the spectral types A and B9V.
Alpha Delphini is sometimes known as Sualocin. The name was given to it by the Italian astronomer Niccolò Cacciatore. It is the Latinized version of his name, Nicolaus, spelled backwards.
β Delphini (Beta Delphini)
Beta Delphini was also given a proper name by Cacciatore. Rotanev, spelled backwards is Venator, which is the Latinized version of his family name, which means “the hunter.”
Beta Delphini was discovered to be a binary star in 1873 by the American astronomer S. W. Burnham. The system is about 1.8 billion years old and it consists of a pair of stars belonging to the spectral types F5 III and F5 IV, a giant and a subgiant, approximately 101 light years distant. The stars orbit each other with a period of 26.66 years. They are separated by only 0.44 arc seconds, which makes them difficult to resolve with a telescope.
γ Delphini (Gamma Delphini)
Gamma Delphini is another binary star, also about 101 light years distant. The primary component is a yellow-white dwarf of the spectral type F7V and the companion star is an orange subgiant belonging to the spectral class K1IV. The stars have magnitudes of 5.14 and 4.27 respectively.
A planetary candidate was discovered in the orbit of the secondary component in 1999, but it has not yet been confirmed.
δ Delphini (Delta Delphini)
Delta Delphini is a giant star belonging to the spectral class A7IIIp. It has a visual magnitude of 4.434 and is classified as a Delta Scuti variable.
ε Delphini (Epsilon Delphini)
Epsilon Delphini is a blue-white giant belonging to the spectral class B6III. It has a visual magnitude of 4.03 and is 358 light years distant. Epsilon Delphini is a slightly variable star; its brightness occasionally reaches 3.95 magnitudes.
The star’s traditional name, Deneb Dulfim, comes from the Arabic ðanab ad-dulfīn, which means “the dolphin’s tail.” The name was translated into Latin as Cauda Delphini.
Tso Ke – ρ Aquilae (Rho Aquilae)
Rho Aquilae is a white main sequence dwarf of the spectral type A2V. It has an apparent magnitude of 4.94 and is 154 light years distant. The star’s traditional name, Tso Ke, means “the left flag” in Mandarin. In China, Left Flag refers to an asterism formed by Rho Aquilae and several stars in the constellation Sagittarius, and Rho Aquilae itself is known as the Ninth Star of the Left Flag.
Rho Aquilae is believed to be about 50 million years old. The star belonged to the constellation Aquila until 1992, when it moved across the border into Delphinus.
DEEP SKY OBJECTS IN DELPHINUS
NGC 6934 (Caldwell 47)
NGC 6934 is a relatively large globular cluster near the star Epsilon Delphini. It is approximately 50,000 light years distant and has a visual magnitude of 8.83.
The cluster was discovered by William Herschel on September 24, 1785.
NGC 6891 is a small planetary nebula located near the star Rho Aquilae. It has an apparent size of 0.33′ by 0.3′.
The nebula is about 7,200 light years distant from Earth. It was discovered by Scottish astronomer Ralph Copeland on 22 September, 1884.
Blue Flash Nebula – NGC 6905
NGC 6905 is a small planetary nebula, bluish in colour. It can be observed in a six-inch telescope.
NGC 7006 (Caldwell 42)
NGC 7006 is a globular star cluster located approximately 137,000 light years away in the outskirts of the Milky Way.
The cluster is part of the galactic halo, a region of the Milky Way spherical in shape and consisting of gas, dark matter and the occasional star cluster.
The cluster has a visual magnitude of 10.6. It is located close to the star Gamma Delphini.