1. Introduction

Delphinus, the Dolphin constellation, is a small but distinctive grouping of stars located in the northern celestial hemisphere. Its name, derived from Latin, translates to "dolphin," reflecting the constellation's recognizable shape resembling a leaping marine mammal. Delphinus is situated between the larger constellations of Aquila and Cygnus, adding to its charm and visibility in the night sky. The constellation consists of four main stars that make up the body and tail of the dolphin, along with several fainter stars that complete its shape (Figure 1). Despite its small size, Delphinus stands out due to its distinctive appearance and relative proximity to other prominent constellations.

Celestial coordinates place Delphinus in the northern celestial hemisphere, with right ascension ranging from approximately 20 hours to 21 hours and declination ranging from approximately +6 degrees to +21 degrees. Its location makes it visible from most inhabited regions of the world, particularly during the summer months in the northern hemisphere when it is high in the sky.

Figure 1. IAU chart of Delphinus. Source: https://www.iau.org/static/archives/images/screen/del.jpg. Credit: IAU and Sky & Telescope. Reproduced under CC BY 4.0 license.

2. Historical Background and Mythology

In ancient Greek mythology, Delphinus is often associated with the story of Apollo and the sea god Poseidon. According to legend, Apollo sent the dolphin Delphinus to find a suitable location to build a temple in honor of his sister, Artemis. Delphinus successfully located the ideal spot and guided a ship to shore, where Apollo's temple was eventually constructed. As a reward for its help, Apollo placed Delphinus among the stars as the constellation we know today.

Another prominent myth involving Delphinus is the story of the musician Arion. According to legend, Arion was a talented lyre player who was saved from drowning by a group of dolphins after being thrown overboard by pirates. In gratitude, Arion promised to dedicate a song to the dolphins, which he later performed while riding on the back of one of the dolphins. Inspired by Arion's bravery and musical prowess, the god Dionysus placed both Arion and the dolphins among the stars as constellations. While Arion's constellation is not specifically identified, the association with dolphins has led some to link the story with Delphinus.

In Roman mythology, Delphinus is often associated with the sea god Neptune (the Roman equivalent of Poseidon). Dolphins were considered sacred animals to Neptune, and they were often depicted accompanying him in artwork and mythology. It is believed that the Romans adopted the Greek myth of Apollo and Delphinus and incorporated it into their own religious beliefs, further solidifying the association between dolphins and the sea gods.

Delphinus also holds cultural significance in other ancient civilizations. In Hindu mythology, the dolphin is associated with the god Vishnu, who is often depicted riding on the back of a dolphin. In Native American cultures, dolphins were revered as symbols of intelligence, harmony, and protection.

Throughout history, Delphinus has been an important navigational marker for sailors and seafarers. Its distinctive shape and prominence in the night sky made it a useful reference point for determining direction and location while at sea. The constellation's association with dolphins, creatures known for their intelligence and agility in water, further reinforced its significance in maritime cultures.

3. Notable Stars

Alpha Delphini (Sualocin): Alpha Delphini, also known by its traditional name Sualocin, is the brightest star in the Delphinus constellation. It is a binary star system located approximately 240 light-years away from Earth. Sualocin consists of two main-sequence stars, with the primary component being a white A-type star and the secondary component being a fainter star. Its name, Sualocin, is derived from the Latin word "Nicolaus" spelled backward, in honor of the Italian astronomer Niccolò Cacciatore.

Beta Delphini (Rotanev): Beta Delphini, also known as Rotanev, is another binary star system in Delphinus. It is located approximately 100 light-years away from Earth. Rotanev consists of two main-sequence stars, with the primary component being a yellow-white F-type star and the secondary component being a fainter star. Similar to Alpha Delphini, the name Rotanev is also derived from a reversal of the name of the discoverer, the English astronomer John Russell Hind.

Gamma Delphini (Aldulfin): Gamma Delphini, also known as Aldulfin, is a multiple star system located approximately 101 light-years away from Earth. It consists of at least four stars, although only two are easily visible to the naked eye. The primary component is a binary star system, while the secondary component is a binary star system with two fainter stars. Gamma Delphini is notable for its variability in brightness, which has been observed and recorded by astronomers over the years.

Delta Delphini: Delta Delphini is a solitary star located approximately 93 light-years away from Earth. It is a yellow-white F-type main-sequence star, similar to the Sun but slightly hotter and more massive. Delta Delphini is relatively faint compared to some of the other stars in Delphinus but is still visible to the naked eye under dark sky conditions.

4. Deep-Sky Objects

NGC 6891: NGC 6891 is a planetary nebula located in Delphinus, approximately 7,200 light-years away from Earth. It is often referred to as the "Diamond Ring Nebula" due to its striking appearance resembling a sparkling gemstone when observed through telescopes. The nebula is formed from the outer layers of a dying star that have been expelled into space, creating a colorful and intricate structure.

NGC 6934: NGC 6934 is a globular cluster located in Delphinus, approximately 51,800 light-years away from Earth. It is one of the more distant globular clusters visible in the night sky. NGC 6934 contains hundreds of thousands of stars densely packed together in a spherical shape. Observers with telescopes can see individual stars within the cluster, with some of the brighter ones resolving into distinct points of light.

NGC 7006: NGC 7006 is another globular cluster in Delphinus, situated approximately 137,000 light-years away from Earth. It is one of the more distant globular clusters in the Milky Way galaxy. NGC 7006 is notable for its high metallicity, meaning it contains a higher proportion of elements heavier than hydrogen and helium compared to other globular clusters. This characteristic suggests that NGC 7006 may have formed later in the galaxy's history.