Delphinus, the Dolphin, is a small northern constellation whose stars form one of the most recognizable animal shapes in the night sky. Its compact diamond-shaped body and short tail make it relatively easy to identify, even though none of its stars is particularly bright.
Covering approximately 189 square degrees, Delphinus is the 69th-largest of the 88 recognized constellations. It was among the original 48 constellations recorded by the Greek astronomer Ptolemy in the second century AD.
Delphinus is especially interesting for its distinctive star pattern, unusual stellar names, and several distant globular clusters.
Where and When to Find Delphinus
Delphinus is best observed from the Northern Hemisphere during July, August, and September, when it appears prominently in the evening sky.
The easiest way to locate it is to first find Altair, the bright star in Aquila that forms the southern corner of the Summer Triangle. From Altair, look roughly 10 degrees northeast for a small diamond-shaped group of stars. This is the body of Delphinus, with a short line of stars extending southwest to form the Dolphin’s tail. The constellation lies near Sagitta, Vulpecula, Equuleus, and Pegasus.
Although its principal stars are only around fourth magnitude, the constellation’s compact shape makes it easier to recognize than many larger, brighter constellations.
Job’s Coffin — The Dolphin’s Diamond
The four stars forming Delphinus’s distinctive diamond are sometimes known as Job’s Coffin, a traditional asterism within the constellation. The pattern consists of Alpha, Beta, Gamma, and Delta Delphini, with Epsilon Delphini extending outward to form the Dolphin’s tail.
The origin of the name Job’s Coffin is uncertain, but the star pattern has been recognized for centuries. Like the Summer Triangle, Job’s Coffin is an asterism rather than a separate constellation. Its small size makes it a convenient target for binoculars and a useful landmark when exploring this part of the sky.
Mythology
Two Greek myths are commonly associated with Delphinus.
In the first, Poseidon, the god of the sea, fell in love with the sea nymph Amphitrite. When she fled his advances, Poseidon sent a dolphin to find her. The dolphin persuaded Amphitrite to return, and Poseidon rewarded it by placing its image among the stars.
Another story concerns Arion, a celebrated poet and musician. While returning from a performance tour, Arion was threatened by sailors who intended to steal his wealth. He asked permission to sing one final song before being thrown overboard. His music attracted dolphins to the ship, and when he jumped into the sea, one carried him safely to shore. The dolphin’s rescue was commemorated in the night sky.
Notable Stars
- Rotanev (Beta Delphini) is the brightest star in Delphinus, shining at approximately magnitude +3.6. Located around 100 light-years away, it is a binary system containing two evolved stars that orbit each other approximately every 27 years.
- Sualocin (Alpha Delphini) is the constellation’s second-brightest star, with a combined apparent magnitude of around +3.8. It is a multiple-star system approximately 240 light-years away.
- Gamma Delphini is an attractive double star approximately 100 light-years away. Through a small telescope, its yellow-white and orange components provide a pleasing color contrast.
- Delta Delphini is a white evolved star shining at approximately magnitude +4.4. It forms one corner of the Job’s Coffin diamond and is classified as a variable star.
- Deneb Dulfim (Epsilon Delphini) is a blue-white giant shining at around magnitude +4.0. Its name refers to the Dolphin’s tail, which it marks in the constellation’s traditional outline.
The Curious Names of Sualocin and Rotanev
Two of Delphinus’s brightest stars have names with an unusual origin.
In the early 19th century, the astronomer Niccolò Cacciatore introduced the names Sualocin and Rotanev into a star catalog. Read backward, the names become Nicolaus and Venator, the Latinized form of his name, Niccolò Cacciatore. The word cacciatore means “hunter” in Italian, while venator has the same meaning in Latin. It was a clever way of placing his own name among the stars, and both names remain officially recognized today.
Notable Deep-Sky Objects
Although Delphinus contains no Messier objects, it has several interesting globular clusters and planetary nebulae.
- NGC 6934 (Caldwell 47) is a globular cluster approximately 50,000 light-years away, shining at around magnitude +8.8. It appears as a faint, rounded glow through small telescopes, while larger instruments begin to resolve some of its individual stars.
- NGC 7006 (Caldwell 42) is a much more distant globular cluster, approximately 135,000 light-years away. It lies far out in the Milky Way’s halo and appears as a small, faint patch even through moderately sized telescopes.
- NGC 6905, commonly called the Blue Flash Nebula, is a planetary nebula formed from gas expelled by an aging star. Its compact appearance and bluish color make it an interesting target for experienced observers.
- NGC 6891 is another small planetary nebula in Delphinus. It appears almost star-like through modest telescopes, although higher magnification can reveal its tiny disk.
Observing Delphinus
Delphinus is an excellent constellation for observers learning to recognize smaller star patterns.
Start with Altair in the Summer Triangle, then move northeast to find the diamond of Job’s Coffin. Binoculars help bring out the surrounding stars, while a small telescope makes Gamma Delphini an appealing double-star target.
Observers with larger telescopes can explore the globular clusters NGC 6934 and NGC 7006, making this small constellation surprisingly rewarding.