Delphinus the Dolphin

Delphinus the Dolphin
Delphinus (the Dolphin) isn't a southern sea constellation invented by early European navigators. It's an ancient northern constellation first catalogued in the 2nd century by Greek astronomer Ptolemy. Although small and made up of faint stars, its diamond is easily visible in a clear dark sky.

Mythology
Like other old constellations, Delphinus has a rich mythology.

In one tale, Greek poet and musician Arion, famed for his lyre playing, was sailing home after a profitable concert tour in Italy. But the sailors plotted to kill and rob him. Surrounded by the rogues, and facing death, Arion asked to play one last song. The beauty of his playing attracted some dolphins. When he suddenly leapt overboard, one of them carried him back to Greece. Apollo, the god of poetry and music, honored the dolphin with a place in the sky. Sometimes the constellation Lyra is said to be Arion's instrument, but it's usually associated with Orpheus.

In an older tradition, the sea god Poseidon tried to court the nymph Amphitrite. Rejecting this, she hid. The god sent out messengers to find her. The dolphin not only found her, but persuaded her to accept Poseidon's proposal. When he then carried her to Poseidon, in gratitude the god put him in the sky when he died.

Job's Coffin
The Dolphin's four brightest stars form the asterism Job's Coffin. Its kite shape looks a bit like a coffin, but why Job's coffin, no one knows.

All four stars are binary, i.e., made up of two stars orbiting each other. With a telescope you can see the stars of some binaries separately. But a spectroscopic binary only shows the presence of a second star if a spectrum is taken.

Some amateur telescopes can separate the stars of both Gamma Delphini and Beta Delphini (Rotanev), each about 100 light years away from us.

Gamma Delphini represents the celestial dolphin's nose. It's made up of a yellow-white dwarf seven times more luminous than the Sun, and an orange subgiant over twenty times more luminous than the Sun. They orbit each other about every three thousand years, and are easily separated even in quite small telescopes. Beta Delphini, the brightest of the four stars, needs a larger amateur telescope to separate its giant (Rotanev) and subgiant (Beta Delphini B).

Alpha Delphini (Sualocin) and Delta Delphini are spectroscopic binaries.
Sualocin's primary star is a blue subgiant some 140 times more luminous than the Sun. The secondary is so faint and close to the primary that it was only resolved using a highly specialized technique. A further five faint stars are associated with Sualocin, but they're probably in the line of sight, not gravitationally bound.

Delta Delphini comprises two chemically peculiar stars. This means they have more elements heavier than helium in the surface layers. They orbit each other about every 41 days, and are very bright, each about 65 times more luminous than the Sun. They seem dim to us because they're over 220 light years away. Delta Delphini would look as bright as Arcturus if it were at the same distance.

Deep sky objects
There aren't any Messier objects in Delphinus, but there are some deep sky objects attractive to amateur observers.

Globular clusters
A globular cluster is a big group of stars that formed at about the same time, and held together in a roughly spherical shape by their mutual gravity. These are truly ancient stars, some of the oldest known. Many of the clusters are found in the outer limits of the Milky Way, the Galactic halo.

NGC 6934 was discovered by German-British astronomer William Herschel in 1785. It contains about a quarter of a million stars, but Herschel couldn't resolve any of them. Today it looks bright in a 6” telescope, and a seasoned observer might see individual stars near the edges. The cluster is about 50,000 light years away.

Globular cluster NGC 7006 appears much fainter than NGC 6934. This isn't surprising since it's over 135,000 light years away in the Galactic halo. The shape of its orbit suggests that it formed outside the Milky Way before being captured by our Galaxy.

Planetary nebulae
A planetary nebula is created when a dying sunlike star sloughs off its outer layers. William Herschel saw a number of them that seemed rounded like the disk of a planet. He dubbed them planetary nebulae, and the name stuck even though they can be various shapes.

NGC 6905 is a planetary nebula discovered by Herschel in 1784, only acquiring its nickname Blue Flash Nebula in the mid-twentieth century. Although small, it can be seen with a 4" telescope in dark skies, even though seeing the bluish color and central star needs at least a 10" scope. NGC 6891 is similar to NGC 6905, but somewhat farther away. It was discovered by Scottish astronomer Ralph Copeland in 1884.

A puzzle solved
Unlike many star names, Sualocin and Rotanev aren't derived from Arabic or Greek. They first appear in the 1814 Observatory of Palermo star catalog. Yet it wasn't until 1859 that the names were explained by English amateur astronomer T.W. Webb. If you read the star names backwards, you get Nicolaus Venator, the Latinized form of Niccolò Cacciatore, who was Giuseppe Piazzi's assistant and successor at the Observatory of Palermo. Was Cacciatore himself the joker, or perhaps his mentor? No one knows.



You Should Also Read:
Milky Way – Our Galaxy
ABC of Astronomy – D Is for Double Star
White Dwarfs

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