Cygnus, the Swan
By Ellis Myers
|The finest star fields to be found in the northern sky are those of Cygnus,
although at lower latitudes Scorpius and Sagittarius are more than equal
in splendor. Wonderful views are here for binoculars; and for small telescopes
worthy targets are nearly endless.
Premiere among these objects is Albireo, Beta Cygni, at the bill of the swan, or the base of the Northern Cross. It's probably the prettiest of all the double stars, for it is a magnificent blue and gold. Alpha Cygni, the brightest star in the constellation-18th brightest of all stars-and the most distant of first-magnitude stars, is Deneb; a luminous white supergiant.
Of deep-sky objects, the North America Nebula is the best known. The glow of the nebulosity, larger than the Moon, can be seen using large-aperture binoculars. M29 and M39 are both open clusters of stars. M39 is both the larger and brighter. It is said that Aristotle recognized it as early as 325 bc.
With a 6-inch or larger telescope, you may be fortunate to enjoy the Veil Nebula, which is part of the visible aftermath of a supernova that exploded sometime in the far distant past. In the recent past, other novae have appeared in the constellation. One happened in August 1975 (well, not really - it happened about 3700 bc, but was first observed in 1975), when a 21st magnitude star increased its luminosity 40 million times to an apparent magnitude of 1.8; nearly as bright as nearby Deneb. Within two weeks Nova Cygni 1975 was no longer visible to the naked eye.
Many times more massive than the Sun, P Cygni's nuclear furnace burns much hotter-so hot that the star can't hold itself together. Radiation pressure pushes the gas in its outer layers into space. As the expelled shell of gas gets thicker or thinner, the star's brightness appears to change. For over a century, until 1715, the star flared up and cooled down with a irregular period of about three years, but since then it has been quiet at about magnitude five.
Double star 16 Cygni is one of special interest. As one of the nearest stars to the Solar System, this was among the first systems to be searched for possible extra-solar planets. And in July 1997, Paul Butler and Geoff Marcy, at San Francisco State University, along with astronomers from the University of Texas, announced the co-discovery of a planet, somewhat larger than Jupiter, in orbit around one of the Sun-like pair. To date more than 58 planets have been found around stars other than our Sun.
Assyrians, Arabs, and Greeks described this grouping of stars as some form of bird, but the name we use today comes fom Roman mythology. Two major stories are connected, along with lesser stories. In one of Jupiter's many lustful liaisons, he changed his form into that of a swan in order to seduce Leda, wife of King Tyndareus of Sparta. From this union was born the twins Castor and Pollux, Castor the mortal son of the king, Pollux the divine son of Jupiter. Leda was also the mother of Helen of Troy.
Phaethon was the son of Apollo, god of the Sun, and the nymph Clymene. When one of his friends challenged him to prove he was the son of a god, Phaethon asked his father to be allowed to drive the chariot of the Sun for one day across the sky. But as he spurred the mighty steeds to the noon heights, Phaethon became frightened and lost control. The chariot plunged toward the claws of Scorpius. To prevent an impending catastrophe, Jupiter threw a thunderbolt that knocked Phaethon from the chariot and he fell into the river Po. Apollo was able to rescue the raging horses in mid-flight and lead them to their western stables at twilight.
Cycnus was Phaethon's beloved brother and he was nearly overcome with grief over his brother's death. He vowed to give Phaethon a proper burial, and he repeatedly dove into the waters of the Po to find the charred body. The gods of Mount Olympus were touched by this devotion while they were amused by the likeness of Cycnus to a swan diving for food. They decided to reward the brotherly love by placing him among the stars.
Eight stars of this constellation form an asterism that is popularly known as the Northern Cross. The base of the cross is Albireo, the beak of the swan, while the top of the cross is Deneb, the tail. Deneb is also part of another asterism, the Summer Triangle, joined by Vega in Lyra and Altair in Aquila.