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Cygnus the Swan

Cygnus the swan is one of the oldest constellations. When Ptolemy described it in the second century it was already ancient.

There are many stories attached to Cygnus. The best known is that of the liaison between Zeus disguised as a swan and Queen Leda. The unusual outcome was Leda's giving birth to an egg. In some versions it's two eggs, with twins hatching from each: Castor and Pollux from one and Helen of Troy and her sister from the other.

Cygnus is one of the constellations whose shape is a plausible representation of what it represents, a swan with outstretched wings, as shown in 19th century Urania's Mirror. Yet probably more people would recognize it as the Northern Cross, which is an asterism. An asterism is a recognizable star group that isn't a constellation.

The cross is opposite in orientation to the swan. At the top of the cross, the swan's tail, is Deneb. Deneb (Alpha Cygni) is a supergiant, a hot young star with a diameter a hundred times that of the Sun. Although it still looks bright to us, Deneb is so far away that its light takes some 1500 years to reach us. It's at least 50,000 times brighter than the Sun.

In looking for Cygnus, it's helpful that Deneb is one star of the Summer Triangle. (Move your mouse over the Summer Triangle picture to show the stars.) The brightest star of the triangle is Vega which can be seen early in the evening nearly overhead. The Summer Triangle is another asterism.

At the foot of the cross - the swan's head - is Albireo. It's about halfway between Vega and Altair and seems to be a single star. However it is one of the most beautiful double stars in the sky. Even a small telescope can resolve it into two stars, one amber and one blue.

Cygnus contains a number of nebulae, which are great clouds of gas and dust. One of them is the Veil Nebula, discovered by William Herschel in the 18th century. It is a supernova remnant, the remains of a giant star that exploded thousands of years ago. This will be Deneb's fate one day.

Another nebula discovered by Herschel is the North America nebula. It was just a fuzzy object in Herschel's telescope, but the reason for its name is quite evident in this photo taken by Jason Ware. It is an emission nebula. An emission nebula glows because it's energized by a nearby bright star. This is common in regions where new stars are forming.

Several extrasolar planets were discovered in Cygnus. One of the most interesting of them orbits a sun-like star in 16 Cygni, which is a triple star system.

A new development has been NASA's Kepler Mission. It is monitoring part of Cygnus in its hunt for extrasolar planets. By March 2012, it had increased the number of confirmed exoplanets by sixty, but has found over two thousand exoplanet candidates.

But this isn't quite the end of the tour, for I've saved something special for last: a black hole. Cygnus X-1 is a strong x-ray source and a dense unseen object. Its mass is at least eight times the mass of the Sun, making it a black hole.

The black hole forms a binary system with a blue supergiant. They both orbit a common center of mass about once every five and a half days. The black hole's strong gravity pulls gas from its companion into a disk of gas called an accretion disk. As matter spirals from this disk into the black hole itself, it heats up so much that high energy radiation such as X-rays and gamma rays is given off.

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