Cygnus (pron.: /ˈsɪɡnəs/) is a northern constellation lying on the plane of the Milky Way. Its name is the Latinized Hellenic (Greek) word for swan. One of the most recognizable constellations of the northern summer and autumn, it features a prominent asterism known as the Northern Cross (in contrast to the Southern Cross). Cygnus was among the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations.
[edit] Notable features
The constellation Cygnus as it can be seen by the naked eye.
There are several bright stars in Cygnus. Alpha Cygni, called Deneb, is the brightest star in Cygnus. It is a blue-white hued supergiant star of magnitude 1.3, 3200 light-years away. The traditional name of Alpha Cygni means "tail" and refers to its position in the constellation. Albireo, designated Beta Cygni, is a celebrated binary star among amateur astronomers for its contrasting hues. The primary is an orange-hued giant star of magnitude 3.1 and the secondary is a blue-green hued star of magnitude 5.1. The system is 380 light-years away and is divisible in large binoculars and all amateur telescopes. Gamma Cygni, traditionally named Sadr, is a yellow-tinged supergiant star of magnitude 2.2, 1500 light-years away. Its traditional name means "breast" and refers to its position in the constellation. Delta Cygni is another bright binary star in Cygnus, 171 light-years with a period of 800 years. The primary is a blue-white hued giant star of magnitude 2.9, and the secondary is a star of magnitude 6.6. The two components are divisible in a medium-sized amateur telescope. The fifth star in Cygnus above magnitude 3 is Gienah, designated Epsilon Cygni. It is an orange-hued giant star of magnitude 2.5, 72 light-years from Earth.
There are several other dimmer double and binary stars in Cygnus. Mu Cygni is a binary star with an optical tertiary component. The binary system has a period of 790 years and is 73 light-years from Earth. The primary and secondary, both white stars, are of magnitude 4.8 and 6.2, respectively. The unrelated tertiary component is of magnitude 6.9. Though the tertiary component is divisible in binoculars, the primary and secondary currently require a medium-sized amateur telescope to split, as they will through the year 2020. The two stars will be closest between 2043 and 2050, when they will require a telescope with larger aperture to split. Omicron Cygni is a contrasting double star similar to the brighter Albireo. The components, 30 Cygni and 31 Cygni, are divisible in binoculars. The primary, 31 Cygni, is an orange-hued star of magnitude 3.8, 1400 light-years from Earth. The secondary, 30 Cygni, is a blue-green hued star of magnitude 4.8, 720 light-years from Earth. 31 Cygni itself is a binary star; the tertiary component is a blue star of magnitude 7.0. Psi Cygni is a binary star divisible in small amateur telescopes, with two white components. The primary is of magnitude 5.0 and the secondary is of magnitude 7.5. 61 Cygni is a binary star divisible in large binoculars or a small amateur telescope. It is 11.4 light-years from Earth and has a period of 650 years. Both components are orange-hued dwarf (main sequence) stars; the primary is of magnitude 5.2 and the secondary is of magnitude 6.1. 61 Cygni is significant because Friedrich Wilhelm Bessel determined its parallax in 1838, the first star to have a known parallax. 16 Cygni is a binary star 70 light-years from Earth. Both components are white and of the 6th magnitude.
Cygnus is also home to several variable stars. Chi Cygni is a red giant and the second-brightest Mira variable star at its maximum. Its maximum magnitude is 3.3 and its minimum magnitude is 14; it has a diameter of 300 solar diameters. Chi Cygni has a period of 400 days and is 350 light-years from Earth. P Cygni is a large, unstable blue supergiant that is evolving into a red supergiant. In 1600, it brightened suddenly to 3rd magnitude, though its normal magnitude is approximately 5. It began to brighten gradually in the 1700s and has continued to brighten to the present day; this is part of the process of stellar evolution. W Cygni is a semi-regular variable red giant star, 618 light-years from Earth. It has a maximum magnitude of 5 and a minimum magnitude 8; its period of 130 days.
There are several asterisms in Cygnus. Patchik 56 is one less prominent, a small asterism that measures 3.3 by 2.2 arcminutes. Named for its discoverer, Dana Patchik, an amateur astronomer, it appears in small amateur telescopes as a group of six stars described as resembling a dolphin.[2]
http://en.wikipedia.org/wiki/Cygnus_(constellation)
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