Cygnus is a northern constellation lying on the plane of the Milky Way, deriving its name from the Latinized Greek word for swan. Cygnus is one of the most recognizable constellations of the northern summer and autumn, and 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.
Cygnus contains Deneb (ḏanab, tail) – which is one of the brightest stars in the night sky and is the most distant first-magnitude star as its “tail star” and one corner of the Summer Triangle. It also has some notable X-ray sources and the giant stellar association of Cygnus OB2. Cygnus is also known as the Northern Cross. One of the stars of this association, NML Cygni, is one of the largest stars currently known. The constellation is also home to Cygnus X-1, a distant X-ray binary containing a supergiant and unseen massive companion that was the first object widely held to be a black hole. Many star systems in Cygnus have known planets as a result of the Kepler Mission observing one patch of the sky, an area around Cygnus. In addition, most of the eastern part of Cygnus is dominated by the Hercules–Corona Borealis Great Wall, a giant galaxy filament that is the largest known structure in the observable universe, covering most of the northern sky.
History and mythology
In Eastern and World Astronomy
In Hinduism, the period of time (or Muhurta) between 4:24 AM to 5:12 AM is called the Brahmamuhurtha, which means “the moment of the Universe”; the star system in correlation is the Cygnus constellation. This is believed to be a highly auspicious time to meditate, do any task, or start the day.
In Polynesia, Cygnus was often recognized as a separate constellation. In Tonga it was called Tuula-lupe, and in the Tuamotus it was called Fanui-tai. In New Zealand it was called Mara-tea, in the Society Islands it was called Pirae-tea or Taurua-i-te-haapa-raa-manu, and in the Tuamotus it was called Fanui-raro. Beta Cygni was named in New Zealand; it was likely called Whetu-kaupo. Gamma Cygni was called Fanui-runga in the Tuamotus. Deneb was also often a given name, in the Islamic world of astronomy. The name Deneb comes from the Arabic name dhaneb, meaning “tail”, from the phrase Dhanab ad-Dajājah, which means “the tail of the hen”.
A very large constellation, Cygnus is bordered by Cepheus to the north and east, Draco to the north and west, Lyra to the west, Vulpecula to the south, Pegasus to the southeast and Lacerta to the east. The three-letter abbreviation for the constellation, as adopted by the IAU in 1922, is “Cyg”. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined as a polygon of 28 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 19 h 07.3m and 22 h 02.3 m , while the declination coordinates are between 27.73 degrees and 61.36 degrees. Covering 804 square degrees and around 1.9% of the night sky, Cygnus ranks 16th of the 88 constellations in size.
Cygnus culminates at midnight on 29 June, and is most visible in the evening from the early summer to mid-autumn in the Northern Hemisphere. Normally, Cygnus is depicted with Delta and Epsilon Cygni as its wings. Deneb, the brightest in the constellation is at its tail, and Albireo as the tip of its beak.
There are several asterisms in Cygnus. In the 17th-century German celestial cartographer Johann Bayer’s star atlas the Uranometria, Alpha, Beta and Gamma Cygni form the pole of a cross, while Delta and Epsilon form the cross beam. The nova P Cygni was then considered to be the body of Christ.
Bayer catalogued many stars in the constellation, giving them the Bayer designations from Alpha to Omega and then using lowercase Roman letters to g. John Flamsteed added the Roman letters h, i, k, l and m (these stars were considered informes by Bayer as they lay outside the asterism of Cygnus), but were dropped by Francis Baily.
There are several bright stars in Cygnus. Alpha Cygni, called Deneb, is the brightest star in Cygnus. It is a white supergiant star of spectral type A2Iae that varies between magnitudes 1.21 and 1.29, one of the largest and most luminous A-class stars known. It is located about 3200 light-years away. Its traditional name 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 visible 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 (the proper name is Fawaris) 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 visible in a medium-sized amateur telescope.The fifth star in Cygnus above magnitude 3 is Aljanah, designated Epsilon Cygni. It is an orange-hued giant star of magnitude 2.5, 72 light-years from Earth.
Cygnus is also home to several variable stars. SS Cygni is a dwarf nova which undergoes outbursts every 7–8 days. The system’s total magnitude varies from 12th magnitude at its dimmest to 8th magnitude at its brightest. The two objects in the system are incredibly close together, with an orbital period of less than 0.28 days Chi Cygni is a red giant and the second-brightest Mira variable star at its maximum. It ranges between magnitudes 3.3 and 14.2, and spectral types S6,2e to S10,4e (MSe) over a period of 408 days; it has a diameter of 300 solar diameters and is 350 light-years from Earth. P Cygni is a luminous blue variable that brightened suddenly to 3rd magnitude in 1600 AD. Since 1715, the star has been of 5th magnitude, despite being more than 5000 light-years from Earth. The star’s spectrum is unusual in that it contains very strong emission lines resulting from surrounding nebulosity. W Cygni is a semi-regular variable red giant star, 618 light-years from Earth.It has a maximum magnitude of 5.10 and a minimum magnitude 6.83; its period of 131 days. It is a red giant ranging between spectral types M4e-M6e(Tc:)III, NML Cygni is a red hypergiant semi-regular variable star located at 5,300 light-years away from Earth. It is one of largest stars currently known in the galaxy with a radius exceeding 1,000 solar radii. Its magnitude is around 16.6, its period is about 940 days.
Cygnus is one of the constellations that the Kepler satellite surveyed in its search for extrasolar planets, and as a result, there are about a hundred stars in Cygnus with known planets, the most of any constellation. One of the most notable systems is the Kepler-11 system, containing six transiting planets, all within a plane of approximately one degree. With a spectral type of G6V, the star is somewhat cooler than the Sun. The planets are very close to the star; all but the last planet are closer to Kepler-11 than Mercury is to the Sun, and all the planets are more massive than Earth. The naked-eye star 16 Cygni, a triple star approximately 70 light-years from Earth composed two Sun-like stars and a red dwarf, contains a planet orbiting one of the sun-like stars, found due to variations in the star’s radial velocity. Gliese 777, another naked-eye multiple star system containing a yellow star and a red dwarf, also contains a planet. The planet is somewhat similar to Jupiter, but with slightly more mass and a more eccentric orbit. The Kepler-22 system is also notable, in that its extrasolar planet is believed to be the first “Earth-twin” planet ever discovered.
Cygnus A is the first radio galaxy discovered; at a distance of 730 million light-years from Earth, it is the closest powerful radio galaxy. In the visible spectrum, it appears as an elliptical galaxy in a small cluster. It is classified as an active galaxy because the supermassive black hole at its nucleus is accreting matter, which produces two jets of matter from the poles. The jets’ interaction with the interstellar medium creates radio lobes, one source of radio emissions.
Cygnus is also the apparent source of the WIMP-wind due to the orientation of the solar system’s rotation through the galactic halo.