And as he went, his voice grew thinner, shriller,

White feathers hid his hair, and his neck lengthened,

A web began to join his ruddy fingers,

Wings came along his sides, his lips extended

Into a blunted beak: what once was Cygnus

Was a new bird, the Swan.

- Ovid: Metamorphoses (Book II)

Cygnus the Constellation

Cygnus is an easily-recognisable Northern Hemisphere constellation, one of Greek astronomer Ptolemy's original 48, and is very prominent throughout the summer months. Constellations are formed by joining up the dots (stars) with imaginary lines, and, much like the child's game, you end up with a recognisable shape. The cross-like pattern of its main stars suggests the outline of a flying swan. A less common alternative name for the constellation is indeed the 'Northern Cross'. Cygnus has more than its fair share of double and variable stars, and it boasts the first-discovered 'black hole'.

The brightest star, Deneb, is one of the three stars of the Summer Triangle, along with Altair (in Aquila) and Vega¹ (in Lyra). Deneb is at the north-eastern apex. The triangle lies almost directly overhead to European observers at Midsummer, and can hardly be mistaken, since it is also spanned by one of the densest sections of the Milky Way.

The Milky Way in Cygnus is riven by a dark band of dust sometimes known as the Cygnus Rift (or, in some sources, as the 'Northern Coalsack', echoing the more familiar Coalsack within the Southern Cross). When looking in this direction, an observer is looking out along the spiral arm of the galaxy in which our own Sun lies.

Le Gentil 3 is a dark nebula in Cygnus named after its discoverer, the French astronomer Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière. One of history's long-forgotten astronomers, poor le Gentil had some real bad luck attempting to record the Venus  transits of 1761 and 1769.

Cygnus A is one of the earliest radio galaxies discovered. At 700 million light years distance, it is the most powerful in our part of the universe. Shaped like an American football, it emits powerful x-rays - and there's a black hole in the nucleus.

Mythology

A sudden blow: the great wings beating still

Above the staggering girl, her thighs caressed

By the dark webs, her nape caught in his bill,

He holds her helpless upon his breast.

- William Butler Yeats

Cygnus is associated with several myths. According to one of these, it represents Zeus in the form of a swan, one of his legendary disguises. This is the one he used to visit King Tyndareus' wife Leda, Queen of Sparta. This encounter provided the inspiration for many paintings and sculptures. The story goes that after Zeus had impregnated Queen Leda, she also slept with her husband Tyndareus, who fathered a daughter, Clytemnestra and a son, Castor. Another two children, Helen and Pollux, were sired by Zeus. These offspring caused a lot of trouble: Helen, reputedly the most beautiful woman in the world, was kidnapped by Paris, so the Greeks besieged the city of Troy in what became known as the Trojan War². Clytemnestra was married to Agamemnon, King of Argos, but after he returned from the war she killed him. Castor and Pollux have their own story.

In an alternative story, Cygnus is one of three birds hunted by Hercules, whose constellation is close by. (Another is Aquila, the Eagle, and the third is a vulture, comprised of the stars we now know as Lyra. The group were identified with the Stymphalian Birds of Greek legend).

The most usual folklore concerns Phaeton, mortal son of the sun-god Helios, who unwisely allowed Phaeton to drive the sun-chariot. Zeus thought that the world was threatened, and struck Phaeton down. His body fell into the River Eridanus, from where his devoted friend Cygnus strove to recover it. Zeus took pity, turned Cygnus into a swan, and placed him in the sky.

Stars

The scientific star names are simple to understand (if you know your Greek alphabet). 'Alpha Cygni' means that it is the brightest star in the constellation Cygnus. The next brightest is designated 'beta', etc. Some are named stars, like alpha Cygni is Deneb; others are known by their catalogue numbers or 'designation'.

• Deneb (alpha Cygni) forms the tail of the Swan, and is a blue supergiant of stellar magnitude 1.3, making it the 20th brightest star in the sky. At approximately 3,200 light-years distant, however, it is one of the most luminous stars known, with an absolute magnitude of -8.7.

• The star which marks the beak of the Swan, Albireo (beta Cygni), is a particularly beautiful binary, comprising a 3rd magnitude orange giant and a blue companion of 5th magnitude. Powerful telescopes can resolve the brighter component, which turns out to be a double star itself.

• Sadir (gamma Cygni) is at the centre of the cross and the Swan's breast; a 2nd magnitude yellow-white supergiant which is also believed to comprise a binary system. It is the designated 'gamma' but it is actually brighter than the beta star Albireo. Sadir is surrounded by a diffuse nebula.

• Rukh (delta Cygni) is a triple star system which is sometimes the (north) pole star, thanks to the 26,000-year cycle of precession.

• Gienah (epsilon Cygni) marks the wing tip. It's a fabulous 2nd magnitude orange giant, just over 70 light-years distant.

• 61 Cygni has two dwarf components of the 6th magnitude at 11 light years distance and is of significance as its distance was the first to be measured of any star.

• P Cygni is a nova-like variable which periodically dims from 3rd to 6th magnitude. It has given its name to a class of stars with similar and characteristic spectra, and its behaviour was first noted by Kepler.

• Chi Cygni varies regularly from 3rd to 14th magnitude, with a period of about 400 days.

Star Chart

Quantum Singularity

Close to the star Eta Cygni lies an x-ray object designated Cygnus X-1, which in 1973 was the first star to be tentatively identified as a quantum singularity. More commonly known as black holes, they are collapsed red giant stars, and nothing that strays too close escapes them, not even light. We can't see the event horizon, but we can detect their presence by the reaction of surrounding objects. Luckily for us, this intergalactic heavyweight with an insatiable appetite is over 8,000 light years distant.

Messier Objects

Two open clusters from the Messier catalogue lie within the bounds of Cygnus. M29, in the vicinity of Sadir, is faint and unremarkable. M39 is a large naked-eye open cluster that lies close to the eastern boundary of the constellation and slightly to the north of Deneb.

New General Catalogue Objects

Cygnus contains some rather more interesting nebulae, including a number in a tight area about three degrees south of Epsilon Cygni (also known as Gienah) in the eastern wing of the Swan. To the east of the group lies the Cirrus Nebula (NGC6992) and to the west the fainter and more diffuse Veil Nebula  (NGC 6992). The complete system is the remains of a 5,000 year-old supernova. Though difficult to observe without resort to long-exposure photography, it has yielded strikingly beautiful images.

Between M39 and Deneb lies the North America Nebula (NGC 7000), which gets its name from its shape, easily revealed by a small telescope. Also close by are the two nebulae IC5067 and IC5070, which are collectively known as the Pelican.

Wolf-Rayet Stars

Wolf-Rayet (WR) stars are rare blue giants named after their discoverers Charles Wolf and Georges Rayet. The stars are massive, over 20 times the size of our Sun. The stellar wind they produce has been measured at speeds of several million kilometres per hour. They have a high rate of mass loss, equivalent to an Earth mass per year. This shortens the star's life and will eventually cause them to go supernova.

Meteor Showers

A couple of meteor showers originate from the direction of Cygnus, though they are modest ones. The October Cygnids is a sparse, irregular shower visible between 26 September and 10 October, by which time Cygnus is falling lower in the northern sky. The Kappa Cygnids (with a different virtual point) was once a concentrated shower peaking in early August, but has now broadened and lessened in intensity. It is active between mid-July and the end of August. During the Kappa Cygnids of 1977, Elvis Presley died. Ever since then, the meteor shower has been dedicated to him by his faithful fans.

Cygnus in Modern Culture

• Cygnus is the family name (genus) of large waterbirds which we know as swans. These stately, graceful creatures enjoy royal protection in the UK. Swan Upping takes place on the River Thames annually in July.

• 'Cygnet Committee' is a song by David Bowie, from his 1975 album Space Oddity.

• The USS Cygnus was the name of the spacecraft in the 1979 Disney film The Black Hole.

• The Cygnoids are minor players in a few episodes of Futurama.

• 'Cygnus X-1' is an interminable prog-rock classic by Canadian rockers Rush.

• The Ugly Duckling is a children's story by Hans Christian Andersen - it's about a hatchling being raised by ducks, but he's different from his siblings. He wakes one morning to find he's turned into a beautiful swan. 'The Ugly Duckling' has also been immortalised in song by the likes of Mike Reid and Danny Kaye (from the 1952 film Hans Christian Andersen).

The Swan and the Bard

Go thither; and, with unattainted eye,

Compare her face with some that I shall show,

And I will make thee think thy swan a crow.
- Romeo and Juliet

The swan was William Shakespeare's favourite bird and he included references to it in many of his plays such as Antony and Cleopatra, As You Like It, The Merchant of Venice, Othello and Romeo and Juliet. Out of all his theatres, the one named The Swan Theatre in Paris Gardens, Surrey, was built by Francis Langley around 1594. One visitor in 1596, Johannes de Witt, made a drawing of the interior of the Swan playhouse, and it survives today, having outlived the theatre.

The most outstanding of all the theatres, however, and the largest, is that whose sign is the Swan as it seats 3,000 people. It is built out of flint stones stacked on top of each other (of which there is great store in Britain), supported by wooden pillars which, by their painted marble colour, can deceive even the most acute observers. As its form seems to bear the appearance of a Roman work, I have made a drawing of it.

- Johannes de Witt, Utrecht, Randstad, Netherlands.

Amazingly, Shakespeare's plays didn't go down well with everyone. One neighbour of the theatre, William Wayte, went so far as to issue a writ for Sureties of the Peace against 'William Shakspeare, Francis Langley, Dorothy Soer wife of John Soer, and Anne Lee, for fear of death and so forth'.

The play watched at The Swan by de Witt was Twelfth Night; we know this because his drawing was identified as Scene 4/Act III in Twelfth Night by the Oxfordian scholar, Mrs Eva Turner Clark, in 1937.