|Location||RA: 21 h DEC: +40°|
|Brightest Star||Deneb (α Cyg)|
Cygnus is a very large, bright constellation in the northern sky. It covers 804 square degrees – or around two percent of the sky – and is usually depicted as a swan, flying along the plane of the Milky Way. In fact, it is positioned in the brightest part of the northern Milky Way, and as such, contains many targets that are admired by observers and astrophotographers alike.
Cygnus has been identified with a number of celestial swans in Greek mythology. The favoured story is that the constellation represents Zeus, who took on the form of a swan in order to court Leda, wife of the Spartan king, Tyndareus. As a result of their union, Leda had several offspring: the Gemini twins (Castor and Pollux), Clytemnestra (later wife to Agamemnon), and Helen of Troy.
In addition to this, several characters named “Cycnus” appear throughout Greek myths, many of whom were transformed into swans and placed in the night sky.
The Stars of Cygnus
Due to its large size and its position along the axis of the Milky Way, Cygnus contains several bright stars. Foremost among these is Deneb, with an apparent magnitude of 1.25, making it the sixteenth brightest star in the night sky. Unlike some other stars, Deneb’s brightness does not stem from its proximity to our own solar system, rather it is due to the intrinsic brightness of the star itself, which shines with the light of over 60,000 Suns.
Deneb also features prominently in northern sky asterisms such as the Northern Cross and Summer Triangle.
The Northern Cross largely replicates the form of Cygnus itself, with Deneb sitting at the top of the cross and Albireo (β Cyg) at the foot, while Gienah (ε Cyg) and Delta Cygni (δ Cyg) make up the crossbar.
The Summer Triangle is much wider and features Deneb at one vertex, with the bright stars Altair (α Aquilae) and Vega (α Lyrae) forming the other two.
The other well-known star in Cygnus is Albireo (Beta Cygni) which, as it turns out, is not a single star at all. In fact, Albireo is perhaps the most well-known and oft-photographed double star in the entire night sky.
Albireo’s popularity is not surprising, as it is both easy to find and spectacular to view. It forms the head of the swan of Cygnus (and the base of the Northern Cross), and a good pair of binoculars or a small telescope will resolve the pair into its component stars. The brighter of the two companions (which is itself a tightly bound binary system) shines relatively brightly with an amber-gold colour and an apparent magnitude of 3.1, while the dimmer star is blue-green in colour and magnitude 5.1. The colour difference between the two stars in the Albireo pair is one of the most extreme of any binary in the sky, and it makes for an excellent visual demonstration of star colour.
Deep Sky Objects in Cygnus
NGC 6826 is located near θ Cygni in the left wing of the swan and is known as the Blinking Planetary Nebula. This is due to the fact that, when viewed using averted vision, the brightness of the nebulosity appears to overwhelm that of the central star, however, when viewed directly, the reverse is true. Thus, the nebula appears to “blink”.
The Crescent Nebula (NGC 6888) is a fairly dim emission nebula that can be found around two degrees south of Sadr (Gamma Cygni), which is the star at the juncture of the Northern Cross’ two arms. It probably requires a telescope with an aperture of around 8″ in order to get a decent view, however, a larger aperture and/or an OIII filter would definitely be beneficial.
The Crescent Nebula is the result of intense stellar winds being emitted from a massive Wolf-Rayet star. These winds are colliding with slower-moving winds emitted by the same star a few hundred-thousand years ago, and the resulting collision has generated a shockwave that excites the gas.
The Veil Nebula lies between the star Geinah (Epsilon Cygni) and the border between the constellations Cygnus and Vulpecula. It is made up of several catalogued nebulae, which comprise the visible portions of a large but dim supernova remnant known as the Cygnus Loop.
All told, the Veil Nebula/Cygnus Loop covers almost three degrees of the sky (or around six times the diameter of The Moon. While some areas of nebulosity can be seen with binoculars or a small telescope, the full spectacle only really becomes apparent to those observing through a large-aperture instrument.
Accompanying Deneb (α Cygni) through the night sky is the North America Nebula – NGC 7000. This large emission nebula encompasses approximately three degrees of the sky and bears a strong resemblance to the continent from which it takes its name.
Under dark skies, the North America Nebula is large enough to be faintly seen with the naked eye, but with the aid of binoculars or a small telescope, the shape of the “continent” starts to become apparent.
The Pelican Nebula (IC 5070) is part of the same interstellar cloud of gas and dust as the North America nebula and can be seen to the right of the accompanying image.
Although not visible to amateur astronomers, in the X-ray spectrum Cygnus X-1 is one of the brightest objects in the night sky. It has a mass approximately fourteen times that of our Sun, and is orbited by a blue supergiant star at a distance of around 0.2 AU – or about half to two-thirds the orbital distance of Mercury.
At this close proximity, Cygnus X-1 is slowly stripping away material from its companion star. This material falls into an accretion disk and is heated to millions of degrees.
Given its mass and tiny size, Cygnus X-1 was one of the first – and still most likely – candidates in the night sky to be a stellar mass black hole.