The constellation of the Swan (Cygnus - Cyg)


I was talking about the Milky Way: already from the picture of Stellarium you can see that our swan with its wings spread out seems to fly along this whitish, milky river. And from the photo you can notice that parallel to the body of the Swan (Deneb, Sadr, η Cyg and Albireo) there is a darker area, called "Swan's Slit", formed by dark interstellar powders that hide the stars behind.

To speak then about other infinite characteristics of this constellation we would need an encyclopedia, so I will choose some absolutely unexpected peculiarities, that makes it jump in head to a particular classification, with respect to the other constellations analyzed so far. But let's go in order. In the meantime, let's launch the program in another window, so we can follow my considerations together.

The first thing we can notice is that this constellation corresponds almost perfectly to the bird it represents: also in this case with the "f" key we can see another swan figure by H.A.Rey who was really very able to represent some constellations in a different and completely new way. In this case he was able to see also the two little legs, besides the well stretched wings: he could have also added some of the body, since the swan's neck is (and must remain) thin and flexuous.

If we turn our sheet and cut it all to the left, we can see the distances of the stars that I have inserted in the diagram: here there is a star at just 12 light years (61 Cyg, which we will talk about in another article), then there are 4 under 100 Cyg, τ Cyg, Giena and μ1 Cyg) then others up to 1000, others up to 2000 (35 Cyg), one over 3000 (Deneb), one over 4500 (σ Cyg) and one no less than 5200 al (the variable KY Cyg). So compared to the other constellations seen so far, there are a few stars near and a lot of stars really far away: the farthest of this group of stars is also very particular because it has a really huge size. Just think: 1420 times the Sun's ray and therefore almost twice the size of the monster that until now seemed unbeatable to us, the wonderful Antares, in Scorpio. But... it doesn't end here: in fact, in the study of the constellations we will meet even bigger stars! And in the initial summary I said that this is a particular constellation: it is since in it we find another ten or so huge stars, of which we will see a lot of photos. Meanwhile I tell you that (and perhaps it is a record), the three stars α, β and γ, the brightest of the Swan, are also monsters. It is a real saraband of giant stars never seen before: at most we had found a couple of them and who knows that in the next articles we will not meet other constellations with so many stars so big.

The name, history and myth of the Swan

The representation of the stars of the Swan in the form of a bird has previous origins. It is thought that the constellation was originally known in Mesopotamia as Urakhga, the antecedent of the Arabic Rukh, known to us as "Roc", the gigantic bird that appears in some novels of the voyages of Sindbad the sailor, a fictional character of The Thousand and One Nights inspired by the stories of the Baghdad merchants.

In his second trip Sindbad comes across a huge egg of Roc with a circumference of 50 paces. When the parent bird returns to the place where it laid the egg, Sindbad clings to its claws and is transported to the Valley of Diamonds, from where it returns home with a large booty.

In some Arabic representations the group of stars would represent a simple hen.

Many Greek tales tell of men turned into swans. The constellation of the Swan is linked first of all to the vision of an undefined bird, and only secondly to the swan. Like many other constellations, this one is finally connected to the infidelities of the god Jupiter who, in order to convince the beautiful wife of the king of Sparta Tindaro, Leda, turned into a swan. In this way she managed to 'mate' with Leda, who gave birth to two eggs. From the first egg were born Castor and Clytemnestra, from the second Pollux and Helen, the future wife of Menelaus who was the cause of the Trojan War.

According to other versions, it was Leda who transformed in swan, being a creature able to change its appearance, in order to escape Zeus, who, seeing it in those features, decided to take them in turn to possess it anyway.

Another mythology sees the Swan as a friend of Phaethon, son of Apollo, killed by Zeus for burning the fields. Phaethon's body fell into the river, and his friend plunged repeatedly into the vain hope of finding him. The behavior of this boy moved Zeus to compassion: he turned him into a swan to facilitate the work of recovery in the water.

Information about stars

At this point I would like to make a technical clarification: all the information I provide is usually taken from official sources on the Internet (SIMBAD, which I have already mentioned several times). But among these data the dimensions of the stars are not found, since they must be calculated in a rather complex way according to the physical characteristics of the single stars. All this is already done by Celestia, whose programmers have always set themselves the goal to be as scientific and adherent to reality as possible, except to correct any errors or make updates based on new discoveries and reports coming from the forum. So the numbers, which I report, may be slightly (and in some cases quite a lot) different from those found in other sources. Just think that Celestia's creators don't just need a certain information to be reported on Wikipedia to be accepted: they try to check it on more than one authoritative source (according to certain criteria) and only after confirmations are inserted in the program. Since these creators are a pool of scientists and great experts in Astronomy, Astrophysics, Cosmology, Star Dynamics, Planetology, etc., I would say that you have to absolutely trust them and that's what I do very willingly. Beyond the mere figures, what counts is the substance behind certain information: for example, having found so many giant stars in a single constellation was also an absolute novelty for me!

Ancient and modern representations

As usual we see together how the Swan was depicted in the Uranometria, according to Hevelius and how it is represented more recently by Stellarium.

In all three cases practically the same design, but it is obvious, since it is a very common bird, not mythological, unless we think of it as one of the many transformations of Jupiter in order to be able to concuperate its beloved (in this case Leda). In front of a constellation that represents a slender bird, vice versa we would never have thought about the many enormous stars that find place in it.

The table of 11

In this table I have reported the stars having an important size, inside the constellation of the Swan (Ro=sunray): some of these stars were already among the brightest (and therefore they had a classical name, with Greek letters), while others I chose them at random in the navigator "Celestia" and therefore it is not excluded that there are some others that can enter in the ranking...

We are ready to travel in the cosmos with our spaceship Celestia: we have to visit 11 very big stars that we will then compare both between them and with the stars known so far. From these photos you can already understand that we are really in front of really big stars, since I chose as common distance the 10 UA that represent Saturn's distance from the Sun: from this planet our Sun is very bright (magnitude about -22), but it has a diameter of little more than 3′ (first of arc). For each star we analyze now, click on the corresponding link, maybe opening the photo in another window to be able to observe it while reading the text.

  • KY Cyg : as I said at the beginning of the article, among the constellations visited so far, it is the biggest star, a red supergiant, that from 10 UA covers 47° of sky, occupying almost completely the frame. In this other photo we see it from more than 600 UA: the star has a radius of even 6.6 UA: we will see after that if it was at the center of the Solar System it would incorporate inside it the orbit of Jupiter. That's amazing!
  • 47 Cyg : it has a radius of 1 UA and that is the distance of the Earth from the Sun, absolutely unimaginable. It appears as a 10° disc, practically the angular distance in the sky between Deneb and δ Cyg.
  • ο2 Cyg, Sadr (γ Cyg) : only slightly smaller than the previous star, with a diameter close to the Earth's orbit. Sadr has a blue coloration, definitely more annoying for our eyes.
  • χ Cyg : the above considerations apply also in this case, except that the star is definitely redder and is also a so-called variable star of Mira Ceti type. It is a variable that in a period of about 400 days changes its brightness from a maximum value of magnitude 3.62 (therefore visible also in cities not too illuminated) up to a minimum of 14.40 (this time visible only with medium power telescopes): this variation is one of the biggest existing.
  • ξ Cyg : we are decreasing in size and now the apparent diameter is only 7°, but still incredibly huge, so much that it would almost reach the orbit of Venus, if placed in the center of the Solar System.
  • Deneb (α Cyg) : It is a bluish-white star, shining in our sky with a remarkable magnitude of 1.25, considering the fact that it is much more than 3000 light-years away from us.
  • 63 Cyg : practically as big as Deneb, but definitely more orange and absolutely less known because of its low luminosity (4.55).
  • 19 Cyg : we have fallen below 100 times the sun's radius, but think how big a star can still be with 91 times the radius of our Sun!
  • Albireous (β Cyg) : this star is part of a pair of stars very beautiful to observe with a telescope, even of small power. It shines a beautiful orange colour, while its companion is decidedly bluish-white. Small personal note: after 40 years that I called it Albiréo, I knew that the correct pronunciation is Albiréo. I still have to get used to it.
  • 35 Cyg : in this ranking of greatness is the taillight, but it still defends itself very well, being almost twice the size of Aldebaran.


As it is now customary, at this point I propose the comparison between the stars of the constellation examined compared to the others analyzed in the previous episodes: I already had to thin out the comparison stars leaving only the biggest ones. All this because this time I had to insert 11: KY Cyg I couldn't draw concentric at Betelgeuse and Antares because it didn't fit in the sheet, while the others I had to superimpose them all. I hope that the diagram can still be readable: you can see how Rigel and Aldebaran, compared to the Swan's stars, have definitely resized...

Instead in this other diagram we can compare the immense size of the star KY Cyg with the orbits of the planets of the Solar System and with the usual Betelgeuse and Antares: here is that the Swan star, if it were placed at the center of our Solar System, would encompass all the planets up to well beyond Jupiter, leaving Saturn as the first uncovered planet. To think that there are even bigger stars makes one realize even more the immensity of the cosmos.

A correction on the fly

You know very well that for my articles I use two very well-known and powerful programs, Stellarium and Celestia. Both use (quite obviously) the data provided by the Hipparcos star catalog, currently the non plus ultra in this field. This catalog contains the data of more than one hundred thousand stars of which the Hipparcos satellite has calculated the parallax and therefore the distance.
Starting from this data, using the calculation algorithms used in Celestia by means of a Java program and an Excel board, I created a large table containing for each star also the data of its ray, compared to that of the Sun: you know very well that this is information on which I draw many cues for my articles. Now that I have all this data, I no longer have to click in Celestia on the stars of a certain constellation to get the ray information: it was a hard work that involved errors or forgetfulness...
In fact from my table I immediately found in the constellation of the Swan an even bigger, enormous star, the variable P Cyg (34 Cyg) that has a radius of 1900x compared to the Sun! I added it to the comparison diagram between the stars and in the diagram of the Solar System, where it would almost reach the orbit Saturn: on this star I add that it is of stellar class B2 (like Bellatrix of the constellation Orion) and therefore its light is decidedly bluer (and annoying to our eyes) than that of the other monster...

Not only monsters...

... but also stars more normal if not smaller than the Sun: in particular we find 61 Cyg, one of the closest to us, just over 11 light years away and with a radius (finally!) equal to 65% of the Sun's radius. This is a double star that we will analyze in more detail when I talk about the stars closer to the Earth, in an article in the category of virtual travel. But since it is the closest to us in this constellation, it deserves a trip to its parts to photograph it and then look back and see our Sun, to understand where it is in the sky and what magnitude it has.

In one of the rare photos where the Sun is visible in the center of the photo, here is our star of magnitude 2.55 in a decidedly austral sky area, where we find Rigel Kentaurus (better known as Alpha Centauri), Suhail, Regor and Naos (respectively λ and γ Velorum, that is the constellation of the Sails and ζ Puppis, that is the Poppa, always of the ship of Argos), while on the top right dominates the brilliant Sirius of the Big Dog).

I had talked about the star θ Cyg at a distance of 61 light years, as one of the four stars of the Swan nearest to the Sun: after 61 Cygni it is the only one from which the Sun (now of magnitude 6.1) appears in a zone of sky full of stars, unlike the other two nearby stars, from which the Sun appears in the so-called cosmic nothingness. From the photo we can see that the Sun is in the company of stars of our southern hemisphere to which we can add a couple of famous intruders, absolutely out of position, that is Vega of the Lyre) and Altair of the Eagle): this intrusion is due once again to the fact that all the stars are to be thought placed in their correct position inside a three-dimensional space and not stuck to the celestial vault, apparently all at the same distance.

Many giant stars...

...but few names: that's the meaning of stars that have received a name:
  • DenebCyg): from the Arabic hen's tail, which the Arabs saw instead of the swan.
  • AlbireusCyg): name that has no meaning and is the result of misinterpretations by the amanuensis of other names assigned to the star and the constellation.
  • SadrCyg): from the Arabic breast, referring to the hen's breast
  • GienaCyg): from the Arabic wing, it derives the name from another star of the Raven, also representing the wing of the bird
  • Azelfafage (π1 Cyg): another erroneous transliteration of a name that the Arabs had given to this star. Definitely one of the most likeable names, like Sadalmelik of Aquarius and Menkalinan of Auriga.

When to observe her?

The constellation of the Swan is typically visible in summer and autumn: at 9 p.m., a convenient time to make observations, it is visible in the months from June (when it will be seen low on the horizon at NE) until January of the following year (when it will be about to set in NW this time). The peak, with the Swan in the South, is between September and October, when at that time and at our latitudes it will be at its zenith: beware of the wrynecks always lurking!

Audio Video The constellation of the Swan (Cygnus - Cyg)
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