The constellation of Cassiopeia is easily recognizable and therefore can be found in the sky at any time of the year: beyond the letters "W" or "M", no one ever thought that it could be seen as a stylized "3" or as a letter "Σ", well known to engineers and mathematicians, as well as a stepladder made by a couple of steps. Mind you: anything but a female figure! In the intentions of the ancient Astronomers, it is instead Cassiopeia, the mythical bride of the King of Ethiopia Cepheus (who happens to be looking at a constellation at her side) and mother of Andromeda (another constellation nearby): recognizing the figures of this cheerful little family requires a lot of imagination but at least recognizing one of them, the other two are "free". Since we are here, let's immediately see how the ancients interpreted this sequence of five stars.
The name, history and myth of Cassiopeia
Mythologically, Cassiopeia was the wife of the King of Ethiopia, Cepheus. For boasting of the beauty of her daughter Andromeda, Cassiopeia sent the daughters of the god Nereo into a rage. One of them was married to Poseidon, who - informed of the fact - decided to punish Ethiopia by sending the monster Cetus (the whale).
Cassiopeia as a punishment for her vanity, was sent to heaven by Poseidon, but in a singular and somewhat indecorous attitude and forced to rotate around the North Star forever.
In the words of the Greek poet Aratus (3rd century B.C.) it is quoted as follows: "she no longer shines on a throne... but throws herself, like a diver, upside down, with her knees in the air".
This dive refers to the circumpolar movement so the constellation goes up and down in rapid succession.
According to a variant of the myth, Cassiopeia was placed inside a basket, which rotates and throws her inside, forcing her into completely clumsy poses, appearing ridiculous when she shows upside down.
And again, in a distinct version, the punishment was administered to her by Athens, who placed her figure in the sky so that, due to the celestial rotation, she would come to visit for long hours upside down and her robe would fall down, discovering her intimate parts, mortifying her.
She regains some dignity in the representations reported in the European star maps of the 13th-17th centuries, in which she is depicted as a mature woman, sitting composed on a chair with a mirror in her hand raised to the height of her face, through which she contemplates her beauty.
Cassiopeia in antiquity
Here we see the representation of the constellation according to Uranometria and according to Hevelius. In both cases I turned the drawing so as to see the queen "straight", sitting on a throne, but still very restless.
Let's get back serious
Let's go now to analyze our rotating and three-dimensional map of the constellation, in which once again stands out this strange succession of zig-zag stars. Just going to look behind the map, we discover (and I will never fail to underline it!) that the 5 main stars of Cassiopeia are all less than a physical group of stars: you can easily see that they are respectively 442, 99, 613, 228 and 54 light years away from us. Perhaps if even the ancients had had the feeling that the stars are at very different distances from us and between them, maybe they would have thought of the constellations in a different way recognizing them as random groups of stars: maybe astrology would not have been born, based on erroneous foundations among which the randomness of the arrangement is certainly the most striking. But let's go back to true science...
Even in this case, placing the cut sheet on the left, no apparent physical grouping appears: there are two or three nearby stars (Achird at 19 light years, μ Cas at 25 and Caph at 54), while the others are anyway scattered in the cosmos, with ρ Cas down there at the bottom at an astronomical distance of more than 11000 light years. We already know that the Rocassiopeian Astronomers do not even know about the existence of the Sun, a tiny star of 17.59 but magnitude: given the distance it is also useless to send them messages or emails to signal our presence ... Think about how bright this star must be to appear fourth in our sky: then we will see that it is in fact a monstrous hypergiant star!
Also in this constellation we meet (and we will see closely) three very big stars: let's start from the smallest one, just 130 times the radius of the Sun and that is 0.6 UA, something less than the orbit of Venus. Seen from 10 UA (Saturn's distance from the Sun) the variable star V509 Cas appears the same colour as our Sun, which we had never met before, but much brighter and bigger (6° and a half).
The second celestial monster is another variable, R Cas, which from its name tells us that it is the first variable star discovered in Cassiopeia: it is a variable of type Mira Ceti and that is a star that changes its brightness over a long period of time, between very different values. In this case, R Cas changes its brightness from a maximum of 4.7 (visible to the naked eye in a city not too bright) to a minimum of 13.7 (therefore visible only with good telescipo) in a time equal to 431 days. This star has a radius of 340 times the solar one, practically as much as the orbit of Mars and therefore if it was placed in the center of the Solar System it would incorporate everything up to the red planet, Earth included.
The third godzilla of the constellation of Cassiopeia is the star ρ Cas, which touches a little bit the thousand sun rays, touching the frightening value of 950 times the ray of our very small Sun: in this case from 10 UA the star is remarkably threatening, dazzling yellow, with a diameter of more than 35°. Let's think for a moment about the values at stake: this star has a diameter that is 950 times that of the Sun. How far do we have to go to see this monster the size of the Sun, with a radius of 31′ ? I'll give you 10 seconds to think about it, then I'll answer with another photo comparing the two stars.
The 10 seconds have passed and the answer is obvious: we must put ourselves at 950 UA from this stellar monster. Let's think together: the Voyager I probe, launched in 1977, so 34 years ago, after having explored Jupiter and Saturn is moving away from the Solar System and at the date I write (end of April) it is 117 UA from the Sun. The distance we have to place ourselves from ρ Cas to see it as big as the Sun is currently more than 8 times the distance travelled in 34 years by NASA's probe: as if to say that to reach that distance, the probe will take more than 8 times as long, almost 270 years, year less. Terrifying! And from that distance this star monster is practically as bright as the Sun, with a -25 compared to -26.
A reflection after so many figures: I find that this kind of analysis makes us understand more and more how varied and wonderful the universe is, with surprises around the corner, hidden behind apparently anonymous dots. Among other things, I confess that much of the information I provide is totally new for me too, a 40-year Astronomy enthusiast: all this thanks to the fact that I think of the stars as objects endowed with their own size and immersed in a boundless three-dimensional space. You know that I am very fond of astronomical distances and I smile when I hear about the possibility of interstellar travel (other than the science fiction ones of Star Trek & C). Do you have any idea how many light years correspond to the famous value of 950 astronomical units? Ten? A hundred? Three hundred and fifty? No, no, that's just fifteen thousandths of a light year, that is 5 and a half light days, which correspond exactly to the space traveled by light in a 5 and a half day trivia.
Five and a half days compared to 270 years... A big difference!
There are even normal stars!
In this whirlwind of astronomical figures it almost seems that the other stars of Cassiopeia are much less interesting: but for once let's leave aside the brightest stars of the constellation, they have been talked about since ancient times awakening images of queens with more or less long feet. Of these 5 main stars I only add that among them there are some little monsters, Tsih 62 times the Sun and Shedir, 42 times our star: we will find them in a little while in the diagram where I make the graphic comparison of the sizes of various stars.
I spoke earlier about the three Cassiopeian monsters and how they would take their place within the Solar System. As always I made a diagram (according to the logic that a drawing is worth more than many figures) in which I placed the monsters that we meet, exactly in the place of the Sun: so here is the red R Cas that you eat everything beyond the orbit of Mars and the yellow ρ Cas that instead abundantly exceeds the sacred monsters Betelgeuse and Antares. But still the star KY Cyg seems unreachable... Will any star be able to overcome it?
For now we take a little stroll around the parts of that star that is the closest among those of Cassiopeia, with 19 light years and a diameter of (hear hear) 1.1 times that of the Sun (Achird A), practically its twin, accompanied by a star half the size (Achird B) and placed at a ridiculous distance of just 88 UA. From this small distance of 19 light-years, the Achirdian astronomers (A or B, you do it) know well our star, of 3.7, which certainly does not disfigure inside the Southern Cross, which probably over there they call Dus led Ecorc. Have you noticed that all the extrasolar astronomers, that we have met so far, all have the same funny way, opposite to ours, to call the objects of the sky?
I add that from the other two nearby stars (μ Cas and Caph) the Sun is obviously less bright and also for the astronomers Mucassidi and Caphiani the star Elos belongs to the same constellation with the inverted name.
In this other diagram instead I made the usual comparison between the most famous and significant stars encountered so far with those seen in this episode: there is to reiterate the concept that even the shining Rigel is increasingly going to hide in front of stars that make her literally eat dust, like V509 Cas which is almost double, not to mention R Cas of which we already know everything. Below I've added the main stars of the constellation and let's say that in lean times they too would make their figure: I have the feeling that in the next diagram we'll find a few of them... we'll see. What about the Sun? I remind you that the dot is a poor homage to our star, which in scale would actually be much smaller, practically invisible.
Let's do the roll call
Here is the meaning of the stars that have a name, but that perhaps is not used much: in this case just point up there the 5 stars of Cassiopeia, definitely difficult to remember.
- Shedir (α Cas): from the Arabic, the queen's chest
- Caph (β Cas): from Arabic, the stained hand of the Queen
- Tsih (γ Cas): from Arabic, the whip (of the queen? boh!)
- Ruchbah (δ Cas): from the Arabic, the queen's knee
- Segin (ε Cas) and Achird (η Cas): quoted from various parts, but without explanation about the origin of the name
- Marfak (θ and μ Cas): from the Arabic, the queen's elbow
When and where is it visible?
The answer is simple: Anytime, anywhere. On the opposite side of the Big Dipper to the polar star, it should be easy to find and recognise this circumpolar constellation later on.