The Zuben family
We have arrived at Libra, a zodiacal constellation (and therefore crossed by the Sun in its apparent annual motion) inconspicuous and difficult to find in the sky, due to the fact that the brightest stars that are part of it are all below magnitude 2. To find it immediately it is enough to exploit its near constellation of Scorpio: its stars in antiquity were part of that constellation (they were the claws) and only recently they have been detached, proof of it is the fact that the names of the main stars begin for "zuben " Arabic term that indicates a "claw". Exactly prolonging in Scorpio the three traits that connect Antares with the claws (β, δ and π Sco), we find with good precision the three stars β, α and σ Lib.
I was saying that Libra is traveled by the Sun on its path: in particular, it enters the constellation around 31 October each year and leaves on 23 November. Thanks to Stellarium I created an image that represents the two indicated moments. Since I hadn't done it, here you can find similar photos of the other two zodiacal constellations we have met so far, Taurus and Scorpio. Obviously these dates should not be confused with the wrong ones and always present in horoscopes and astrology in general.
The name, the history, the myth of Libra
Libra is a mysterious constellation of the Zodiac, to which the only inanimate sign is linked. Its stars have been known since ancient times, although in fact the constellation may not have been autonomous but may have represented the claws of Scorpio. To confirm this primitive belonging there is the name of the main stars of Libra since Zubel el Shamali means 'northern chela' while Zuben el Genubi means 'southern chela'. It was Aratus of Soli, in the III century B.C., to see in the stars of today's Libra the continuation of the constellation Scorpio.
Only from the 1st century B.C. did Libra gain autonomy, and today it is seen as a symbol of Justice: it weighs the souls of men after death on their plates. In fact it would be the instrument used by the goddess of Justice, Astrea or Dike, represented by the neighboring Virgin, according to some myths related to her.
Known since ancient times, Mesopotamian finds (Assyrian era) dating back to 2200 B.C. show a priest holding zibanitu, the scales, understood as the scales of justice.
Actually, Libra is pre-existing to Scorpio and it was not an integral part of it: it became it only temporarily and for few people, if it is true that the alternative names of the two brightest stars are Kiffa Australis and Kiffa Borealis (Southern Plate and Boreal Plate), The constellation Libra symbolized for the Latins the equal length of the day and of the night to the equinoxes: two millennia ago about, in fact, the Sun passing through Libra marked the autumn equinox.
Four jumps on the scales
Thanks to the spaceship Celestia we go to visit some stars: this time the constellation contains human stars as size, not those monsters we have met so far... Today we start with the largest of the Zuben family, Zubenelakrabi (σ Lib), a beautiful star of spectral class M (so definitely reddish orange) 130 times the size of the Sun. In the photo we see it from 10 UA, the distance of Saturn from the Sun, as bright as the Sun, but with an apparent diameter of almost 6 and a half degrees. The photo has a beautiful three-dimensional effect, don't you think?
At the same distance we find α1 Lib, which even with binoculars you can see well, of 5.1 compared to 2.8 of the neighbor. They are a couple of stars just 0.85 light years apart and in this picture we can see both of them, having ordered our spaceship to bring us close to α1 to be able to frame them both: this star is also small, just 1.4 times our Sun.
Another star of the family, Zubenelgenubi (α2 Lib), is a star with a diameter of only 2.8 times that of the Sun: I chose it to have a chromatic contrast with the previous and the next one. Here we have a decidedly blue star (spectral class A), which this time we see from a distance equal to that of Mercury (0.3UA). This is the star at 77 light years, mentioned above.
Another reddish colored Libra star is ν Lib, which with its diameter of 63 times that of the Sun is almost twice the much more famous Aldebaran (which we always bring up when it comes to a bigger star!). We are in the presence of another star decidedly redder than the first and here we see it from the usual distance of 10UA, decidedly big and bright as our Sun from Earth.
Let's compare the stars on the scale
As usual I have reported in this diagram some stars of Libra, in order to compare their dimensions with other stars that I carry with me as the analysis of the constellations continues. In particular I have inserted only the stars of a size greater than 5 times that of the Sun, just to avoid overcrowding and to make the diagram illegible. Is it just me or is Alberaran getting smaller and smaller?! It's better not to talk about the Sun, but who cares...
Would you like to compare earth to earth? You have a small car (a Panda, a Smart car) and you see a Mercedes, a Lamborghini or an SUV with anti-antilope protection... Who cares! In the meantime, your car consumes less. And personally, I wouldn't even change it: I see them there, far away, I know they exist, but I'm keeping mine! So let's enjoy our beautiful Sun and be content to think about what our life would be like if the Earth revolved around another star, much bigger or much smaller than our yellow dwarf.
Zuben the father, Zuben the mother...
... Zuben was his sister's daughter... Zuben was also Zuben...
All right, enough of this nonsense. We already know that in Arabic, Zuben means "chela": certainly a little more imagination wouldn't hurt!
- Zubenelgenubi (α2 Lib): Arabic word meaning southern chela (also called Kiffa Australis)
- Zubenelschemali (β Lib): Arabic word meaning northern chela (also called Kiffa Borealis)
- Zuben-elakrab (γ Lib): Arabic word meaning crab chela
- Zuben-elakribi (δ Lib): can't you imagine?
- Zubenakrabi (ε Lib): I have some suspicions
- Zubenelakrabi (η Lib): I think it means crab chela, but I have to check it out.
Libra is visible at 9 p.m., when it is easier to show it to friends, from April (when we find it low on the horizon, at ESE) until mid-September, when it appears low towards WSW.The culmination in the South, always at that hour or rather an hour later, waiting for a slightly darker sky, is instead in July: we find it as always chased by a menacing Scorpio.