It is a small constellation that is easily recognizable even in the city, thanks to the bright Arturo that allows easy identification in the sky. Once in a while we have in front of our eyes an unmistakable constellation formed by 7 stars arranged in the shape of a diadem with a beautiful bright star in the center, to represent the precious stone, and the others well visible in the bright city sky.
According to the parameters chosen by me to classify the constellations and talk about them in this series of articles (in particular the number of nearby stars, the number of the biggest ones of a certain value and the apparently very low number of deep sky objects), the Boreal Crown is at the bottom of the list. But we know that for us fans any constellation is always fascinating! The easy recognizability makes it jump forward of many positions ... and then we know that Nature has many surprises in store!
The name, history and myth of the Northern Crown...
Persians and ancient Arabs called the constellation of the Boreal Crown the "Dervish Tray", or "the bowl of the Elemosina", or for the incompleteness of the circle of its stars, the "Broken Tray".
Note therefore since very ancient times - references are already attested in the Odysseus - the legend from which the name is taken is not only one but all lead to the character of Ariadne.
According to the first legend, Dionysus gave a crown to Ariadne, the work of Hephaestus, for the wedding, but when the woman died, the god took the crown and threw it into the sky.
According to another legend, Ariadne (daughter of King Minos) is the heroine who helped Theseus defeat the Minotaur, Asterio, and escape from the labyrinth thanks to her famous thread. To repay him, Theseus took Ariadne and took her with him to the island of Naxos, but he abandoned her there (hence the expression 'Dropping out') during the night. Ariadne cried so much that the god Dionysius came down to the island and married her, giving her the crown as a pledge of love.
And in another interpretation, the constellation represents the golden thread given by Ariadne to Theseus to guide him through the labyrinth.
Actually, a third legend sees in the asterism a bowl, rather than a valuable tiara and precisely because of this story Gemma, the brightest star, is also known as Alphecca, which means just bowl.
The ancient Greeks and Romans, instead, saw in the constellation the laurel wreath offered to athletes and military commanders.
In Uranometry it appears more like a garland
while according to Hevelius
and according to Stellarium
is represented as a royal crown.
Three stars of important size
Having already said that no star among the closest to the Sun is present in this constellation, on the contrary, from the comparison diagram with other bigger and better known stars encountered during the various episodes of the column, we see that there are three of them that immediately jump to the eyes.
The largest of the three is μ CrB, a red giant of spectral class M2 with a diameter equal to 82 times that of the Sun: it is very difficult to imagine how majestic and disturbing can be a star 82 times larger than our Sun and then in our help comes the photo taken with Stellarium from a distance of 10 UA (that of Saturn from the Sun). As a comparison, on the right side I added just the Sun as you can see on the planet with the rings: really impressive!
Only slightly smaller is the other red supergiant, ν1 CrB, also of spectral class M2 and with a diameter equal to 76 times the solar one.
The last of the trio doesn't joke about size either: it is the supergiant ν2 CrB of spectral class K5, with a diameter equal to 50 times that of the Sun. Together with the star mentioned above, it forms a pair of visual stars (visible with good binoculars): the two components ν1 and ν2 are respectively at 640 and 594 al from the Sun and Celestia confirms that it is not a binary star since the distance between them is 46.7 al.
Moving on as usual to the factions, I only add that the photo of the biggest star of the Boreal Crown was sent to me by my Microbes friends, who, despite the name, have a disproportionate amount, so much so that they wear (you look at the cases of nature) exactly the number 82.
Deep sky objects
For the Boreal Crown we have only one photo, taken by the fantastic Hubble Space Telescope (HST): this fact could make mistakenly think that this constellation is small and insignificant compared to the others, for some of which vice versa I had published dozens of photos of splendid galaxies, star clusters and nebulae.
And no! Analyzing better the photo we realize that what we see is not just any cluster formed by stars, but a very particular cluster, called Abell 2065
the Abell 2065 Galaxy Cluster
instead of containing a set of stars, we can ascertain that this cluster consists of more than 400 galaxies, all in just 1° square of sky. Let's reflect for a moment on this small difference! While in a star cluster each bright spot is a star, in this case the bright spots are galaxies!
As I said at the beginning this is a pleasant surprise: try to count how many objects (mainly elliptical) you can see in the picture! Can you imagine how many stars are actually present (even if not visible!) in this picture?
Once again Nature shows us some absolutely unpredictable wonders!
Names of stars and visibility
Among the stars of the Boreal Crown, only two have received names, of which the first is quite known.
- Alphecca (α CrB): the light of the broken ring, also called with the Latin name Gemma
- Nusakan (ß CrB): the two series
As far as the visibility of the constellation is concerned, at the usual convenient time of 9 p.m., it is low on the horizon, to the North-East, in the first days of March, culminating in the South in the second decade of July, and then low on the horizon, to the North-West, at the beginning of October.