Costellazione Orsa Minore, caratteristiche, stelle principali e mitologia
Per noi uomini moderni dotati di smartphone, navigatori e prima ancora dell'ausilio di una bussola, il Nord non è mai un mistero; ovunque ci troviamo basta buttare un occhio alla strumentazione. Nonostante ciò la stella Polare e l'Orsa Minore per l'importanza rivestita nei millenni scorsi, ancora oggi rappresentano i corpi celesti più segnificativi per l'Umanità stessa.
For thousands of years they have provided orientation for all travellers, especially sailors and caravaners in the deserts, where there are no points of reference.
Ursa Minor is a constellation typical of the far north of the northern hemisphere. Visible throughout the year and timeless, it is a scale version of the Big Dipper. Unlike the latter, it is composed of stars that are not very bright and cyclical, that is, their brightness increases and decreases in certain periods. It is therefore much less visible than the Big Dipper in terms of size and intensity of light, but the brilliance of the Polar Star and its proximity to the Big Dipper still allow it to be immediately identified.
The main stars of Ursa Minor
The main stars that represent it are:
- Polaris (α Ursa Minoris), the North Star. It is the brightest with a magnitude of 1.97 and is a yellow star. It represents the tip of the bear’s tail and is considered the extension of the earth’s axis, being perfectly coincident with this. The ancient Chinese named it the “Magnetic Star” having noticed a correlation between it and the compass needle perpetually oriented to the North. To easily identify it even on nights that were not perfectly crystal clear, we relied on the greater visibility of the Big Dipper. Extending the imaginary line of the shorter side of the big dipper, after about five lengths the Polar Star is intercepted. It is 431 light years from Earth and is a double star composed of Mv 2.1 which is the primary component, very bright and yellow in color, while the secondary companion Mv 9.1 is white but much less bright. The two are separated by 18.2 arcseconds and a position angle of 321 degrees. A medium-sized telescope should be enough to distinguish the doubleness and resolution of the two stars. This is used as a reference for testing small achromatic telescopes.
- Kochab (β Ursa Minoris) is the second brightest object in Ursa Minor. It is located in the opposite position to the Polar Star, it is an orange giant of the K4III class, it has a radius 42 times that of the Sun and is 390 times brighter. It is 131 light years from Earth and is considered the former Polar Star, in fact due to the Earth’s precession motions it once had to be positioned exactly on the Earth’s axis, like the Polar Star today. As proof of this it must be said that its name probably derives from the Arabic “Al Kaukab al Shamaliyy” which literally means “Northern Star”.
- Pherkad (γ Ursa Minoris) of magnitude 3.0 is 480 light years from Earth and is a white Delta Scuti variable star; its brightness also varies by 0.05 magnitudes in 3.43 hours and in an irregular manner. It is easily observable with the naked eye only at high latitudes, being a circumpolar celestial body at 18° N. It is classified as A3lab, i.e. it is an intermediate supergiant but according to other classifications it would be an A3II-III, i.e. halfway between a giant and a giant brilliant. It is approximately 85 times the Sun and rotates at a very high speed (170 km/sec). It is a binary star and pairs with Pherkad minor or 11 Ursa Minor with which it shares the orbit. The chromatic contrast of the two allows them to be identified even with small telescopes, one being a white star and the other orange.
Pherkad and Kochab are called “The Guardians of the Pole”.
- Yildun (δ Ursi Minoris) is like the others a peripolar boreal star, its name derives from Turkish and simply means “star”. Fairly visible even in the presence of partial haze, being of magnitude 4.3, it is the closest to the Earth’s axis other than the Polar Star, from which it is only 3.5°. It is a white main sequence star of class A1vn, it is 2.8 times larger than the Sun and 47 times brighter. It has a rotation speed of approximately 180 km/sec, compared to the Sun its complete rotation lasts 19 hours while for the Sun it takes 26 Earth days.
History and Mythology of Ursa Minor
There are no further particularly interesting celestial objects within Ursa Minor and perhaps also due to its partial visibility, Ursa Minor has not been the subject of particular mythological stories, if not by reflection. In fact, the nymph Callisto, after having joined with Zeus and having attracted the ire of Artemis, was transformed into a bear (the Great Bear). To put an end to the revenge, Zeus transformed everyone into stars but Hera was not satisfied and asked Thetis, the sea Goddess, for help. This placed a curse on all the circumpolar constellations so that they would rotate forever without ever resting, in fact these never set.
It would probably never even have been noticed, in fact the Polar Star was highlighted, ideally prolonging one side of the Big Dipper.
The stars that compose it are not all equally visible and not at all latitudes. It was only thanks to Ptolemy that it was officially included in the list of the 48 official constellations (Telemaic).
Today it constitutes a natural device for classifying night visibility. Being made up of stars with a magnitude that fluctuates between 2 and 5, depending on those visible at a specific time and place, a value is expressed that identifies the clarity of the night sky.