The constellations

Constellations

A constellation is a group of stars of the vault heavenly which enough close relations looked the appearance imagination of a figure onto the sky. A constellation is thus a asterism astronomy, an asterism is a remarkable figure drawn by particularly brilliant stars.  particular. In the sky, the stars of a constellation are very distant from each other but appear grouped into figures, so that the constellation has no proper distance. The western constellations are grouped together in two left, dividing the sky by following more or less both hemispheres of the earth, southern sky for the South and the boreal sky for the North.
The boreal constellations are the most the former and correspond to the plan of visible sky since the regions of the Mediterranean Sea by the astronomers of the Antique. The southern constellations were not named by the western astronomers before at least the XVth century. At present, the international astronomical Union ( UAI) divides the sky into 88 official constellations with precise borders, so that any point of the sky belongs to a constellation.
The man is fascinated by this cosmic decoration which represents the sky observed except any artificial lighting. Stars seem quite hung on on the same distance of the earth.
The sky changes day and night all year round. At first sight the sky is strewed with thousand brilliant and twinkling stars.

In theory we could see at one go of eye 3800 stars, really 2600 only are visible because of the lights parasites. It is the Earth which creates the horizon cutting in two the heavenly sphere.
In the pole all the stars turn around the top, at the same time as the horizon.
In the equator all stars get up and go to bed, the heavenly poles are on the horizon. From then on all the stars from a pole to the other one are visible at the certain moments of the year.
In average latitudes, certain stars are circumpolar A heavenly object (typically a star) is said circumpolar with regard to a given place of observation if it is visible in quite the periods of the year. In the north hemisphere, a star disappears under the horizon when the sum of its declension and the latitude of the place of observation is lower than 90 °. The 'circumpolar' notion is thus connected instead of observation. So, in the North Pole, all the stars of positive declension are circumpolar, while there is no circumpolar star in the equator. A constellation is said circumpolar if the totality of its main stars are circumpolar.  or always above the horizon while the others get up and go to bed.
Stars differ by their color, their brilliancy and their luminosity. The most brilliant stars are of magnitude 1. An observer can see stars hundred times weaker, that is of magnitude 6. Every star has a characteristic color according to its temperature:
Red (Temperature 2000° Celsius) as Ras Algethi, Proxima Centauri, Bételgeuse, Antares.
Orange as Alpha Centauri, Aldebaran, Arcturus.
Yellow (Temperature between 4000 and 6000° Celsius) as the Sun, Vindemiatrix.
White as Procyon, Sirius
Blue (Temperature between 40 000 and 100 000° Celsius) as Vega, Rigel.

It is at first the most brilliant stars which draw the attention of the observer. The Greek mythology left us in the sky of the images formed by imaginary lines that we call constellations.
These constellations have names according to their forms. These groupings of stars have no link with their distances with regard to the Earth. There are southern, equatorial or boreal constellations. Those who are observable of the north hemisphere are the equatorial and boreal constellations.
It is easy to recognize the said constellations circumpolar: it are the ones that we can observe in any seasons. These constellations are constituted by the Great Bear, by the Small She-bear, by the Dragon, by the Giraffe, by Cassiopeia, by the Lynx and by Cephee.

Image: By agreement the names of the stars of a constellation are prefixed by a Greek letter followed by the first 3 letters of the constellation, the example: α Ori, β Ori,... (Greek letters: α β γ δ ε ζ η θ ι κ λ μ ν ξ ο π ρ ς σ τ υ φ χ ψ ω).

The Great Bear constellation (Ursa Major)

One of the biggest and of the easiest to identify among the constellations circumpolarA heavenly object (typically a star) is said circumpolar with regard to a given place of observation if it is visible in quite the periods of the year. In the north hemisphere, a star disappears under the horizon when the sum of its declension and the latitude of the place of observation is lower than 90 °. The 'circumpolar' notion is thus connected instead of observation. So, in the North Pole, all the stars of positive declension are circumpolar, while there is no circumpolar star in the equator. A constellation is said circumpolar if the totality of its main stars are circumpolar. . 
The Great Bear formed by 8 stars is of which a double (Mizar and Alcor), Alkaid (η UMa magnitude 1,87 to 100,70 AL), Mizar (ζ UMa magnitude 2,25 to 78,16 Al) and Alcor (magnitude 4,1 to 81,15 AL), Alioth (ε UMa magnitude 1,78 to 80,93 AL), Megrez (δ UMa magnitude 3,34 to 81,84 Al), Phad (γ UMa magnitude 2,43 to 83,65 Al), Dubhé (α UMa magnitude 1,83 to 123,64 Al) and Merak (β UMa magnitude 2,36 to 79,42 Al). We also find in the Great Bear two important galaxies carrying the numbers M81 and M102 in Charles Messier's catalog. According to the Greek mythology, this constellation would represent Callisto, a nymph was loved by Zeus.

When Héra, the wife of Zeus, discovered their relation, she changed Callisto in Great Bear and her son Arcas in Small She-bear.
Offended by this insult in her honor, she asked for justice in the Ocean, and the she-bears were then condemned to turn perpetually around the North pole, never authorized to rest.
The Great Bear is at the origin of the "northern" term. The Roman called this constellation septentriones (seven beefs of plowing).
In the United Kingdom, we call it the Plough ( the plow), in the United States of America, the Big Dipper ( the big spoon), in Scandinavia, Karlavagen (Charles's car, probably Charlemagne), in the Indian astronomy, Sapta Rishi (seven wise men).

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The Small Bear constellation (Ursa Minor)

The Small She-bear is formed by 7 stars and the constellation has a shape looking like that of the Great Bear but in smaller.
The Small She-bear is a weakly brilliant constellation, which owes its celebrity to its most brilliant stud, the polar huge Star or Polaris (α UMi of magnitude 1,99 to 431,43 Al).
Other considerable stars of the constellation are Kocab (β UMi of magnitude 2,09 to 126,47 AL), Pherkad (γ UMi of magnitude 3,02 to 480,35 AL). This constellation gave several words.

The 'arctic' word which comes from the Greek word ' arktos ', 'bear', and the word 'septentrion' which indicates seven stars of this constellation.
A myth according to which the constellation would not be a bear but a dog, the Pole star is sometimes named cynosure, 'tail of the dog', term also meaning 'object of interest' in English.
The small bear finds a way generally with regard to the Great Bear. 

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The Dragon constellation ( Draco)

The Dragon is one of 88 constellations of the sky, the eighth by the size. it consists of a long suite of stars which goes along a part of the Small Bear.
Between both She-bears winds the tail of the constellation of the Dragon.
Only 4 of its 12 stars are rather brilliant. The main star of the constellation of the Dragon is called Thouban (α Dra of magnitude 3,69 situated to 308,86 AL). Other stars of constellations are Rastaban (β Dra of magnitude 2,81 to 361,60 AL), Etamin (γ Dra of magnitude 2,26 to 147,58 AL), Altais (δ Dra of magnitude 3,09 to 100,23 AL), Edasich (ι Dra of magnitude 3,31 to 102,18 AL), Giansar (λ Dra of magnitude 3,84 to 334,18 AL).

In the Greek mythology, the constellation would result or from the dragon which attacked Athéna in the war between the gods of the Olympe and the Titans, or of the dragon killed by Cadmos near the place where it based Thebes, or of the one who guarded the Golden fleece either still of the dragon Ladon which guarded the golden apples of the garden of Hespérides and was killed by Héraclès.
The constellation of the Dragon surrounds the Small She-bear in the direction of the star Vega. The head of the Dragon is situated between Vega by the constellation of the Lyre and the Small She-bear.

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Cassiopeia constellation

Opposite to the constellation of the Great Bear, the other side of the star Polaris, we see a big "W" easily recognizable which bounds in the Milky Way, 5 stars of the constellation of Cassiopeia.
The main star of the constellation of Cassiopeia is called Shedir (α Cas of magnitude 2,26 situated to 228,56 AL).
Other stars of constellations are Caph (β Cas of magnitude 2,30 to 54,46 AL), γ Cas of magnitude 2,17 to 613,08 AL), Rucnbach (δ Cas of magnitude 2,68 to 99,41 AL), ε Cas of magnitude 3,84 to 334,18 AL. Cassiopeia is one of 88 constellations of the sky, visible in the north hemisphere.
Contrary to the Great Bear with regard to the Small She-bear.

The central point of W clocks towards the Pole star. Originally considered by Ptolémée during the editorial staff of its Almageste, the constellation represents, in the Greek mythology, queen Cassiopeia chained to its throne, woman of Céphée and mother of Andromeda.

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Cepheus constellation

Between the constellation of Cassiopeia and the Dragon is Cepheus.
This constellation is not very visible but it reminds child's drawing representing a small house with a roof. The main star of the constellation of Cepheus is called Alderamin (α Cep of magnitude 2,47 situated to 48,80 AL). Other stars of constellations are Alfirk (β Cep of magnitude 3,25 to 595,18 AL), Roamed (γ Cep of magnitude 3,23 to 44,99 AL).

Cepheus is a weak constellation, his wife Cassiopeia is much more brilliant.
In good conditions of visibility we can find the general shape, the sort of rectangle marking the bust of Cepheus with a petit point glittering in the middle, surmounted by a sort of sharp hat directed to the pole star (α UMi).

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