|The term "constellation" derives from Latin constellatio (from cum + stellatus) and it can be defined as a portion of the vault of heaven which groups a certain number of stars.
The tradition to divide the sky into constellations is very old. The Greeks drew forty-eight constellations, twelve of which were crossed by the Sun during the year (they consitute the zodiac, so called because a lot of these constellations represent an animal): every constellation was explained and described with mythological stories. During the centuries, considering that shapes and borders were not sure, the sky cartographers were free to modify the existing constellations and to introduce new constellations.
This mutable and confused situation finished in 1930, year in which the International Astronomical Union established once for all the exact borders of the constellations, which were (and are) eighty-eight. There is not a particular reason for which the constellations must be just eighty-eight or must have such shapes: in fact the stars of a same constellation don't have, generally, any bond among them, but we see them near only because of the perspective. The shapes and the names of the constellations derive therefore from history and from human imagination.
|The declination (Dec.) is the angular distance between a point of the sky and the equator. It is measured in north and south degrees, from 0 degrees at the equator to +90 degrees at the North Pole and -90 degrees at the South Pole.
The other celestial coordinate is the right ascension.
|Looking with the telescope what to the naked eye seems a star, you can sometimes see that is not a single star, but two or more very near stars. It is a double or multiple star.
It is necessary, however, to distinguish clearly two types of double stars:
There are very many double stars: they are practically in all the constellations.
|A galaxy is an enormous stellar agglomerate, containing from some million to some hundred milliards of stars, together with big clouds of gases and dusts. The dimensions of a galaxy vary from about 2.500 light-years to about 200.000 light-years of diameter.
The galaxies are divided, on the basis of their shape, into three classes:
|The globular cluster is a dense aggregation of spherical shape, which can contain hundreds of thousands of stars: toward the center of the cluster the stars are so dense that it is often difficult to resolve them singly.
The globular clusters are scattered in a halo that winds our Milky Way and they contain a lot of the oldest stars that we know.
|The brightest stars of a constellation are generally indicated with the method introduced in 1603 by Johann Bayer: a letter of the Greek alphabet followed by the genitive of the Latin name of the constellation (for example, Sirius is alpha Canis Maioris).
This is the complete Greek alphabet:
|In the II century before Christ, Ipparchus of Nicea divided the stars into six classes, according to their brightness: to the most shining stars he attributed the first magnitude, while to the weakest stars that he could see he assigned the sixth magnitude.
Today there is a well precise relationship between brightness and magnitude: a difference of 5 magnitudes is equivalent to a difference of brightness of 100 times. Then, since the relationship is set on a logarithmic scale, the correspondences will be as follow:
Difference of Difference of magnitude brightness 0,5 1,6 1,0 2,5 1,5 4,0 2,0 6,3 2,5 10 3,0 16 3,5 25 4,0 40 5,0 100 6,0 250 10,0 10.000 12,5 100.000 15,0 1.000.000
To the stars which are 2,5 times brighter than those of first magnitude, negative magnitudes are assigned; to the stars which are weaker than those of sixth magnitude, greater positive magnitudes are assigned.
|Milky Way is the name of the spiral galaxy in which there is the solar system. It has a 100.000-light-year diameter, a 10.000-light-year maximum thickness and it contains about one hundred milliard stars and big clouds of gases and dusts. Besides, it is surrounded by a halo in which there are many globular clusters and it is accompanied by two small satellite galaxies, the Magellanic Clouds (in the constellations of Dorado and Tucana). The Sun is 30.000 light-years away from the Milky Way center and completes a galactic revolution in about 250 millions years.
In the sky the Milky Way seems a slightly bright strip (wherefore its name), which crosses several constellations, the principal of which are Cassiopeia, Cygnus, Auriga, Carina, Crux, Scorpius, Sagittarius. In this last constellation there is the galactic center.
|The nebulas are clouds of gases and interstellar dusts.
Other nebulas are dark and visible only because they intercept the light coming from bright objects which are behind them: an example of dark nebula is the Horsehead Nebula, in the Orion's belt.
|The open cluster is a group of a certain number of stars (from about ten to some thousands), gravitationally tied up. Usually the open cluster is of irregular shape and contains very young stars (few millions of years) or even forming stars.
|The planetary nebulas are clouds of gases thrown to the outer space by a dying star, which often we can see at the center. The adjective "planetary" doesn't mean that the nebulas has something to do with planets, but it is justified because to the telescope they resemble a planet disk.
A famous example of planetary nebula is the Ring Nebula, in the constellation of Lyra.
Precession of the equinoxes
|Because of the Earth's millennial motions (precession of the terrestrial axis and move of the apsides line), every year the equinoxes fall with about 61" of arc of advance, that is approximately 20 minutes before, in comparison with the precedent sidereal year (the year reported to stars).
|The right ascension (R.A.) is the angular distance between a point of the sky and the fundamental meridian (that is the meridian which intersects the equator at the point of the spring equinox, that today is in Pisces).
The right ascension is measured eastwards from the equinoctial point, in hours from 0 to 24: every hour is divided into 60 minutes and every minute into 60 seconds.
The other celestial coordinate is the declination.
|Besides the Bayer catalog, that uses the Greek alphabet, many stellar catalogues exist, lik for instance that of Flamsteed, which classifies the stars using an Arab numeral followed by the genitive of the Latin name of the constellation (so, Sirius is 9 Canis Maioris).
There are then some lists which contain particular types of stars, as the double or the variable. The variable stars, for example, are indicated with one or two Roman letters followed by the genitive of the Latin name of the constellation (W Virginis, RR Lyrae, etc.); when the letter combinations are exhausted, the variable stars are indicate with the letter V followed by an Arab numeral (V 1500 Cygnis).
As far as globular and open clusters, nebulas and galaxies are concerned, these objects are listed in special catalogues: the most important are those by Messier and by Dreyer.
|A star is variable when its brightness is not constant, but oscillates with periods that can go from few hours to some years.
There are different types of variable stars