Little dictionary

Constellation

Greek alphabet

Planetary nebula

Declination

Magnitude

Precession of the equinoxes

Double star

Milky Way

Right ascension

Galaxy

Nebula

Sky catalogues

Globular cluster

Open cluster

Variable star

Constellation
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.

Declination
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.

Double star
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:

  1. the optical double stars, that are two independent stars which we see near only because of the perspective, since they lie on the same visual line;
  2. the physical double stars, that are two stars tied together by gravity, which rotate round each other, with a period that depends on their distance.

There are very many double stars: they are practically in all the constellations.

Galaxy
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:

  • spiral galaxies, disk-shaped, with a spheroidal central nucleus from which the spiral branches start: an example of spiral galaxy is M31, the galaxy of Andromeda;
  • elliptical galaxies, deprived of branches and formed by old stars: an example of elliptical galaxy is M8 in Virgo;
  • irregular galaxies, which are galaxies that, not having a well defined shape, are not included in the two preceding categories: examples of irregular galaxies are the two Magellanic Clouds, in Dorado and in Tucana.

    Our galaxy is called Galaxy (with the capital letter) or Milky Way.

Globular cluster
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.

Examples of globular clusters are M13, in Hercules, and omega Centauri.

Greek alphabet
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:

Greek alphabet

Magnitude
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
So, for instance, considered that
- Vega (in Lyra) is of magnitude 0,0;
- Adhara (in Canis Maior) is of magnitude 1,5;
- Ras Algethi (in Hercules) is of magnitude 3,5;
this means that
- Vega is 4 times brighter than Adhara;
- Vega is 25 times brighter than Ras Algethi;
- Adhara is 6,3 times brighter than Ras Algethi.

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.

Magnitude scale

Milky Way
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.

Nebula
The nebulas are clouds of gases and interstellar dusts.

Some of them, thanks to the light of the stars which are in them, are bright and often many-coloured: shiny nebulas are for instance that of Orion and the Tarantula, in Dorado.

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.

Open cluster
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 open clusters are in the spiral branches of our Milky Way: some examples of famous clusters are the Pleiades and the Hyades in Taurus and the Double Cluster in Perseus.

Planetary nebula
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).

Because of such phenomenon, already discovered by Ipparchus of Nicea (II cent. b.C.), if in the ancient times the spring equinox was in Aries, today its position is "early" in Pisces.

Right ascension
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.

Sky catalogues
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.
The Messier catalogue, compiled in the XVIII century, contains few more than hundred objects, which are marked with the letter M followed by an Arab numeral (for example, the nebula of Orion is M42).
The Dreyer catalogue is entitled New General Catalogue of Nebulae and Clusters of Stars and it was published in 1888: the classified objects are indicated with the letters NGC followed by an Arab numeral (for example, the nebula around to eta Carinae is NGC 3372). In 1895 and in 1908 two supplements were published to the Dreyer catalogue, entitled Index Catalogues: the listed objects are marked with the letters IC followed by an Arab numeral.

Variable star
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

  • the cepheids, so called by their prototype, delta Cephei: they are yellow supergiants which pulsate with regularity;
  • the red giants and supergiants, which have some form of variability, due to volume change, that however don't follow a rigid periodicity (an example of this type is Mira, in Cetus);
  • the eclipse binaries: they are particular cases of phisical double star, since they are constituted by two stars that, seen from the Earth, periodically pass before each other, provoking variations in the total brightness; (a famous eclipse binary is Algol, in Perseus);
  • the novae: they are stars which have sudden and violent explosions, that make their brightness to increase, without any periodicity (an example of recurrent nova is in Pyxis).

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