The galaxy par excellence is the star system of which our Sun is a part; It is described in detail in the Milky Way voice. Galaxies in general are systems of billions of stars held together by the force of gravitational attraction.

Seen from Earth, these systems appear as tiny spherical or elliptical nebulas, or as glands or irregular clouds. Until the 19th century, neither its structures nor its situation with respect to our Galaxy were clear. Only at the beginning of the twentieth century could it be determined with certainty that these faint and small clouds are star systems completely similar to our Galaxy, but much farther away, and their distances and their distribution in space could be measured with various methods.

Of all the outer galaxies, only three are visible to the naked eye: it is the famous Andromeda nebula, located in the homonymous constellation, cataloged with the number M 31 in the Messier catalog; of the Small and Large Magellanic Cloud. These last two are well visible only in the Southern Hemisphere, from latitudes greater than 20º and are called that because they were first described by the Portuguese navigator Fernando de Magallanes.

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Astronomy Glossary: ​​GGalileo (mission)