Nebulae are clusters of gases and dusts in space, which have a notable cosmological importance because they consider the places where they are born, due to condensation and aggregation of matter, solar systems similar to ours.
Nebulae can become visible if they are in the vicinity of stars, or remain completely wrapped in the darkness of space.
In the first case, a nebula can shine either because it reflects the light of nearby stars, as happens to the Mérope nebula in the Pleiades (and it is referred to as a reflection nebula), or because, excited by the radiations of the stars neighbors, it emits radiation itself, like the famous Orion nebula (and then it speaks of emission nebulae).
In the second case, however, the nebula does not emit any light; However, its presence is deduced by a kind of black region that stands out against the background of the starry sky. These nebulae are called dark ones and a typical case of them is represented by the so-called Carbon Bag in the Southern Cross.
Galaxies are also improperly called nebulae, that is, the star systems such as the one of which our Sun is a part, which nevertheless have nothing to do with the nebulae of which we speak. It is a heritage of nineteenth-century astronomy, which has left its sign in contemporary astronomical language.
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