Star Associations

Star Associations

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With the name of stellar associations groups of stars with similar physical characteristics and that are gathered in a certain region of space are indicated.

They should not be confused with star clusters because, unlike these, they have a rather lower density and are not characterized by a particular structure. However, there is a relationship between associations and clusters, since open clusters have been discovered at the center of many associations.

Star associations are of two types:

The best known and numerous are those constituted by giant stars of very high temperature, also called "Associations O", because their spectral category goes from O to B2. The stars that are part of them, a hundred on average, are young stars (some millions of years), animated by an expansion movement of some km / sec. and they are usually immersed in clouds of hydrogen. A typical example is represented by the association contained in the so-called "rosette nebula", classified with catalog number NGC 2244.

Less numerous are the associations formed by dwarf variable stars of a very young age. They are also called "T Associations" of T Tauri, that is, the variable star prototype that best represents the class. These also appear associated with nebulae. A typical example of these associations is found in the Orion trapezoid.

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