Star association

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A star association is that loose form of an open star cluster in which the stars are least gravitationally bound to one another, so that over time they are dispersed by forces within the Milky Way . Their 5 to 100 young stars have physically similar properties due to their joint formation in an H-II region . They are generally only slightly condensed towards a center, so that they usually hardly stand out against the star background .

Star rows within associations could have arisen in "elephant trunks" of the original gas cloud .

Although associated stars move together, star associations do not count among the stellar currents , which are torn globular clusters and dwarf galaxies .

Example Big Bear

Among the dozen or so star associations known today, the group of bears was one of the first to be discovered and investigated: five of the seven main stars of the Ursa Major constellation , which make up the Big Dipper , not only have almost the same brightness and size, but also have the same direction of movement relative to the Milky Way.

Courvoisier found another 6 stars in this community in 1915; two are even in the southern sky , from which it can be concluded that our solar system lies in the vicinity of this group, although it does not belong to it.

Types of associations

Groups of star associations are distinguished on the basis of certain criteria:

  • OB association (also O association )
    An OB association mainly consists of hot stars of the spectral classes  O and B. About 70 such associations are known, e.g. B. in the constellation Perseus and the Orion belt .
  • R association
    A collection of stars around 1 million years old embedded in a reflection nebula . The nebula consists of the matter from which the stars emerged.
  • T association
    A T association consists primarily of stars of the variable type T Tauri . These very young stars have not yet reached the state of the main sequence , so they are not yet in internal equilibrium. A well-known T association is around the trapezoidal stars ( Orion constellation ).