A star association is that loose form of an open cluster, in which the stars are the least gravitationally bound together, so that they disperse over time by forces within the Milky Way system. Your 5 to 100 young stars have similar physical properties due to the common origin in an H II region. They are generally only slightly compressed against a center, so they usually hardly stand out against the star background.
Star rows within associations may have arisen in " elephant trunks " of the original gas cloud.
Although associated stars move together, stellar associations do not count the star streams in which it is torn apart globular clusters and dwarf galaxies.
Example Ursa Major
Among these groups of stars, of which one knows several dozen today, the so-called group of bears was one of the first to be discovered and studied. Five of the seven main stars of the constellation Ursa Major, which form the Big Dipper, not only have nearly the same brightness and size, but also the same direction of movement relative to the Milky Way. Courvoisier found 1915 more 6 star this star stream; two are even in the southern sky, which suggests that our solar system lies within this group, although it is not one of them.
Types of associations
On the basis of certain criteria groups of stellar associations are distinguished:
- OB association (or trolley Association ) An OB association consists mainly of hot stars of spectral types O and B. We know about 70 of such associations, for example, in the constellation Perseus and the Orion belt.
- R association A collection of stars with an age of around 1 million years ago, which are embedded in a reflection nebula. The fog is made that the matter of which the stars are formed.
- T- Association A T- association consists mainly of stars of the variable type T Tauri. This very young stars have not yet reached the state of the main series, so they are not yet in inner equilibrium. A well-known T- association is located in the vicinity of the Trapezium stars ( Orion ).