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A stellar association is a very loose star cluster, whose stars share a common origin and are still moving together through space, but have become gravitationally unbound. Associations are primarily identified by their common movement vectors and ages. Identification by chemical composition is also used to factor in association memberships.
For example, in the vicinity of a typical star the ratio of radiation-to-gravity force on a hydrogen atom or ion, = =,, hence radiation force is negligible in general, except perhaps around a luminous O-type star of mass , or around a black hole accreting gas at the Eddington limit so that its luminosity-to-mass ratio / is defined by =.
The regions with higher density of stars are shown; these correspond with known star clusters (Hyades and Coma Berenices) and moving groups. This is a list of nearby stellar associations and moving groups. A stellar association is a very loose star cluster, looser than an open cluster. A moving group is the remnant of such a stellar association ...
Scutum Star Cloud with open cluster Messier 11 at lower left. Technically not star clusters, star clouds are large groups of many stars within a galaxy, spread over very many light-years of space. Often they contain star clusters within them. The stars appear closely packed, but are not usually part of any structure. [18]
Globular cluster masses can be determined by observing the proper motion of nearby stars influenced by the cluster [1] [2] or by estimating the cluster's relaxation time. [ 3 ] Open clusters
Nearest globular cluster to the Earth. Also the first globular cluster known to have exoplanets (PSR B1620-26b) Messier 12: 74.4 [28] Messier 70: 68 [29] NGC 290: 66 [30] Open cluster: Messier 28: 60 [31] Globular cluster: Messier 18: 52.4 [32] Open cluster: The following notable star clusters are listed for the purpose of comparison. MGC1: 49 ...
A stellar association is a very loose star cluster, looser than both open clusters and globular clusters. Stellar associations will normally contain from 10 to 100 or more visible stars. An association is primarily identified by commonalities in its member stars' movement vectors, ages, and chemical compositions.
The moving-cluster method relies on observing the proper motions and Doppler shift of each member of a group of stars known to form a cluster. The idea is that since all the stars share a common space velocity, they will appear to move towards a point of common convergence ("vanishing point") on the sky.