An OB association is a loosely organized, gravitationally unbound group of young, massive, main sequence stars primarily of spectral types O and B, characterized by their high temperatures, blue color, luminous, and massive stars with masses between 10 and 90 times that of the Sun. Sometimes these late O and Early B stars are referred as OB stars.[1] These associations typically contain 10 to 100 (or more) massive stars alongside numerous lower-mass stars, all sharing common motion vectors, ages, and chemical compositions, indicating a shared origin. Unlike denser star clusters, OB associations lack sufficient gravitational binding and can disperse over millions of years.[2]
The concept of stellar associations, including OB associations, was introduced by Armenian astronomer Victor Ambartsumian in 1947. He distinguished them from bound clusters, categorizing them into OB associations (for O and B-type stars) and T associations (for cooler, variable T Tauri stars). Since their identification, OB associations have been observed not only in the Milky Way but also in nearby galaxies, contributing significantly to studies of galactic star formation.[3]
Formation and characteristics
OB associations form within giant molecular clouds, where dense regions of gas and dust collapse under gravity to produce stars. The massive O and B stars form in a relatively small volume, but stellar winds, radiation pressure, and supernovae from these stars expel surrounding gas, reducing gravitational cohesion and causing the group to expand as an unbound system. This process results in lower star formation efficiency compared to bound clusters, contributing to their dispersed nature.[4][5]
OB associations are notably sparse, often spanning 700 to 1,500 light-years in diameter, and are visually distinct from compact clusters. They are typically very young, with ages of a few million years, as O-type stars have lifespans of 1 to 15 million years, while B-type stars last somewhat longer due to their rapid nuclear fuel consumption. These associations are often found in the spiral arms of galaxies like the Milky Way and are associated with nearby open star clusters. The massive stars within them are extremely luminous, up to 100,000 times brighter than the Sun, and frequently end their lives as supernovae.[6][7][8]
↑Preibisch, T.; Mamajek, E. (2009). "The Nearest OB Association: Scorpius-Centaurus (Sco OB2)". Handbook of Star-Forming Regions. 2: 0. arXiv:0809.0407. Bibcode:2008hsf2.book..235P.
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