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.
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
, OB associations lack sufficient gravitational binding and can disperse over millions of years.
History
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.
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.
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.
Examples
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Scorpius-Centaurus Association: It is the closest OB association to Earth, located approximately 400 light-years away, Containing bright stars in the constellations of Scorpius, Centaurus, Lupus, and Crux, including one of the nearest red supergiant to Earth, Antares.
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Orion OB1: It is a prominent group in the constellation of Orion. It is linked with Orion Nebula and active star- forming region of Orion Molecular Cloud Complex.
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Some examples of nearby OB association are Camelopardalis OB1, Carina OB1, Carina OB2, Cepheus OB1, Collinder 121, Columba association, Cygnus OB2, Cygnus OB7, Cygnus OB9, Perseus OB1, Vulpecula OB1, Vela OB2, Circinus OB1, Auriga OB1 and other examples includes in the nearby Large Magellanic Cloud and the Andromeda Galaxy, illustrating their prevalence in other galaxies.
Gallery
File:OB global.png|Map of OB association
File:Rich Young Cluster Cep OB 3b (noao1303a).jpg|Image of Cepheus OB3
See also
