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, and luminosity, and by the presence of massive stars with masses between 10 and 90 times that of the Sun. Sometimes these late O and early B stars are referred to 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 star clusters, 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.
Table
+NameRADECConstellationDistance(Ly)NotesAra OB1Ara4,560Auriga OB1Auriga5,700Auriga OB2Auriga20,540Camelopardalis OB1Camelopardalis3,300Camelopardalis OB3Camelopardalis13,000Canis Major OB1Canis Major3,260Canis Major OB2Canis Major1,920Carina OB1Carina8,700Carina OB2Carina7,200Cassiopeia OB1Cassiopeia6,500Cassiopeia OB2Cassiopeia11,050Cassiopeia OB4Cassiopeia7,800Cassiopeia OB5Cassiopeia7,200Cassiopeia OB6Cassiopeia6,800Cassiopeia OB7Cassiopeia6,500Cassiopeia OB8Cassiopeia8,500Cassiopeia OB14Cassiopeia3,600Centaurus OB1Centaurus7,820Cepheus OB1Cepheus11,400Cepheus OB2Cepheus2,900Cepheus OB3Cepheus2,350Cepheus OB4Cepheus2,755Cepheus OB5Cepheus6,850Cepheus OB6Cepheus880Circinus OB1Circinus10,700Crux OB1Crux8,150
Scorpius-Centaurus association
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.
Orion OB1 association
Orion OB1 is a prominent OB association located in the constellation of Orion. It is linked with Orion Nebula and active star- forming region of Orion Molecular Cloud Complex.
Gallery
See also OB stars Cepheus OB2 Region Cepheus OB3 Region Cepheus OB4 Region Stellar associations Sagittarius OB7
References