OB association
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 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 and T associations. 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
| Name | Galactic Arm | Association Notable Objects |
| Sagittarius OB1 | Sagittarius Arm | Messier 8, Sh 2-34 |
| Sagittarius OB4 | Sagittarius Arm | |
| Sagittarius OB5 | Sagittarius Arm | |
| Sagittarius OB6 | Sagittarius Arm | Messier 24 |
| Sagittarius OB7 | Sagittarius Arm | Sh 2-35, Sh 2-37 |
| Serpens OB1 | Sagittarius Arm | Messier 16, Messier 17, NGC 6611 |
| Serpens OB2 | Sagittarius Arm | Messier 16, NGC 6604 |
| Scutum OB2 | Sagittarius Arm | |
| Scutum OB3 | Sagittarius Arm | Sh 2-50 |
| Vulpecula OB1 | Orion Arm | Sh 2-86, Sh 2-87, Sh 2-88 |
| Vulpecula OB4 | Sh 2-82, Sh 2-91 | |
| Cygnus OB1 | Cygnus X | |
| Cygnus OB2 | Cygnus X | |
| Cygnus OB3 | ||
| Cygnus OB4 | ||
| Cygnus OB7 | NGC 7000, IC 5070 | |
| Cygnus OB8 | ||
| Cygnus OB9 | Cygnus X | |
| Cepheus OB1 | Sh 2-132 | |
| Cepheus OB2 | IC 1396, Sh 2-134 | |
| Cepheus OB3 | Sh 2-157, Sh 2-160 | |
| Cepheus OB4 | ||
| Cepheus OB5 | NGC 7380 | |
| Cepheus OB6 | ||
| Lacerta OB1 | Sh 2-126 | |
| Cassiopeia OB1 | ||
| Cassiopeia OB2 | ||
| Cassiopeia OB4 | ||
| Cassiopeia OB5 | ||
| Cassiopeia OB6 | ||
| Cassiopeia OB7 | ||
| Cassiopeia OB8 | ||
| Cassiopeia OB14 | ||
| Perseus OB1 | ||
| Perseus OB2 | ||
| Perseus OB3 | ||
| Camelopardalis OB1 | ||
| Camelopardalis OB3 | ||
| Auriga OB1 | ||
| Auriga OB2 | ||
| Gemini OB1 | ||
| Orion OB1 | ||
| Orion OB2 | ||
| Monoceros OB1 | ||
| Monoceros OB2 | ||
| Monoceros OB3 | ||
| Canis Major OB1 | ||
| Canis Major OB2 | ||
| Puppis OB1 | ||
| Puppis OB2 | ||
| Puppis OB3 | ||
| Vela OB1 | ||
| Vela OB2 | ||
| Carina OB1 | ||
| Carina OB2 | ||
| Crux OB1 | ||
| Scorpius OB1 | ||
| Scorpius-Centaurus Association | ||
| Scorpius OB4 | ||
| Centaurus OB1 | ||
| Circinus OB1 | ||
| Ara OB1 |
Examples
- 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: 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.
- 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.