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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 to 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]
History
editThe 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
editOB 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.[4][5]
Examples
edit- 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.[6][7][8][9]
- 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.[10]
- Some Examples of nearby association are 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.[11]
See also
editReference
edit- ^ "Star Clusters: Inside the Universe's Stellar Collections". NASA. Retrieved 1 March 2025.
- ^ "OB Association". Brittanica. Retrieved 1 March 2025.
- ^ Israelian, Garik (1997). "Obituary: Victor Amazaspovich Ambartsumian, 1912 [i.e. 1908] -1996". Bulletin of the American Astronomical Society. 29 (4): 1466–1467. Bibcode:1997BAAS...29.1466I.
- ^ "Unlocking Stellar Secrets: OB Associations". Number Analytics. Retrieved 1 March 2025.
- ^ "Cosmography of OB stars in the solar neighbourhood". Astronomy & Astrophysics. Retrieved 1 March 2025.
- ^ Melnik, A. M.; Dambis, A. K. (2020). "Distance scale for high-luminosity stars in OB associations and in field with Gaia DR2. Spurious systematic motions". Astrophysics and Space Science. 365 (7): 112. arXiv:2006.14649. Bibcode:2020Ap&SS.365..112M. doi:10.1007/s10509-020-03827-0. S2CID 220128144.
- ^ Rizzuto, Aaron; Ireland, Michael; Robertson, J. G. (October 2011), "Multidimensional Bayesian membership analysis of the Sco OB2 moving group", Monthly Notices of the Royal Astronomical Society, 416 (4): 3108–17, arXiv:1106.2857, Bibcode:2011MNRAS.416.3108R, doi:10.1111/j.1365-2966.2011.19256.x, S2CID 54510608.
- ^ Mark J. Pecaut; Eric E. Mamajek & Eric J. Bubar (February 2012). "A Revised Age for Upper Scorpius and the Star Formation History among the F-type Members of the Scorpius–Centaurus OB Association". Astrophysical Journal. 746 (2): 154. arXiv:1112.1695. Bibcode:2012ApJ...746..154P. doi:10.1088/0004-637X/746/2/154. S2CID 118461108.
- ^ 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.
- ^ Blaauw, Adriaan (1964). "The O Associations in the Solar Neighborhood". Annual Review of Astronomy and Astrophysics. 2 (1): 213–246. Bibcode:1964ARA&A...2..213B. doi:10.1146/annurev.aa.02.090164.001241.
- ^ "Mapping the distribution of OB stars and associations in Auriga". Oxford Academic. Retrieved 1 March 2025.
