Organization: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.[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
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]
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.[9][10][11][12]
- 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.[13]
- 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.[14]
Table
| Name | RA Dec |
Distance (ly) | Galactic Arm | Association Notable Objects |
|---|---|---|---|---|
| Sagittarius OB5 | 17h 55m −28° |
3,000 | Sagittarius Arm | |
| Sagittarius OB1 | 18h 10m −22° |
1,500 | Messier 8, Sh 2-34 | |
| Sagittarius OB7 | 18h 17m −19° |
1,750 | Sh 2-35, Sh 2-37 | |
| Sagittarius OB4 | 18h 13m −16° |
2,400 | ||
| Sagittarius OB6 | 18h 21m −15° |
Messier 24 | ||
| Serpens OB1 | 18h 21m −15° |
1,700 | Messier 16, Messier 17, NGC 6611 | |
| Scutum OB3 | 18h 26m −14° |
1,700 | Sh 2-50 | |
| Serpens OB2 | 18h 26m −12° |
1,700 | Messier 16, NGC 6604 | |
| Scutum OB2 | 18h 35m −09° |
|||
| Vulpecula OB1 | 19h 45m 24° |
2,300 | Orion Arm | Sh 2-86, Sh 2-87, Sh 2-88 |
Gallery
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Map of OB association -
Image of Cepheus OB3
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See also
- OB stars
- Stellar associations
References
- ↑ "Star Clusters: Inside the Universe's Stellar Collections". NASA. 3 May 2023. https://science.nasa.gov/universe/star-clusters-inside-the-universes-stellar-collections/.
- ↑ "OB Association". Brittanica. https://www.britannica.com/science/OB-Association. 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.
- ↑ Bouy, H.; Alves, J. (December 2015). "Cosmography of OB stars in the solar neighbourhood". Astronomy & Astrophysics 584: A26. doi:10.1051/0004-6361/201527058. ISSN 0004-6361. Bibcode: 2015A&A...584A..26B. http://www.aanda.org/10.1051/0004-6361/201527058.
- ↑ Quintana, Alexis L. (2024). "OB associations: from stellar to galactic scales". arXiv:2412.10769 [astro-ph.GA].
- ↑ "Unlocking Stellar Secrets: OB Associations". Number Analytics. https://www.numberanalytics.com/blog/ultimate-guide-to-ob-associations.
- ↑ Bouy, H.; Alves, J. (2015). "Cosmography of OB stars in the solar neighbourhood". Astronomy & Astrophysics 584: A26. doi:10.1051/0004-6361/201527058. Bibcode: 2015A&A...584A..26B. https://www.aanda.org/articles/aa/full_html/2015/12/aa27058-15/aa27058-15.html. Retrieved 1 March 2025.
- ↑ McKee, Christopher F.; Williams, Jonathan P. (1997-02-10). "The Luminosity Function of OB Associations in the Galaxy" (in en). The Astrophysical Journal 476 (1): 144–165. doi:10.1086/303587. ISSN 0004-637X. Bibcode: 1997ApJ...476..144M. https://iopscience.iop.org/article/10.1086/303587.
- ↑ 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. doi:10.1007/s10509-020-03827-0. Bibcode: 2020Ap&SS.365..112M.
- ↑ 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, doi:10.1111/j.1365-2966.2011.19256.x, Bibcode: 2011MNRAS.416.3108R.
- ↑ 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. doi:10.1088/0004-637X/746/2/154. Bibcode: 2012ApJ...746..154P.
- ↑ Preibisch, T.; Mamajek, E. (2009). "The Nearest OB Association: Scorpius-Centaurus (Sco OB2)". Handbook of Star-Forming Regions 2: 0. Bibcode: 2008hsf2.book..235P.
- ↑ Blaauw, Adriaan (1964). "The O Associations in the Solar Neighborhood". Annual Review of Astronomy and Astrophysics 2 (1): 213–246. doi:10.1146/annurev.aa.02.090164.001241. Bibcode: 1964ARA&A...2..213B.
- ↑ L, Quintana; J, Wright; D, Jeffries (21 April 2023). "Mapping the distribution of OB stars and associations in Auriga". Oxford Academic 522 (2): 3124. doi:10.1093/mnras/stad1160. Bibcode: 2023MNRAS.522.3124Q. https://academic.oup.com/mnras/article/522/2/3124/7131461. Retrieved 1 March 2025.
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