Orion OB1 Association

The Orion OB1 stellar association is the group of hot giant stars in a large cluster. They are spectral types O and B. There are are thousands of lower-mass stars also in this group, and some protostars.

Orion OB1 Association

The association is part of the larger Orion Molecular Cloud Complex. It is the most closely studied OB association.[1]

The Orion OB1 association has these subgroups:[1][2]

  • Orion OB1a - the group of stars northwest of the Orion Belt stars with an average age of about 12 million years. Within this group is a subgroup known as the 25 Orionis group.[3][4] They are near the star Bellatrix.
  • Orion OB1b - the three bright stars ζ Ori (Alnitak), ε Ori (Alnilam), and δ Ori (Mintaka) which make up the asterism known as "Orion's Belt", and minor stars. This group has an average age of about eight million years and has three subgroups.[5][6]
  • Orion OB1c - the stars in Orion's Sword: 42 Orionis, θ Orionis, and ι Orionis. These stars are about 3-6 million years old.
  • Orion OB1d - the stars of the Orion Nebula and M43 (the youngest stars)

Signs of stellar debris disk have been found in the 1a and 1b subgroups. That is a circumstellar disk of dust and debris in orbit around a star.[7][8]       


  1. 1.0 1.1 Brown A.G.A.; de Geus E.J.; de Zeeuw P.T. (1994). "The Orion OB1 association. 1: Stellar content". Astronomy and Astrophysics. 289 (1): 101–120. arXiv:astro-ph/9403051. Bibcode:1994A&A...289..101B. ISSN 0004-6361.
  2. 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.
  3. Briceño, César; Hartmann, Lee; Hernández, Jesús; Calvet; et al. (2007). "25 Orionis: A kinematically distinct 10 mya-old group in Orion OB1a". The Astronomical Journal. 661 (2): 1119–1128. arXiv:astro-ph/0701710. Bibcode:2007ApJ...661.1119B. doi:10.1086/513087.
  4. Harper, Graham M.; Brown, Alexander; Guinan, Edward F. (2008). "A New VLA-Hipparcos distance to Betelgeuse and its implications". The Astronomical Journal. 135 (4): 1430–40. Bibcode:2008AJ....135.1430H. doi:10.1088/0004-6256/135/4/1430.
  5. Warren W.H., Jr.; Hesser J.E. (1977). "A photometric study of the Orion OB 1 association. I - Observational data". Astrophysical Journal Supplement Series. 34: 115–206. Bibcode:1977ApJS...34..115W. doi:10.1086/190446.
  6. Warren W.H., Jr.; Hesser J.E. (1978). "A photometric study of the Orion OB 1 association. III - Subgroup analyses". Astrophysical Journal Supplement Series. 36: 497–572. Bibcode:1978ApJS...36..497W. doi:10.1086/190510.
  7. N. Calvet et al; The Astronomical Journal 129:935–946 (2005)
  8. J. Hernandez et al., Astrophys. J. 652:472-481 (2006)