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 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. 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.
- 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)
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- N. Calvet et al; The Astronomical Journal 129:935–946 (2005)
- J. Hernandez et al., Astrophys. J. 652:472-481 (2006)