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 thousands of lower-mass stars also in this group, and some protostars.
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]
↑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)