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 association is part of the larger Orion Molecular Cloud Complex. It is the most closely studied OB association.
The Orion OB1 association has these subgroups:
- 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)
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.