group contains three Herbig–Haro objects HH 111 one of the most well-known Herbig–Haro objects Orion OB1 association Orion's Belt Sigma Ori cluster 25 Ori cluster Lambda Orionis molecular ring (Sh2-264) Lambda Ori cluster Barnard 30 Barnard 35 Orion-Eridanus superbubble Barnard's Loop (Sh2-276) Eridanus Loop A more complete list can be found for example in Maddalena et al. (1986) Table 1 Individual components Orion A The giant molecular cloud Orion A is the most active star-forming region in the local neighbourhood of the Sun. In the last few million years about 3000 young stellar objects were formed in this region, including about 190 protostars and about 2600 pre-main sequence stars. The Orion A cloud has a mass in the order of 105 . The stars in Orion A do not have the same distance to us. The "head" of the cloud, which also contains the Orion Nebula is about 1300 light-years (400 parsecs) away from the Sun. The "tail" however is up to 1530 light-years (470 parsecs) away from the Sun. The Orion A cloud is therefore longer than the projected length of 130 light-years (40 parsecs) and has a true length of 290 light-years (90 parsecs). Orion Molecular Clouds The Orion Molecular Clouds (OMC 1 to OMC 4) are molecular clouds located behind the Orion Nebula. Most of the light from the OMCs are blocked by material from the Orion Nebula, but some features like the Kleinmann-Low Nebula and the Becklin-Neugebauer object can be seen in the infrared. The clouds can be seen in the far-infrared and in radio wavelengths. The Trapezium Cluster has a small angular separation from the Kleinmann-Low Nebula, but the Trapezium Cluster is located inside the Orion Nebula, which is closer towards earth. Orion B Orion B is about 1370 light-years (420 parsecs) from Earth. It has a size of about 1.5 kpc² and a mass in the order of 105 . It contains several star forming regions with the star cluster inside the Flame Nebula being the largest cluster. Orion OB1 association The Orion OB1 association represents different stellar populations that are superimposed along our line of sight. The oldest group with 8-10 million years is Orion OB1a, northwest of Orion's Belt, and the youngest group with less than 2 million years is Orion OB1d, which contains the Orion Nebula cluster and NGC 2024. Lambda Orionis molecular ring The Lambda Orionis ring is a large