The external illumination provides cleaner and more direct diagnostics of the mass outflows from YSOs. A portion of the Mayall 4-m image showing HH 444 and HH 445, two irradiated jets lying about 10' east of the ionizing star, Orionis.
Most young stars produce powerful collimated winds or jets during their birth.
Optical emission lines are often seen in the vicinity of young stars in the form of Herbig-Haro objects, which are collisionally excited nebulae powered by jets and outflows from young stars. Until the early 1990s, it was generally believed that HH objects and stellar jets were small-scale phenomena with typical dimensions of 0.1 to 0.3 parsecs. The new externally illuminated jets seen by Reipurth et al. were discovered in the vicinity of the Orionis sub-group of the Orion OB Association, which is believed to be at least 1.7 million years old. They found about a half dozen examples in the outskirts of the Orion Nebula on the 36' FOV images obtained at the Mayall 4-m. Since the jets are photo-ionized, their densities can be reliably estimated.