One of a number of peculiar nebulous objects associated with newly forming stars. The first three were discovered on images of the nebula NGC 1999 in Orion in 1946/7 by the American astronomer George Herbig and the Mexican Guillamero Haro. Many more similar objects have since been identified. Herbig-Haro objects are thought to result from the interaction between a strong bipolar outflow from a protostar and the interstellar gas, which is heated and compressed. They are typically between 500 and 4,000 AU in size and have masses in the range 0.5-30 Earth masses, making them among the least massive objects to have been detected outside the solar system. Their velocities are high and, in many cases, their motion can be traced back to T Tauri stars or infrared-emitting objects, suggesting that they have been ejected by young stars. Hubble’s Variable Nebula is an example of an HH object.