Herbig-Haro objects, first observed by George Herbig and Guillermo Haro in 1946/1947, are bright sources of light found where high velocity gas collides with other material. This collision creates ionized gas. They are often found near protostars that emit jets of material at high speeds from their poles. This material can then interact with slower moving material released earlier to produce a Herbig-Haro object. Alternatively, it may collide with a molecular cloud surrounding the protostar creating a significantly brighter emission. These objects are dynamic and have been observed to change over the course of a few years, with new ones appearing and old ones becoming brighter or fainter.
Herbig-Haro objects often have temperatures in the range of 8,000-12,000 kelvin and densities between 2,000 to 20,000 particles per cubic centimetre. They can have masses up to 20 times greater than the Earth. They tend to be found within 0.5 parsecs of the protostar. They were once thought to be rare, but now over 600 such objects are known.