Compact nebulae with peculiar spectra that were first studied in the 1950s by George Herbig and, independently, by Guillermo Haro. Herbig-Haro (HH) objects are typically located within or adjacent to star-forming regions and appear as small patches of nebulosity with highly variable luminosity. They exhibit spectroscopic emission lines that are characteristic of highly ionized gas that has been shocked upon interaction with the surrounding interstellar medium. Doppler shifts of the spectral lines reveal that the gas is moving at supersonic velocities of 100 km/s (60 mi/s) to more than 1000 km/s (600 mi/s). Typical masses of an individual clump of gas are 1–20 times that of Earth with temperatures of 8000–12000 K. They are composed almost entirely of hydrogen and helium with less than 1% consisting of heavier elements. Over 400 individual Herbig-Haro objects are known. Many of the closest ones are observed to emanate from a very young stellar source with proper motions that are discernible in Hubble Space Telescope images taken over periods of several years.