<<Herbig–Haro (HH) objects are small patches of nebulosity associated with newly born stars, and are formed when narrow jets of partially ionized gas ejected by those stars collide with nearby clouds of gas and dust at speeds of several hundred kilometres per second. Herbig–Haro objects are ubiquitous in star-forming regions, and several are often seen around a single star, aligned with its rotational axis. Near to the source star, about 20–30% of the gas in HH objects is ionized, but this proportion decreases at increasing distances. This implies the material is ionized in the polar jet, and recombines as it moves away from the star, rather than being ionized by later collisions. Shocking at the end of the jet can re-ionize some material, however, giving rise to bright "caps" at the ends of the jets.
HH objects are transient phenomena that last less than a few thousand years. They can evolve visibly over quite short astronomical timescales as they move rapidly away from their parent star into the gas clouds of interstellar space (the interstellar medium or ISM). Hubble Space Telescope observations have revealed the complex evolution of HH objects over the period of a few years, as parts of the nebula fade while others brighten as they collide with clumpy material of the interstellar medium.
The temperatures observed in HH objects are typically about 8000–12,000K, similar to those found in other ionized nebulae such as H II regions and planetary nebulae. Densities, on the other hand, are relatively higher; ranging from a few thousand to a few tens of thousands of particles per cm3
, compared to generally less than 1000/cm3
in H II regions and planetary nebulae. Densities also decrease as the source evolves over time. HH objects consist mostly of hydrogen and helium, which account for about 75% and 24% respectively of their mass. Around 1% of the mass of HH objects is made up of heavier chemical elements, including oxygen, sulfur, nitrogen, iron, calcium and magnesium. Abundances of these elements, determined from emission lines of respective ions, are generally similar to their cosmic abundances. Several chemical compounds have been identified in the outflows, including water (ice), methanol, methane, carbon monoxide, carbon dioxide (dry ice) and various silicates. Many chemical compounds found in surrounding interstellar medium, which are not present in the source material, such as metal hydrides, are believed to have been produced by shock induced chemical reactions.>>