Portal:Astronomy/Featured/September 2007

Herbig-Haro objects are small patches of nebulosity associated with newly-born stars, and are formed when gas ejected by young stars collides with clouds of gas and dust nearby 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 along its rotational axis.

HH objects are transient phenomena, lasting only a few thousand years at most. They can evolve visibly over quite short timescales as they move rapidly away from their parent star into the gas clouds in interstellar space (the interstellar medium or ISM). Hubble Space Telescope observations reveal complex evolution of HH objects over a few years, as parts of them fade while others brighten as they collide with clumpy material in the interstellar medium.

The objects were first observed in the late 19th century by Sherburne Wesley Burnham, but were not recognised as being a distinct type of emission nebula until the 1940s. The first astronomers to study them in detail were George Herbig and Guillermo Haro, after whom they have been named. Herbig and Haro were working independently on studies of star formation when they first analysed Herbig-Haro objects, and recognised that they were a by-product of the star formation process.

The stars which are behind the creation of Herbig-Haro objects are all very young stars, the youngest of which are still protostars in the process of forming from the surrounding gases. Astronomers divide these stars into classes 0, I, II and III, according to how much infrared radiation the stars give off. A greater amount of infrared radiation implies a larger amount of cooler material surrounding the star, which indicates that it is still coalescing. The numbering of the classes arises because class 0 objects (the youngest) were not discovered until classes I, II and III had already been defined.

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