Herbig–Haro object

Herbig–Haro (HH) objects are small patches of nebulosity associated with newly born stars, and are formed when narrow jets of 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.

HH objects are a 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 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 the objects, and recognised that they were a by-product of the star formation process.

The first HH object was observed in the late 19th century by Sherburne Wesley Burnham, when he observed the star T Tauri with the 36-inch (910 mm) refracting telescope at Lick Observatory and noted a small patch of nebulosity nearby. However, it was catalogued merely as an emission nebula, later becoming known as Burnham's Nebula, and was not recognised as a distinct class of object. T Tauri was found to be a very young and variable star, and is the prototype of the class of similar objects known as T Tauri stars which have yet to reach a state of hydrostatic equilibrium between gravitational collapse and energy generation through nuclear fusion at their centres.

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