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Herbig–Haro (HH) objects are small patches of nebulosity associated with newly born stars, and are formed when narrow jets of gas ejected by young stars collide 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 with its rotational axis. HH objects are transient phenomena, lasting not more 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. ==Discovery and history of observations== The first HH object was observed in the late 19th century by Burnham, when he observed the star T Tauri with the 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. However, 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. Fifty years after Burnham's discovery, several similar nebulae were discovered which were so small as to be almost star-like in appearance. Both Haro and Herbig made independent observations of several of these objects during the 1940s. Herbig also looked at Burnham's Nebula and found it displayed an unusual electromagnetic spectrum, with prominent emission lines of hydrogen, sulfur and oxygen. Haro found that all the objects of this type were invisible in infrared light. Following their independent discoveries, Herbig and Haro met at an astronomy conference in Tucson, Arizona. Herbig had initially paid little attention to the objects he had discovered, being primarily concerned with the nearby stars, but on hearing Haro's findings he carried out more detailed studies of them. The Soviet astronomer Viktor Hambardzumyan gave the objects their name, and based on their occurrence near young stars (a few hundred thousand years old), suggested they might represent an early stage in the formation of T Tauri stars.〔 Studies showed HH objects were highly ionised, and early theorists speculated they might contain low-luminosity hot stars. However, the absence of infrared radiation from the nebulae meant there could not be stars within them, as these would have emitted abundant infrared light. Later studies suggested the nebulae might contain protostars, but eventually HH objects came to be understood as material ejected from nearby young stars that is colliding at supersonic speeds with the ISM, with the resulting shock waves generating visible light. In the early 1980s, observations revealed for the first time the jet-like nature of most HH objects. This led to the understanding that the material ejected to form HH objects is highly collimated (concentrated into narrow jets). A forming star is often surrounded by accretion disc in their first few hundred thousand years of existence. As gas falls onto them, the rapid rotation of the inner parts of these disks leads to the emission of narrow jets of partially ionized gas (plasma) perpendicular to the disk. When these jets collide with the interstellar medium, they give rise to the small patches of bright emission which comprise HH objects.〔 ((HTML version ))〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Herbig–Haro object」の詳細全文を読む スポンサード リンク
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