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An astrophysical jet (hereafter 'jet') is a phenomenon often seen in astronomy, where streams of matter are emitted along the axis of rotation of a compact object. It is usually caused by the dynamic interactions within an accretion disc. When matter is emitted at speeds approaching the speed of light, these jets are called relativistic jets, because the effects of special relativity become important. The largest jets are those seen in active galaxies such as quasars and radio galaxies. Other systems which often contain jets include cataclysmic variable stars, X-ray binaries and T Tauri stars. Herbig–Haro objects are caused by the interaction of jets with the interstellar medium. Bipolar outflows or jets may also be associated with protostars (young, forming stars), or with evolved post-AGB stars (often in the form of bipolar nebulae). While it is still the subject of ongoing research to understand how jets are formed and powered, the two most often proposed origins of this power are the central object (such as a black hole), and the accretion disc. Accretion discs around many stellar objects are able to produce jets, although those around a black hole are the fastest and most active. This is because the speed of the jet is around the same speed as the escape velocity of the central object. This makes the speed of a jet from an accreting black hole near the speed of light, while protostellar jets are much slower. While it is not known exactly how accretion discs manage to produce jets, they are thought to generate tangled magnetic fields that cause the jets to collimate. The hydrodynamics of a de Laval nozzle may also give a hint to the mechanisms involved. One of the best ways of exploring how jets are produced is to determine the composition of the jets at a radius where they can be directly observed. For example, if a jet originates in the accretion disc, its plasma is likely to have electron-ion composition, whereas if it originates in the black hole it will likely be electron-positron in nature. Also, the plasma emits various forms of radiation such as X-rays and radio waves, which aid diagnosis. ==Relativistic jet== Relativistic jets are extremely powerful jets of plasma, with speeds close to the speed of light, that are emitted near the central massive objects of some active galaxies, notably radio galaxies and quasars. Their lengths can reach several thousand or even hundreds of thousands of light years. Because the jet speed is close to the speed of light, the effects of the Special Theory of Relativity are important; in particular, relativistic beaming will change the apparent brightness. The mechanics behind both the creation of the jets and the composition of the jets are still a matter of much debate in the scientific community. In terms of composition, some studies favour a model in which the jets are composed of an electrically neutral mixture of electrons, positrons, and protons in some proportion, while others are consistent with a jet of electron-positron plasma. Similar jets, though on a much smaller scale, can develop around the accretion disks of neutron stars and stellar black holes. These systems are often called microquasars. An example is SS433, whose well-observed jet has a velocity of 0.23c, although other microquasars appear to have much higher (but less well measured) jet velocities. Even weaker and less relativistic jets may be associated with many binary systems; the acceleration mechanism for these jets may be similar to the magnetic reconnection processes observed in the Earth's magnetosphere and the solar wind. The general hypothesis among astrophysicists is that the formation of relativistic jets is the key to explaining the production of gamma-ray bursts. These jets have Lorentz factors of ~100 (that is, speeds of roughly 0.99995c), making them some of the swiftest celestial objects currently known. 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「Astrophysical jet」の詳細全文を読む スポンサード リンク
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