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bipolar outflowA flow of gas from a very young star, forming two oppositely directed beams; it is apparently a common feature of star formation. In the youngest stars the bipolar flow is detected by the radio emission from molecules such as carbon monoxide; when stars have dispersed most of the surrounding gas and dust, the outflow is visible optically as a bipolar nebula or as ‘jets' extending away from a T Tauri star for thousands of astronomical units. Both kinds of outflow can reach speeds of several hundred kilometers per second. The outflow from T Tauri stars can produce a line of Herbig–Haro objects. These flows are too powerful to be driven by the pressure of the star's radiation. They are probably T Tauri winds that are channeled outward from the star's poles, a disk of dense gas around the equator preventing them from escaping isotropically. Such disks are indeed often detected in the centers of the different kinds of bipolar outflow at the correct orientation to account for the observed outflow of less-dense gas.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006