In regions of active star formation, several other phenomena have been observed, again through studying the molecular line emission, which indicate that stars just about to start shining eject a lot of material at hundreds of kilometers per second. These objects are known as T Tauri stars, named after a variable star discovered in the constellation of Taurus. Immediately surrounding a T Tauri star, within an arcminute or so, a small nebula can often be seen. This is made of interstellar dust and gas immediately around the protostellar object. Gas seems to be moving both in and out of the T Tauri star while, incredibly, at some distance further out small nebulae are found streaming away from the star in the same way that a jet in a radio galaxy is pointed away from its nucleus (Chapter 11). These small companion nebulae are known as Herbig—Haro (HH) objects after their discoverers. The HH objects were for a very long time a mystery because they did not appear to have any stellar objects associated with them. The stellar objects are not located in the HH objects, but are light-years away.
It appears that when a T Tauri star, soon ready to turn on its nuclear furnace, begins to stir in its cocoon, the energy it generates, due to the collapse from dimensions of several light-years across to stellar size, is so great that it can hurtle a great amount of matter outward (a millionth of a solar mass per year is typical). This matter travels in two directions because something prevents the material from moving in the other directions. That something is a disk of matter accreting around the star, The accretion disk is shaped like a flat doughnut and gases that are pulled into the accretion disk can escape only by flowing out of the hole in tw o directions at right angles to the plane of the disk. This is called a bipolar flow.
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