The Stages Immediately Following Star Birth

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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