star_formation - STAR FORMATION (Ch. 19) The basics:...

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(Ch. 19) The basics: GRAVITY vs. PRESSURE (heat; but also rotation and magnetic fields) Stages (you don’t have to memorize numbers of stages) 1 . Interstellar cloud—cold (T~10K), large (~1-10pc), massive (~10 3 – 10 5 M 0 ), so gravity wins easily over gas pressure (atomic motions—see Fig. 19.2): cloud must break up, or “fragment.” Other inhibitors: rotation, magnetic fields (Fig.19.3). 2 . Collapsing cloud fragment—still cold, size~0.1-0.01pc. This may fragment further, so have multiple stages of fragmentation (see Fig. 19.4). 3 . Fragmentation ceases—center of fragment dense enough to become opaque to its own radiation, so it heats up, slowing the collapse. (Previously it was transparent and so could stay cool because radiation escaped easily.) 4 . Protostar—now object has a “surface” because it is opaque; this surface is the photosphere (the layer where the photons can finally escape into space). So a protostar is defined by when the object becomes dense enough to have a photosphere. Protostar heats up by gravitational contraction and accretion of remaining material (from a disk). But the temperature in center is too low for nuclear reactions at this stage. 5 . Continued slow contraction (getting fainter), heating. Protostar moves roughly on a vertical track in the H-R diagram
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star_formation - STAR FORMATION (Ch. 19) The basics:...

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