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PCS 181
Margaret Buckby

UNIT TWO STAR FORMATIONS Interstellar space is filled with diffuse gas and dust. Relatively denser and cooler regions, up to 50 pc in diameter and with a million solar masses, are filled with molecules. In these molecular clouds, shock fronts from nearby star formations or a supernova explosion or some other global gravitational disturbance may begin the process of self- gravitational contraction, leading to the formation of new stars. The earliest stages of pre–main sequence evolution are not directly observable, becauseprotostars are hidden behind massive amounts of dust. Consequently, no radiation from the forming star is visible. If one could envision a protostar without the obscuring dust, theory suggests that initially a protostar would be very cool but luminous, with convection very efficiently moving gravitational energy that is released in the interior outwards to the exterior. As the object shrank, the surface area would dramatically decrease and the overall luminosity likewise decrease rapidly. Bok globules, infrared stars, and cocoon stars Some of these earliest stages of evolution are believed to occur in the small, dense, dark dust clouds that often are seen silhouetted against more extended regions of luminous, hot, interstellar nebulae. These are the Bok globules. Observation of radio radiation that penetrates the dust from these sites suggests that internal motions of the interstellar material are in a stage of contraction. Such an object may also be termed a cocoon star, because of the surrounding shroud of dense, opaque dust. When the dust is sufficiently warmed by radiation from the interior protostar, it in turn will radiate in the infrared. Many infrared sources are observed in regions where star formation is taking place. This stage of evolution is also termed Helmholtz contraction—one-half of the energy released by gravitation contraction into the protostellar material results in heating, and one-half of the energy is convected to the surface to be radiated away. As the core temperature of the protostar rises, ionization of the material occurs. Photons are not absorbed as well by ionized mate-rial, thus a transparent radiative core forms in which the energy is transported by photons. Photons, however, cannot directly move to the surface because they are continually colliding with the nuclei and electrons. In a collision, a photon's direction is changed; it is just as likely to be reflected back into the interior as not. Photons slowly drift outwards to the surface, but along the way each undergoes a tremendous number of collisions, and the time to ultimate escape is large, amounting to as long as a million years or more. At this stage the luminosity reaching the surface has declined greatly, but now starts to slowly rise as the protostar continues its contraction. The protostar's surface temperature also increases, thus it now moves parallel to the main sequence in the HR diagram (see Figure 1 ). When the central temperature rises to about 10 K, the first energy generation via nuclear reactions commences. By this time the outer layers of surrounding material may be blown away, revealing a new star. Figure 1 HR diagram showing pre–main sequence evolution for stars of different masses. The dashed line separates those stages (at right) that are hidden within dense clouds of dust and thus not directly observable from those stages (at left) that can be directly seen when the surrounding dust has been blown away and dispersed. T Tauri variable stars A star does not become sta
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