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Lecture

PCS 181 Lecture Notes - Bok Globule, Outer Space, Main Sequence


Department
Physics
Course Code
PCS 181
Professor
Margaret Buckby

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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 premain
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 contractionone-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 ).
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