Star Formation

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The basic idea of star formation is gravitational collapse – the contraction of a region of gas under the influence of gravity. This is a simple process that would be expected to occur in any region of material dense enough for collisions between atoms to radiate away energy. However, the gas must be dense enough for collisions to occur and the temperature must be low enough for the atomic velocities not to be able to escape the system's gravity, so star formation only occurs in a few areas.

The sites of star formation in the galaxy are mostly located within molecular clouds – expansive, cool clouds of mostly hydrogen and helium gas. Molecular clouds are on average too diffuse to contract gravitationally, but within a cloud are regions of locally higher density, which are the sites of active star formation. It is not known exactly what causes molecular clouds and star-forming regions to be distributed as they are. However, it appears to be related to the spiral-arm structure of spiral galaxies, which is thought to be the result of density waves passing through the disk, compressing matter and igniting star formation in their wake, leaving the trails of young, hot, blue stars in their wake that are the primary feature of spiral galaxies.

The distribution of gas in these regions is probably erratic enough that once a sufficient compression comes through a large number of separated regions will be triggered to contract individually. One a sufficiently dense region begins to contract, the process becomes self-sustaining, as contraction only increases the density and makes contraction more rapid. Any net rotation in the region will be flattened by conservation of angular momentum as the cloud contracts and spins more rapidly, making the contracting region attain the shape of a disk.

The center of mass of the cloud will, of course, be the location of highest density, and as matter rains onto the center it will heat up rapidly to very high temperatures. This 'protostar' will emit strong radiation and winds which, while not immediately enough to stop the influx of matter from the disk, generates powerful outflow jets along the poles of the system, which may also be influenced by magnetic force originating in the disk.

Throughout the process the temperature, density, and pressure in the protostar at the center are increasing, and soon approach figures comparable to normal stars. Once the temperature of the center

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