The favoured paradigm for the origin of the solar system begins with the gravitational collapse of part of an interstellar cloud of gas and dust having an initial mass only 10 to 20 percent greater than the present mass of the Sun. This type of collapse could be initiated by random fluctuations of density within the cloud, one or more of which might result in the accumulation of enough material to start the process, or by an extrinsic disturbance such as the shock wave from a supernova. The collapsing cloud region quickly becomes roughly spherical in shape. Because it is revolving around the centre of the Galaxy, the parts more distant from the centre are moving more slowly than the nearer parts. Hence, as the cloud collapses, it starts to rotate, and, to conserve angular momentum, its speed of rotation increases as it continues to contract. With ongoing contraction, the cloud flattens, because it is easier for matter to follow the attraction of gravity perpendicular to the plane of rotation than along it, where the opposing centrifugal force is greatest. The result at this stage, as in Laplace's model, is a disk of material formed around a central condensation.
This configuration, commonly referred to as the solar nebula, resembles the shape of a typical spiral galaxy on a much reduced scale. As gas and dust collapse toward the central condensation, their potential energy is converted to kinetic energy (energy of motion), and the temperature of the material rises. Ultimately the temperature becomes great enough within the condensation for nuclear reactions to begin, thereby giving birth to the Sun. Meanwhile, the material in the disk collides, coalesces, and gradually forms larger and larger objects, as in Kant's theory. Because most of the grains of material have nearly identical orbits, collisions between them are relatively mild, which allows the particles to stick and remain together. Thus, larger agglomerations of particles are gradually built up.
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