The Formation of the Solar Nebula

A cloud which is trying to contract to become a star has several problems to overcome before it can do so. One of these is the outward pressure, discussed above. In this case, the solution to the problem is to heat the cloud to a high enough temperature so that some of the heat is being continually radiated away, reducing the amount of heat left over to create pressure.

Another problem is angular momentum. Even while the cloud is huge, it must have some tiny amount of rotational motion. The random motions which occur in different parts of the cloud will probably nearly cancel each other out, so that any overall motions are small, but it is not likely that they will exactly cancel, and so some tiny rotational motion is to be expected.

In many cases, the rotational motion of the cloud may be so large that it impossible for the gravity of the cloud to overcome it, and the cloud remains as a large cool blob of gases, but the existence of so many stars in our Galaxy shows that there must be various ways in which stars can overcome this problem. One way is for the cloud to break up into two or more blobs, revolving around each other—a binary or multiple star system. Such systems are in fact quite common. Between one-third and one-half of all the stellar systems in our Galaxy are thought to consist of such multiple stars.

Since we do not know of any companions to the Sun, it seems that our Solar System solved the problem in a different way. Presumably the amount of rotation in the cloud was fairly small, and as a result, those parts of the cloud which happened to have a smaller amount of rotation could fall inwards more-or-less uniformly, forming a large, roughly spherical ball near the center, which became the Sun, while those parts of the cloud which were rotating the fastest formed a flattened circular disk rotating around the central ball, the Solar Nebula. As the cloud contracted, parts of it which were moving parallel to the axis of rotation (getting closer to the plane of rotation) would not have had their inward motion affected by the rotation, but parts which were moving in the plane of rotation (getting closer to the axis of rotation) would have gradually increased their rotational speeds, just as ice skaters spin faster by pulling their arms towards their bodies. The same thing can be seen in the motion of the planets around the Sun; Kepler's Law of Areas is mathematically equivalent to the Law of Conservation of Angular Momentum which determines how rotating objects speed up as they get closer to their axis of rotation.

Дата добавления: 2015-09-04