The origin of the sun and the planets

The origin of the sun and the planets – İngilizce ileri düzey okuma parçası (advanced reading)

The suggestion that the material of the earth was indeed derived from an exploding star – a supernova, is supported by strong evidence. The shower of stars must have been surrounded by a cloud of gas – the cloud from which the stars had just condensed. A supernova, undergoing violent disintegration, must have expelled gases that went to join this cloud, the material from the supernova thereby getting mixed with the large quantity of hydrogen of which the cloud was mainly composed. Our problem is then to explain how both the sun and the planets were formed out of this mixture of materials.

It is a characteristic of a good detective story that one vital clue should reveal the solution to the mystery, but that the clue and its significance should be far from obvious. Such a clue exists in the present problem. It turns on the simple fact that the sun takes some 26 days to spin once round on its axis-the axis being nearly perpendicular to the orbits of the planets, which lie in nearly the same plane. The importance of this fact is that the sun has no business to be rotating in 26 days. It ought to be rotating in a fraction of a day, several hundred times faster than it is actually doing. Something has slowed the spin of the sun. It is this something that yields the key to the mystery.

Stars are the products of condensations that occur in the dense inter-stellar gas clouds. A notable cloud is the well-known Orion Nebula whose presence in the ‘sword’ of Onion can easily be seen with binoculars. Stars forming out of the gas in such clouds must undergo a very great degree of condensation. To begin with, the material of a star must occupy a very large volume, because of the extremely small density of the inter-stellar gas. In order to contain as much material as the sun does, a sphere of gas in the Orion Nebula must have a diameter of some 10,000,000,000,000 miles. Contrast this with the present diameter of the sun, which is only about a million miles. Evidently in order to produce a star like the sun a blob of gas with an initial diameter of some 10 million million miles must be shrunk down in some way to a mere million miles. This implies a shrinkage to one ten millionth of the original size.

Now it is a consequence of the laws of dynamics that, unless some external process acts on it, a blob of gas must spin more and more rapidly as it shrinks. The size of a condensation and the speed of its spin keep an inverse proportion with each other. A decrease of size to one ten-millionth of the original dimensions leads to an increase in the speed of spin by 10 million. But the rotation speed of the sun is only about 2 kilometres per second. At a speed of 100 kilometres per second the sun would spin round once in about half a day, instead of in the observed time of 26 days.
Only one loophole remains. We must appeal to some external process to slow down the spin of the solar condensation. Our problem is to discover how such an external process operates. First we must decide at what stage of the condensation the external process acts. Does it act while the condensing blob still has very large dimensions? Or does it operate only in the later stages, as the condensation reaches the compact stellar state ? Or does it operate more or less equally throughout the whole shrinkage?

A strong hint that the process must act mainly in the late stages of the condensation comes from observations of the rates of spin of stars. It is found that the rates of spin have a very curious dependence on surface temperature. Stars like the sun, with surface temperatures less than 6,000° C, rotate slowly like the sun. But stars with surface temperatures greater than 7,000° C rotate considerably more rapidly, their equatorial speeds of rotation being usually greater than 5o kilometres per second. Although this is still much less than what we should expect if no external process were operative, it is considerably greater than the equatorial rotation speed possessed by the sun.

This shows that while the external process must be operative in all cases, it is operative to different degrees that depend on the surface temperature of the final star. Now the difference between one star and another can scarcely show at all during the early stages of the shrinkage. Certainly the difference between two condensations, one yielding a star of surface temperature 6,000° C and the other yielding a star of surface temperature 7,000° C, must be very small indeed during the early stages: much too small for the stars to come to have markedly different rotation speeds if the external process were of main effect during the early stages. The inference is that the process operates mainly during the late stages of condensation.

8. Now what was the external process? We have mentioned that rotary forces must have become important during the late stages of condensation. The effect of these forces was to cause the condensation to become more and more flattened at its poles. Eventually the flattening became sufficient for an external rotating disc to begin growing out of the equator. The sequence of events is illustrated in figure 1.

Once the sun had thus grown a disc the external process was able to come into operation. The process consisted of a steady transference of ‘rotational momentum from the sun to the disc. Two birds were thereby killed with one stone. The sun was slowed down to its present slow rate of spin and the disc, containing the material out of which the planets were subsequently to condense, was pushed farther and farther from the sun. The solar condensation probably first grew its disc when it had shrunk to a size somewhat less than the orbit of the innermost planet, Mercury. The pushing outwards of the main bulk of the disc explains why the larger planets now lie so far from the sun.

It may be wondered why such an obvious theory was not put forward long ago. The answer is that there seemed to be such grave objections to it that not until very recently has it been examined at all seriously. And now it turns out that the objections are not so grave as was previously believed.

(From Chapter VI of Frontiers of Astronomy by Fred Hoyle.)