Heliocentric, Tidal and Nebular Theory Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 16, 2011

The Tidal Theory

According to the tidal theory , the Sun originally had no planets or other satellites. This theory suggests that our Sun formed alone and that the other objects, including the planets and the major asteroids, came later.

Relative Motions Of The Stars

The Milky Way, the spiral-shaped galaxy in which we dwell, is believed to be 100,000 light-years across. A light-year is the distance that light travels in 1 year, approximately 9.5 trillion (9.5 × 10 12 ) km or 5.9 trillion (5.9 × 10 12 ) mi. Our galaxy has roughly 200 billion (2 × 10 11 ) stars, all revolving around the nucleus like an enormous swarm of bees. According to current theories, many of the stars bob up and down, above and below the galactic plane, passing periodically through it. Some stars have highly eccentric orbits around the galactic center.

Although the stars are tiny compared with the space between them, they are in relative motion, and collisions or near misses occur once in a while simply because there are so many stars. On average, however, according to one estimate, an outright collision is an extreme rarity, taking place only about once in every 10 billion (10 10 ) years for a typical spiral galaxy such as ours. This is almost as old as the whole Universe is believed to be! Nevertheless, those people who say that the Sun fell victim to a near catastrophe with another star cannot be discounted completely.

The Scenario

Suppose that another star came close enough to the Sun that it and the other star engaged in a gravitational tug-of-war. What would happen? For one thing, the paths of both stars in the Milky Way would be altered; the two stars would swing around each other. In fact, if they came close enough and the speed was not too great, they would end up in orbit around each other. Suppose, however, that the encounter was extremely close but at high speed so that the two stars did not end up in mutual orbit? They would pull matter from each other and scatter that matter into orbits around either star, where the matter would cool, condense, and form dust, rocky ice chunks, and rocks.

Given time, the particles in orbit around the Sun would coalesce into larger objects because of mutual gravitation. Eventually, several dozen spherical objects, perhaps comparable with the size of our Moon, would be created. These objects would follow all kinds of different orbits because of the chaotic way in which the matter was scattered during the original battle of the stars. The result would be frequent collisions and further coalescing. Computer models can show that the end result would be a few large, massive objects and countless tiny ones. This is, of course, the way we observe the Solar System today.


There are problems with this so-called tidal theory . If this is the way the Solar System formed, the planets would all revolve around the Sun in different planes, and their orbits would be less circular and more elongated than they are (Fig. 9-6). However, the actual state of affairs is far more orderly. The planets all lie in nearly the same plane. With the exceptions of Mercury and the Pluto-Charon system, their orbits are nearly perfect circles. All the planets revolve around the Sun in the same direction. For these reasons, few astronomers today believe that the tidal theory is an accurate representation of what happened. In addition, the fact that such catastrophes in general occur only once every several billion years, in our galaxy at least, is a good reason to doubt that this theory explains how things took place to create our Solar System.

Evolution of the Solar System The Nebular
Theory The Stuff Of Stars

Figure 9-6. If the tidal theory were correct, the planets would have elongated orbits (gray ellipses) in various planes.

The Nebular Theory

If a star has several times the mass of the Sun, ultimately it will explode in a violent outburst called a supernova . These events leave entrails in space—clouds of gas, dust, and rocks of various sizes—in their vicinity. Such mass of debris can appear either light or dark through a telescope depending on how the light of nearby stars shines on it. The cloud is called a nebula .

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