Heliocentric, Tidal and Nebular Theory Help (page 3)

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

The Stuff Of Stars

Most nebulae form near the plane of our spiral-shaped Milky Way galaxy. They are clearly visible in other spiral-shaped galaxies when those galaxies present themselves edgewise to us. Some spiral galaxies are so thick with nebulae that they appear split in two when we see them from within the planes of their disks. Our Solar System is near the plane of the Milky Way, and our galaxy, like all spirals, has plenty of nebulae. This keeps the sky dark at night. If it were not for these obstructing clouds, the sky would be almost as bright when the Sun is “down” as when the Sun is “up.”

According to the nebular theory , also called the rotating-cloud theory , it is from these clouds that second-generation stars, such as our Sun, are born. Evidence suggests that the Solar System formed approximately 4.6 billion (4.6 × 10 9 ) years ago from one of these. The Earth, all the other planets, the asteroids, and the comets are all believed to have formed from a cloud produced a long time ago in one or more supernovae.

As you have already learned, the Sun takes about a month to rotate once on its axis. Because of this, it is logical to suppose that the debris cloud from which the Sun formed had some rotational momentum. Imagine a hurricane forming from the clouds in the tropics. Have you ever seen a time-lapse satellite photo of this process? Think about the eddies or whirlpools that form in the water as you pull a canoe paddle through. According to the rotating-cloud theory, the Sun formed at the center of an eddy in interstellar space.

The Accretion Disk

Astronomers have shown that a cloud of debris, collapsing because of the mutual gravitation among all its particles, would develop one or more vortices, or whirlpools. Near each vortex, the matter would become aligned in a plane, creating a rotating, disk-shaped cloud. It can be demonstrated by computer modeling that the matter in such a cloud would condense into an accretion disk and thence into numerous discrete objects: a large central mass (to become the Sun) and other, relatively small masses in orbit around it (to evolve into the planets and their moons). One theory, proposed several centuries ago, took notice of this fact (without the help of computer modeling, of course) and came to the conclusion that the matter orbiting the Sun would congeal into rings before finally developing into solid planets.

Figure 9-7 is a hypothetical illustration of how the Solar System’s primordial cloud looked from a distance of about 100 astronomical units (AU). In this example, the disk is viewed at an angle, neither face-on nor edgewise, so that the nearly circular rings appear oblong. The Sun is at the center, and it is about to start up its internal nuclear-fusion furnace. The disk-shaped cloud, and in particular its rings, glow from the Sun’s increasing radiance and from the light of other nearby stars. According to the rotating-cloud theory, the particles in the rings gradually pulled themselves together over a period of millions of years into small objects called planetesimals , and these ultimately accreted into the planets. Most of the non-solar matter in the cloud found its way into the planet Jupiter; smaller amounts congealed into the other planets. As the planetesimals aggregated into larger objects, the matter in them swirled just as had the original parent cloud. This explains why the planets rotate. It also explains why most planetary moons orbit in the same sense as all the planets orbit around the sun and why most (but not all) planetary moons orbit near the plane of the planets’ orbits.

Evolution of the Solar System The Nebular
Theory The Accretion Disk

Figure 9-7. The primordial gas-and-dust cloud according to the rotating-disk theory of solar system formation.

The original version of this nebular theory is credited to two men who lived during the eighteenth and early nineteenth centuries: Immanuel Kant, a German philosopher, and Pierre-Simon Marquis de Laplace, a French astronomer and mathematician. In particular, Laplace went into detail concerning the motions of the various planets and moons. In recent decades, the nebular theory has been refined, especially in an attempt to explain why the Sun rotates only once a month and not much faster. In addition, the existence of the rings in the primordial accretion disk has been questioned. Many astronomers believe that the matter simply clumped together into larger and larger “particles,” ending up with the system of planets we now have. The asteroids in the belt between Mars and Jupiter were prevented from accreting into a planet because of the powerful gravitational influence of Jupiter.

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