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An Overview of Astronomy

A Concise Guide to the Universe

The Solar System

The Origin of the Solar System

A. The Solar Nebula Theory: Our solar system was probably formed out of a spinning ball of gas. When the sun became luminous enough, the remaining dust and gas were blown away into space, leaving the planets orbiting the sun This happened about 4.5 billion years ago.

B. The planets move around the sun in orbits that lie nearly in a common plane, and they all revolve about the sun in the same direction (counter-clockwise as seen from the North Pole).

C. By the process of condensation, the heavier elements condensed toward the hotter, inner parts of the nebula, and the lighter, more volatile elements condensed further out. Therefore two distinct types of planets exist. The Terrestrial (Earth-like) planets, Mercury, Venus, Earth and Mars, are small, dense and rocky. The Jovian (Jupiter-like) outer planets, Jupiter, Saturn,. Uranus, and Neptune, are enormous gas giants, and are larger, less dense, and much colder.

D. As the solar nebula cooled, small aggregates of material called planetesimals began to accrete mass. The inner planetesimals accrued heavier material through condensation and the fact that the lighter material had been swept away by the solar wind. The planetesimals further out accrued lighter material. As the planetesimals grew larger, they began to act as vacuum cleaners, ridding the solar system of debris and eventually becoming the current planets.

E. The solar system is left with three types of debris from the solar nebula: asteroids, comets, and meteoroids.

  1. Asteroids are small rocky worlds. Most asteroids orbit the sun in a region between Mars and Jupiter called the Asteroid Belt. This belt is thought to be the remains of a planet that failed to form at a distance of 2.8 AU from the sun. There are about 200 asteroids larger than 60 miles in diameter, about 200 larger than 6 miles, and 500,000 larger than 0.6 miles. The NASA image of Asteroid Vesta, pictured here, shows a set of three craters known as the "snowman."
  2. Comets are large dirty snowballs. They consist of three parts. The nucleus is a ball of frozen water and carbon dioxide, and is usually a few dozen Km in diameter. The coma is the bright area surrounding the nucleus, and may be as large as Neptune. The tail is a luminous trail of debris left behind as the comet melts. As a comet falls into the sun from the Oort Cloud or the Kuiper Belt, it steadily becomes warmer. At about the same distance from the sun as Jupiter (about 5 AU), it begins to melt. The debris it leaves behind behaves similarly to exhaust from a car on a windy day. If there is no wind, the exhaust trails out behind the car. But if it is very windy, the exhaust will go in the direction of the wind, no matter how fast the car is going. A comet travels at about 30 Km/s. The Solar wind, which is continuously emitted from the sun, gusts from 300 to 800 Km/s. The tail of the comet is therefore always blown away from the sun by the solar wind, which also causes it to be luminous. Comet C/1999 S4 LINEAR is pictured.
  3. A meteorite is a space object (a piece of debris. a pebble, or a grain of sand) that survives its plunge through the atmosphere to hit the Earth. Before it hits the Earth, it is called a meteoroid. A meteor is the streak of light across the night sky that one sees as the object burns up in the atmosphere. Meteor showers are events that feature many meteors impinging on the Earth at one time. The usual cause for these showers are events that feature many meteors impinging on the Earth at one time. The usual cause for these showers are the trails of debris left by comets. The showers tend to occur at the same time every year, and have their greatest concentration at the same point in the sky. They are therefore often designated by a name referring to the constellation in which they appear to emanate. For instance, the meteor shower Taurids has its greatest concentration of meteors in the constellation Taurus between November 1-7, and is caused by the Comet Encke. A Leonid meteor is pictured.

Planetary Configurations

  1. An inferior planet is one with an orbit inside that of Earth, i.e., a planet that orbits the sun with a radius less than that of Earth. A planet is in inferior conjunction when it is directly between the Earth and the sun.
  2. A superior planet is one with an orbit outside of Earth. A planet is in superior conjunction when it is directly on the opposite side of the sun from the Earth. A planet is in superior opposition when the Earth is directly in-between the planet and the sun.
  3. An inferior planet will experience points on its trajectory that take it as far from the sun as can be seen from an Earth viewer's perspective. These points are known as the greatest eastern and western elongations.

An Overview of Astronomy

A Concise Guide to the Universe


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