SOLAR SYSTEM

THE SUN

The Sun is the Center around which the Earth and the other planets of our solar system revolve. It is a rather ordinary star of average size. Of course, the Sun appears much bigger and brighter to us because it is much closer to the Earth than is any other star. It is about 93 million miles away. The next-nearest star, Alpha Centuari, is more than 25 trillion miles away.

Our Sun is only one of about 100 billion stars in our galaxy, the Milky Way. It is located in one of the outer, spiral arms of the Milky Way, about three-quarters of the way from the galactic center.

The Sun is a vast ball of hot, glowing gas, some 870,000 miles across - more than 100 times the diameter of the Earth. The Sun's mass, however equals that of 333,420 Earths. This tremendous weight produces a pressure at the center of the Sun of more than 1 million metric tons per square centimeter. 

The Sun's gravity is 28time stronger than that of the Earth. So a man weighing 150 pounds on Earth would weigh 28 x 150 pounds, 4,200 pounds, if he could stand on the surface of the Sun.

In spite of the great mass of the Sun, its average density -  the weigh of standards volume of its matter-is only 1.4 times the weigh of an equal volume of water. The Earth, on other hand, is 5.5 times denser than water. This low solar density is easy to explain. The center of the Sun, because of enormous pressure, is more than 100 times denser than water. but much of the Sun beyond the center is composed of gas that is often thinner than the Earth's atmosphere. When this densities averaged together, the general density of the Sun is quite low.

The Sun i like a huge furnace, fired by nuclear, or atomic, energy at its core. temperatures at the center may be 25,000,000F or more. At the surface, temperatures are much cooler-between 9,000 and 11,000F still hot enough to vaporize nearly all substances that exist as solids or liquids on the Earth.

MOONS

A moon is any natural body that orbits a planet. There are at least 35 known moons in our solar system. The majority of them orbit the giant planets Jupiter and Saturn, and are little more than huge, airless ball of ice, ranging from hundreds to more than a thousand miles across. One of the largest moons, Saturn's TITAN, is so big(3,169miles) that it retains its own atmosphere of nitrogen. Mars has some of the smallest moons, a pair called Deimos and Phobos, each no bigger than an asteroid, which indeed they may have been at one time.

ASTEROIDS

In the 18th century, astronomers calculated astrophysical laws that predicted they would find an as-yet-unseen planet between Mars and Jupiter. And they eagerly searched the skies for it. On the night of January 1, 1801, the Italian astronomer Giuseppe Piazzi discovered a small celestial body, which he took to be planet, in the space between the orbits of Mars and Jupiter. This body, which was later called Ceres, was found to have a diameter of only 478miles. Over the years, many more small, planet-like bodies were found in the gap between mars and Jupiter. Today more than 1,000 of these small bodies have been discovered, leading astronomers to estimate there may be more than 50,000 in all.

Astronomers once thought that asteroids were fragments of a big planet that once orbited between Mars and Jupiter and then broke apart for unknown reason. But in recent years, scientist have come to believe that asteroids are probably debris left over from the solar system's formation, debris that simply never coalesced to form a planet.

COMETS

Comets are among the strangest members of the Solar System. Instead of moving as the planets do, in nearly circular orbits in the same direction, comets revolve around the Sun in every conceivable direction. Much of the time they are so far away from the Sun that they are invisible even to our largest telescopes.

It was once thought that some comets approached the Sun from far beyond the solar system, and that once they withdrew from the Sun, they would never return. Today it is generally agreed that comets are members of the Sun's family. They make up a vast shell of icy debris called the Oort Cloud. Though this region lies 50,000 times farther from the Sun than does Earth, the trillions of icy comet bodies that inhabit it are all gravitationally bound to the Sun.  

When astronomers 1st discovered a comet, it usually appears as a faint, diffused, fuzzy star, with a dense, star-like center and a veil-like region, known as its coma. As the comet approaches the Sun, its coma becomes brighter, as more and more material vaporizes off the surface of the comet's solid, icy nucleus. When they are some 100million miles from the Sun, some comets begin to show a tail streaming behind them, pointing directly away from the Sun. Comets tails appear to consist of very thin gases that fluoresce, or glow, under sunlight, as well as a fine stream of dust particles. This material is forced away from the Sun by the pressure of the solar wind.