We live in a universe of almost unimaginable size. To give you an idea of the scale, we can only use optical and radio methods to observe objects out to about 13,000 million light years away and nobody knows what lies beyond that. Closer to us we can see clusters of galaxies at distances out to about 750 million light years. The other galaxies of what is know as the Local Group of galaxies are all within 2.5 million light years of us. These galaxies are composed of millions and billions of stars.



Our own galaxy, the Milky Way, is a typical spiral galaxy. This means is looks like a great swirling whirlpool with spiral arms extending out from the center. The Milky Way galaxy is 100,000 light years in diameter and our own star, the Sun, lies on the edge of one of the spiral arms. The Sun is about 30,000 light years from the center of the Milky Way galaxy and takes 225 million years to complete one orbit around the galactic center.

According to modern scientific theory the star we call the Sun was formed about 4,500 million years ago. The Solar System originated when gravitational forces caused what is called the solar nebula to collapse and coalesce into a spinning disc with the central mass forming the Sun. The planets formed from large clouds of gas and dust which gradually built up over million of years as they orbited the central mass of the disc.

The central mass continued to build up in size and as it did the gravitational field it exerted grew stronger. As the gravitational field grew stronger the central mass pulled in more gas and dust and grew even larger.

Eventually, the gravitational force became so powerful that the temperature and pressure at the center of the mass rose high enough to cause hydrogen atoms to undergo fusion into helium atoms. This fusion reaction releases enormous amounts of energy and is the source of the Suns power.

Now illuminated by the Suns rays, the planets of our solar system continued to form and transform. All of the planets have undergone a great deal of change since they first accreted. These changes were brought about in a variety of ways, including violent collisions which resulted in craters that can still be seen today. The planets also went through changes caused by volcanism, melting, structural deformation and the release of gases from deep within.

The planets of our Solar System are generally divided into two groups. The inner planets, which include Mercury, Venus, Earth and Mars and the outer planets, Jupiter, Saturn, Uranus, Neptune and until recently the Dwarf Planet Pluto. The inner planets are all characterized by having solid surfaces and are relatively small compared to the outer planets. The outer planets are all characterized by not having a solid surface and are often refered to as gas giants. The inner and outer planets are also separated by a band of asteroids orbiting between Mars and Jupiter.

Comets, meteors and asteroids can also be found orbiting the sun in large numbers from the outer reaches of the Solar System to very close to the Sun. Comets are balls of frozen liquids and dust which often follow eccentric orbits for outside the orbit of Dwarf Planet Pluto and only display their characteristic tails when their orbit brings them closer to the Sun. Asteroids and meteors are both made mainly of rock. The difference between asteroids and meteors is their size, meteors can be as small a grain of dust, asteroids can be miles in diameter and are sometimes called planetoids.

At the outer fringes of the Solar System beyond the orbit of Neptune is the Kuiper belt where at least three dwarf planets orbit. They are Ceres, Pluto and Eris and they are accompanied by millions of smaller objects about which little is known. At this time a variety of spacecraft are either on their way to the far reaches of the Solar System or are planned for the near future. It will be fascinating to see what wonders these space probes will tell us about the Solar System we call home.