Everything that exists - rocks, air, plants, animals, the Earth, the planets, the Sun, the stars, the galaxies and even space itself - makes up what we call the universe. Essentially, the universe consists of matter, energy and space. The universe is vast; we cannot even begin to imagine how big it is. Our little corner of the universe is dominated by the Sun, our local star, which has gathered about it a family of planets and other bodies. They form the solar system. It measures something like 15,000 million km across. To us, this distance seems enormous. But in relation to other distances in the universe, it is tiny. Compare it with the distance to the nearest stars - 40,000,000 million km. Compare it with the distance across our galaxy - 1,000,000,000,000 million km. Compare it finally with the distance to the remotest objects (quasars) we can see in the heavens - over 100,000,000,000,000,000 million km. Distances expressed in kilometres are meaningless, which is why astronomers often use the distance light travels in a year - 10 million million kilometres - as a basic unit, the light-year. Then, it becomes easier to express distances. The nearest stars, for example, lie just over 4 light-years away; our galaxy is 100,000 light-years across. The farthest quasars are over 10,000 million light-years away. The universe, astronomers think, was born about 15,000 million years ago in a cataclysmic event they call the Big Bang. The Big Bang created a universe and set it expanding. We see this expansion today in the outrush of the galaxies. This concept is called the expanding universe.
