THE UNIVERSE HAS A STRUCTURE
All these investigations have convinced astronomers that firstly, the universe is of inconceivably immense size. Secondly, the heavenly bodies are not spread out uniformly in space but are in a series of groups. The basic unit in each group is a star, of which our Sun is an average specimen. The Sun has the Earth and other planets in orbit around it. The stars we can see on a clear night are only the Sun's immediate neighbours in space. The nearest star is 25 trillion miles away and light from it, travelling at 186,000 miles per second takes about 4.3 years to reach us – i.e. at a distance of 4.3 light-years. To help you better envisage this distance, if the distance from the Earth to the Sun (93 million miles) were represented by one inch, then the nearest star would be four miles away.
|Beyond the stars of the Milky Way, our own galaxy, can be seen another similar galaxy and beyond this are more galaxies that appear smaller because they are further away.|
This distance is small in astronomical terms. On a clear night the Milky Way can be seen as a bright hazy band across the sky. With a telescope the Milky Way is seen as millions upon millions of stars, each like our Sun. This cluster of stars is called a galaxy and is a mass of stars formed into a flat disc about 100,000 light-years in diameter. Our Sun with its solar system and the comparatively few stars we can see with the naked eye, are situated towards the edge of this galactic disc.
At one time our galaxy was thought to be the entire Universe but it is now known to be but an infinitely small part of it. There are millions of other galaxies organised in groups. In what is prosaically styled our ‘local group’ are about 20 galaxies but this is a comparatively small group. About 50 million light-years away is a group that contains thousands of individual galaxies.
Your mind may be reeling at the magnitude of all this – but we have not yet described the Universe. These groups of galaxies are themselves aggregated into superclusters of about 150 million light-years across. A large number of these superclusters, separated from each other by immense distances, form the observable Universe.
This then is the modern concept of the Universe. We could summarise our relationship to it as follows:
|many||SUPERCLUSTERS||each of which contains|
|many||GROUPS||each of which contains|
|many||GALAXIES||each of which contains|
|billions of||STARS||one of which is our|
|SUN||which has a planet called|