It is impossible for the human mind to conceive a true picture of the size of the universe. We not only don’t know how big it is, but it is hard for us even to imagine how big it might be.
If we start from the earth and move out, we’ll see why this is so. The earth is part of the solar system, but a very tiny part of it. The solar system consists of the sun, the planets that revolve around it, the asteroids, which are tiny planets, and the meteors.
Now, this whole solar system of ours is only a tiny part of another, much bigger system called a galaxy. A galaxy is made up of many millions of stars, many of which may be much larger than our sun, and they may have solar systems of their own.
So the stars we see in our galaxy, which we call the Milky Way, are all suns. They are all so far away that distances are measured in light-years instead of in miles. Light travels about 6.000.000.000.000 miles in a year. The bright star nearest to the earth is Alpha Centauri. Do you know how far away it is? 25.000.000.000.000 miles!
But we’re still talking only about our own galaxy. This is believed to be about 100.000 light-years in width. This means 100.000 times 6.000.000.000.000 miles! And our galaxy is only a tiny part of a still larger system.
There are probably millions of galaxies out beyond the Milky Way. And perhaps all these galaxies put together are still only a part of some larger system!
So you see why it is impossible for us to have an idea of the size of the universe. Incidentally, it is believed by scientists that the universe is expanding. This means that every few billion years two galaxies will find themselves twice as far apart as they were before!