The solar system is comprised of the sun and other celestial bodies which do not have light of their own. These bodies are gravitationally bound to the sun and revolve in specific orbits around it. As of now, only one solar system is known in the entire universe. Our planet earth is part of this solar system. Astronomers have, however, conjectured that there might be more than a million such solar systems in the vast universe.
A galaxy is a a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction. The galaxy of which we are part is called the Milky Way and it can be seen in the night sky as a hazy band of light. The Milky Way is made up of about 200 to 400 billion stars, and our solar system is located within this system.
The sun, with all its planets, is revolving around the Milky Way Galaxy at an orbital speed of 200 km/s. The Milky Way is so large and spread out that it takes the sun about 200 million years to make a complete revolution around the centre of the galaxy. What is even more astonishing is the fact that there are about 100 billion such galaxies scattered in the vastness of the cosmos, with each galaxy composed of about 100 billion stars.
The stars within a galaxy are located at vast distances from each other. Light travelling from our nearest star, Proxima Centauri, takes about four years to reach the earth.
What is it that is holding together the enormous system of celestial bodies? Astronomers say it is the mutual gravitational attraction of these objects that has held them in place in the vastness of space. It is indeed strange that people understand the meaningfulness of the phrase mutual gravitational attraction of celestial bodies, but do not find meaning when the word God is used to explain the astounding phenomena of the universe.