Soon after entering elementary school, he began to express a strong inquisitiveness about nature.
Sagan recalled taking his first trips to the public library alone, at the age of five, when his mother got him a library card.
He wanted to learn what stars were, since none of his friends or their parents could give him a clear answer:
"I went to the librarian and asked for a book about stars ...
 

And the answer was stunning.

 
It was that the Sun was a star, but really close.  The stars were suns, but so far away . They were just  little points of light. 
The scale of the Universe suddenly opened up to me. It was klind of a religious experience. ..

It Just Grabbed Me. There was a magnificense to it, a grandeur, a scale that has never left me...
Never ever left me."     Carl Sagan