Based on counting the number of galaxies in a small patch of the sky and multiplying by the number of such patches needed to cover the entire sky, the total number of galaxies in the observable universe is estimated to be approximately:

a. 100 million
b. 1 billion
c. 10 billion
d. 100 billion
e. 1 trillion

D) 100 billion

Explanation:

This method is used by astronomers all over the world because it is impossible to count all the stars. Even in parts of the sky that seem to be totally dark, there are dozens of galaxies, each with millions of stars.  

Our universe is so incredibly huge that even a number as big as 100 billion stars seems to be small. But using that technique, astronomers have calculated that there are 100 billion stars in the universe. I believe that number will change in the future as soon as we are able to build better telescopes, and astronomers will be capable of counting more stars in each patch.

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