Question

What is Einstein's theory of relativity? And how is it relevant in Astronomy?

Answer 1

In simple words, the theory of relativity implies that Energy of a particle is proportional to its mass when it reaches a certain speed as `E=mc^2`, where c is constant,i.e, the speed of light.

Explanation:

A simple detailed explanation of how he came to the conclusion can be found here. Now it relates to many aspects of astrophysics and to understand those relation you need to go through his whole paper. There you would find how Einstein predicted the presence of black holes, answers to why humans age slowly in space and many more. There is unfortunately no easy one word answer.