Question

suppose a star which is 100 light year away explodes today It is unlikely that you will be to see the explosion why?

Answer 1

Its because it's very far, 100 light years means even light would take 100 years to travel form the point of explosion to the point of observation .. as far as the light is casuality thus it will be very much difficult even it to observe the explosion like that ..

Answer 2

You cannot see the explosion at least for 100 years because the star is 100 light years away and light will take 100 years to reach here.

There are chances that we may not see the explosion because:
1. Distance is very large, so the light may fade below visible light. Interstellar space contains many particles such as dust, gases, etc. When light collides with these, it loses energy, so it may go below visible light. However, space telescopes that see in infra and radio spectrum will see the explosion.
2. The explosion is not powerful enough.
For eg., Crab Nebula was formed by the explosion of a star. It was observed for many months until it faded away to radio spectrum.