We will see the explosion after 100 light years . Its not like that it travels at a speed of 3 lakh kms per second . It also has variations in it . For example , nova ( smallest explosion in universe )'s light travels at a speed of 35 to 50 thousand km per sec . If your explosion is a supernova , it will travel from 75 thousand to 1 lakh , 50 thousand km per sec and if it is a hypernova , it travels till the limit of speed . It is right that any explosion's speed is as much as light but it travels at that speed till a limit . For example , the light seen by a telescope that came from a nova near or on jupiter's moon was as much as light . But if its light is coming from andromeda galaxy so its speed will be not compared to limit as of light . We are very lucky that in milky way , only nova's burst . If only nova's don't burst so our planet will be in danger . So that is why scientists have estimated the speed of bursts as i mentioned earlier . Please rate me the best if you want !!!!!!!!!