As it is told in the question the star is 100 light years away,so,if it explodes today we would be able to see the explosion but after 100 years from the explosion as light would take 100 years to reach to  the Earth.
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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 !!!!!!!!!

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