(The light from the takeoff event is just now reaching their planet from Earth all those lightyears away)
One of the books in The Culture series has something similar. A group of people chase the light of a star going nova from a war. The war was like 1000 years ago and they travel to different planets and space stations as the light wave propagates out through the universe so that they can see the star explode from each vantage place. I believe it was Look to Windward but I'm not 100% since it's been years since I read the series.