Isso é explicado na Junior Novelisation do filme. A super arma é capaz de disparar um feixe que viaja mais rápido que a luz. Evidentemente, isso também cria um efeito que pode ser visto mais rápido que a luz, explicando por que as pessoas em outros sistemas podem vê-lo em tempo real:
Outside the castle with Chewbacca, Han surveyed a section of Takodana’s sky through a compact ponipin telescope. He’d heard many theories about the origin of the star that had recently blinked into existence. None made any sense. New stellar bodies didn’t just brighten the sky all of a sudden.
Han’s ponipin measured that the new star was also many, many light-years away, which under normal astronomical circumstances meant that it had actually blazed to life years before. Moreover, if the calculations proved correct, the stellar coordinates happened to be the same as those of the Hosnian system, where the capital of the New Republic was located.
Could the Hosnian system have gone nova? Could it—dare he even consider—have been destroyed? And if so, how had it happened so quickly?
Isso foi confirmado em um tweet pelo executivo criativo do filme, Pablo Hidalgo
Aparentemente, as explosões foram ...
... Some weird-ass hyperspace-rip seen-across-the-cosmos kinda shit.