I had forgotten to mention that if there is some way to remove the sun-sucking plot point (seems like it shouldn’t be too difficult–remove all references to it, and the darkening sky could simply be implied as the day wearing on during battle), it should be done. That is lame on a number of levels. And if this is done, clips of that effect could be run backwards, appearing to be the energy beam striking the distant sun, and used to simulate the supernova I’ve suggested.
That idea could also be realized if the planet’s natural rotation would reach ‘target window’ before being able to fire the sun killer blast that destroys a system. The sky darkening could simply be when sunset is reached as the focus cannon comes into line of sight range of D’Qar. It’s just a new twist on the Death Star orbiting Yavin in order to get a clear shot at Yavin IV. The clock simply ticks down during the planetary rotation of Starkiller Base.