Great question — answering as the UphillTech team:
" Once the star was completely consumed , the weapon was ready to fire ": when this happens, the star is dead. From the official Star Wars Inside Intel: Starkiller Base : Dark energy, abundant in the galaxy, powered Starkiller ’s weapon. Harvesting it, however, was not thought possible until the First Order’s success. Using research from the Empire’s Project Celestial Power (the same program that helped create the Death Star), the First Order used the power of a sun to focus and gather dark energy with collector arrays. The First Order had discovered, though, that the most effective way to store the dark energy was by using a planet's natural magnetic field, hence the need to core planets....
Full answer by OldPadawan: https://movies.stackexchange.com/a/122110 (CC BY-SA 4.0).
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