This question already has an answer here:
So I'm writing a novel, where the planet's sun is (although the people who live on the planet don't know this) artificial. It requires a "sacrifice" of blood magic that fuels its core once every 20-30 years, but for plot reasons, things don't go according to plan and it doesn't get its blood. How would a planet react to such a phenomena? It loses about half of its "strength" and it will go on for a year or so, before completely snuffing out.
The planet is like a remnant of old technology. It's actually a terraformed moon, with an artificial sun that is collapsing. It was set to receive "magic blood" to reignite itself after some decades, but it's getting more and more unstable. The civilizations on the planet believe that it's just "magic", as they live in a sort of medieval era since the Sun construction took place millennia ago "By the gods."
My main question would be how would it affect the light available, and also warmth? Would the flora and fauna start to die immediately? Sorry for the vagueness, too, english isn't my main language.