Rev 8:12. The fourth angel blew his trumpet, and a third of the sun was struck, and a third of the moon, and a third of the stars, so that a third of their light might be darkened, and a third of the day might be kept from shining, and likewise a third of the night. A third of the light from the sun is dimmed.
With all these fires from a third of the earth burning and volcanic activity going on, it could be that. This would happen slowly, and I get the impression this happens dramatically because of the use of the word “struck”. If it is an instant event, that would have to be due to the sun not functioning properly. Perhaps it has something to do with the thermonuclear reactions in the sun itself. This may relate to the fifth wrath, and there is some more information there. The sun does have cycles that it goes through. Every four hundred years, it seems to go to sleep. The last time was in the sixteenth century when there was a mini-ice age. That cycle is a four hundred year cycle, so we are due for the next one now.