Stupornova

 As civilizations progress, they get more "complex", that is, more inter-related tasks are required for them to function. At the same time, the tasks get easier to accomplish physically and mentally. Civilization makes life easier for the people who live within it.

These two factors are the heat and gravity in a star. Gravity causes fusion of light elements like hydrogen in a star, which in turn release heat and light and other forms of radiation. The heat of fusion counter-acts the force of gravity and prevents the star from entirely collapsing and either exploding, or forming a black hole, or neutron star.

Once the lighter elements are all used up, though, the fusion process eventually creates Iron, which actually absorbs heat, then the star collapses catastrophically and explodes.

The complexity of a civilization eventually outstrips the ability of the civilization to supply people to service it because the civilization keeps making people dumber. I don't think this is caused by genetics. It's caused by ease and comfort and habit. There's probably nothing in the civilization that can forestall the degradation.

Eventually you get the "fusing iron" moment in the civilization where the catastrophic collapse happens. I think we're there right now. Maybe it happened earlier in this century. I'll say 2005 was the crossover.