How do we know this? Yes, I'm serious.
In the whole HISTORY OF THE GALAXY, this one thing has never happened?
The lifetime of a star is determined by its initial mass. As it turns out, the initial mass of a star tracks with its color, so the massive stars are blue and hot, while the small stars are red and cool. The hotter and more massive stars burn out faster than the small cool stars. I know it’s counter-intuitive, but the way the hydrogen burning rate works out as a function of mass, the burn rate goes up faster than the available mass. The lifetime of hot massive stars is measured in tens of millions of years, an instant on the cosmic scale. Cool red stars, on the other hand, are expected to live for tens of billions of years, longer than the current age of the universe.
Anyway, the other way to check stellar lifetime is to observe globular clusters where all of the stars formed at the same time. A number of these clusters have no hot blue stars because they have all “died,” but the yellow and red stars remain.
MD, temporarily donning his “professional astronomer” hat