The study of the heavens, at some point, brings one to the realization that the sky is not constant. The moon crosses the sky in its monthly pilgrimage, changing its appearence daily. Comets appear, brighten and fade away.
Meteors zip across the sky to the delight of most.
Less obvious change occurs in the stars themselves. Stars with different characteristics behave differently. Some are quite stable, burning their fuel at a liesurely pace throughout the eons. Some race through their fuel with reckless abandon, eventually expiring in supernova explosions. But there are numerous stars that fall somewhere in between.