Variable stars and stellar evolution were not part of the Medieval mind. Until recently, stars were considered eternal and unchanging. Now we know that they are dynamic objects, operating on a time scale of millions and billions of years. We can measure their pulsations on a human time scale, though. Some stars vary their output in just a few hours, and many change in a matter of days. Given the size of a typical star, that is pretty astonishing. Then there are the Mira variables, which dim to the point of completely disappearing from our view, changing brightness sometimes by a factor of 10,000 in a little over a year.
If these objects intrigue you, welcome to the club. They have captivated my imagination since my first astronomy class, and continue to amaze me with their variety. Want to be a part of it? Join the AAVSO and put your finger on the pulse.