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.