These stars are not twinkling - they're varying in brightness! Many of these stars are changing in brightness by more than a factor of 2! The following movie of a portion of M33 is composed of 6 observations taken over 2 years (displayed as a repeating animated gif). Note the quantity of variables (particularly among red stars). We usually think of galaxies as relatively static entities, but if you watch it over time you can see that it is alive with variable stars. The green regions are H-alpha emission nebulae. A few of the brightest stars appear to be varying wildly (a yellow-white star at the upper left in particular) - these stars are actually saturated in the image, their variation is an artifact.



The green box on the above movie shows the field of view of the following zoomed in movie. Most of the stars are long period variable red giants or asymptotic giant branch stars (these are the red variables). The yellower variables are Cepheids - well known for their period-luminosity relation which can be exploited to determine the distance to galaxies such as this one. In fact, our measurement of the Hubble Constant, and by inference, the age of the Universe comes from finding and studying these yellow variables in distant galaxies. The typical period for a Cepheid is between 2 and about 40 days, so even if we were to make a movie with observations spanning only a month the Cepheids would still appear quite variable. To see the longer period red variables you need to span at least a year.