WHAT CAUSES VARIABILITY in STARS?
Over the course of one human lifetime, the vast majority of stars appear to be constant in both location (relative to other stars) and brightness. A few special stars, however, have been known even since ancient times to vary in their brightness, either periodically or sporadically. These stars, collectively termed variable stars , number well over 30,000 in our part of the Milky Way alone, and as stellar observations become increasingly sensitive, the number of known variable stars in both our own Galaxy and other nearby galaxies continues to grow. Variable stars are classified according to the physical mechanism believed to produce their observed variation in brightness. It is believed that virtually all stars will exhibit some level of variability as their internal structure changes in their old age; these are called intrinsic variables, and it is to this class that Zeta (z) Geminorum belongs. Other stars appear to brighten and dim because their light is physically occulted by an or
