Most of us are used to stars being constant, never changing points of light. Our own star Sol is a constant beacon of yellow white light light in the daytime sky. A fixed object, a fact that brings many comfort though as we will see no star is unchanging, and that includes Sol. Luckily for life on Earth our Sun is fairly stable, though some stars have far more violent mood swings.
In true all stars are somewhat variable, that is that their brightness varies over time. For most, this variability is so small that it can generally be ignored as the changes are too small to affect any readings by a non-negligible amount. A few stars show far more pronounced changes in brightness and other characteristics and it is these that are given designations as true variable stars.
There are numerous categories of variable stars based on what is causing the variability: -
A sub-type of eclipsing binary star -
As can be seen in the animation (right) when one star passes in front of the other some of the light from the other star is blocked making the apparent brightness drop. This is most noticeable when ...