Variable stars are stars that experience changes in brightness over time. These fluctuations can occur over various timescales, from minutes to years, and can be caused by different factors, such as pulsations, eruptions, or interactions with other stars. There are several types of variable stars, including Cepheid variables, which are known for their regular brightness changes and are used to measure cosmic distances. Another type is novae, which can suddenly brighten due to explosive events on their surfaces. Understanding variable stars helps astronomers learn more about stellar evolution and the structure of the universe.