There is a special group of stars that change over a relatively short period of time. This can be a change in brightness, mass, density, rotation or composition.
These variable stars can be divided into three categories. There are pulsating, eruptive and eclipsing variables.
Eclipsing variables cannot be compared to the other groups. Although they do change in their brightness, this has external causes. They are binary stars that appear as a single point of light to us as they are very close together. They appear very bright. If these stars occlude each other during their movements, their brightness changes regularly. These regular changes in brightness allow us to calculate the orbital periods of both stars.
Examples of this group are Algol in Perseus, Epsilon Aurigae in Auriga and Beta Lyrae in Lyra.
Pulsating variable stars also change in their brightness. This has internal causes. These stars expand and contract at regular intervals. They pulsate, changing their surface temperature and thus their brightness. One of the most important groups within this category are the Cepheids, stars with a high luminosity, which change in brightness within a few days.
Stars that increase in brightness suddenly and unexpectedly belong to the category of eruptive variables. Among them are novas and supernovas, once thought to be new stars. Today we know that they occur at other stages of a star's development. They are quite rare. A nova, for example, can suddenly flare up to 20,000 times the brightness of our Sun.
Another interesting group of sudden eruptions are flare stars, which go through cataclysmic changes within a few hours.
