They are red giant and supergiant stars with periods ranging from a few months to more than a year. The best known of these stars is Omicron Ceti, also known as Mira. Over a period of about 11 months, it brightens by about 7 magnitudes and then gradually fades. Semiregular variables are stars whose periodic variations are occasionally interrupted by sudden bursts of light. The best-known example is the red supergiant Betelgeuse, in Orion.
Irregular variables show no periodicity in their variations in brightness.
The amplitude of their fluctuations in brightness is in general smaller than the fluctuations of the long-term regular variables. The eruptive variables are highly unstable stars that suddenly and unpredictably increase in brightness. T Tauri stars, also known as nebular variables because they are young objects still embedded in nebulosity, are the least violent of these explosive stars. Novas and supernovas are much more dramatic. Novas are small, very hot stars that suddenly increase thousands of times in luminosity.