Pulsating variables account for more than half of the known variable stars. They are characterized by slight instabilities that cause the star alternately to expand and contract. This pulsation is accompanied by changes in absolute luminosity and temperature. The pulsating variables can be further divided into the following subclasses: short-term, long-term, semiregular, and irregular. Short-term variables have well-defined periods ranging from less than one day to more than 50 days.
Relatively rare among this subclass are the Cepheid variables; these yellow supergiant stars are historically important because, having periods roughly proportional to their absolute brightness, they provide a means of measuring galactic and extragalactic distances. A key research program of the Hubble Space Telescope is the measurement of Cepheid variables in distant galaxies in order to refine our concept of the size and age of the universe. Cepheid variables are classed as either population I Cepheids, which are found in the spiral arms of galaxies, or population II Cepheids, also known as W Virginis stars, which are found in star clusters (see also stellar populations). About 700 Cepheids of both types have been found in our galaxy.
A more common short-term variable is of the RR Lyrae group; about 6,000 of this type are known in our galaxy and are concentrated in globular clusters. They have periods of less than one day, and all have roughly the same intrinsic brightness. The latter feature, along with their wide distribution throughout the galaxy, makes them another useful distance indicator.
The long-term variables are the most numerous of all pulsating stars. 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. Their decline in luminosity is much slower, taking months or even years. Most novas probably repeat their outbursts, the dwarf novas every few months, the recurring novas every few years or decades, and the standard novas over thousands of years. Supernovas, upon exploding, increase millions of times in brightness and are totally disrupted. More than 30 supernovas events are observed annually in distant galaxies. Three supernovas have been seen in our own galaxy, in 1054, 1572, and 1604; in 1987 a supernova erupted in a neighboring galaxy, the Large Magellanic Cloud.