Variable Stars

Stars do not shine uniformly brightly all the time. Stars that show significant changes in brightness over periods we short-lived humans can perceive are of great importance to astronomy because of what we can surmise from those changes. And fortunately for radio astronomy, it has been discovered that stars whose output of visible radiation varies over short periods, either regularly or irregularly, have corresponding variations in their output of radio frequency emissions.

Some variable stars , such as Cepheids, are absolutely regular in their cyclic changes, varying from a few days to a few weeks. It has been found that stars with longer regular periods are always more luminous (emitting more energy) than those with shorter regular periods. Variable stars with very short periods (1.25 to 30 hours) are called RR Lyrae variables. None of these shorter period variables is bright enough to see with the naked eye. Because the intrinsic luminosities of Cepheids and RR Lyraes with similar periods are comparable, variable stars such as these can be used to work out interstellar and even intergalactic distances.

Other variable stars have much longer periods, are less regular in their cycles, and vary by a much greater magnitude. These are called semi-regular variables. The red giant Betelgeuse in the Orion constellation is an example. No period-luminosity relationship has been found for semi-regular variables.

Irregular variables have no set periods at all. They usually are young stars and their luminosities may vary over a very large range.

Flare stars are faint red dwarf stars (older and feebler than white dwarfs) that exhibit sudden increases in brightness over a period of a few minutes due to intense flare activity, fading back to their usual brightness within an hour or so. Typical flare stars are UV Ceti and AD Leonis.

Binary (double) stars may produce apparently regularly varying radiation if the two stars eclipse one another in their orbits. Also, radio emissions from binaries are more common than for single stars. The interaction of stellar winds and magnetospheres, bow shocks, and tidal effects may contribute to the conditions producing radio frequency emissions.