Variable stars are stars that vary in brightness over time. In most cases, these changes occur very slowly over a period of months or even years. In some cases, however, the changes take place in a matter of hours.
The category variable stars encompasses several different types of stars that vary in brightness for entirely different reasons. Examples include red giants, eclipsing binaries, Cepheid variables, and RR Lyrae.
The most common variables, with the longest bright-dim cycles, are red giants. Red giants are stars of average size (like the Sun) in the final stages of life. During the last several million years of its multibillion-year lifetime, a red giant will puff up and shrink many times. It becomes al ternately brighter and dimmer, generally spending about one year in each phase until it completely runs out of fuel to burn.
The apparent variable behavior of a second group of stars, eclips ing binaries, is caused by a very different process. A binary star is a double star system in which two stars orbit each other around a central point of gravity. An eclipsing binary occurs when the plane of a binary's orbit is nearly edgewise to our line of sight (that is, from a viewpoint on Earth). Each star is then eclipsed by the other as they complete their orbits.
A special class of variables, discovered by American astronomer Henrietta Swan Leavitt (1868–1921), consists of blinking yellow supergiants called Cepheid (pronounced SEF-ee-id) variables. They are so named because they were first found in the constellation Cepheus. The pulsing of Cepheids seems to be caused by the expansion and contraction of their surface layers. They become brighter (expansion) and dimmer (contraction) on a regular cycle (lasting 3 to 50 days). For this reason, astronomers use Cepheids as a way of measuring distances in space. If two Cepheids have the same cycle of variation, then the brighter one is closer to Earth.
Similar to Cepheids but older are a group of stars known as RR Lyrae stars. They are so named because one of the first stars of this type was discovered in the constellation Lyra. RR Lyrae are usually found in densely packed groups called globular clusters. Because of their age, RR Lyrae stars are relatively dim. They also have very short light variation cycles, lasting usually less than one day.
Two American astronomers have been instrumental in tracking variable stars. Leavitt, in a search of the southern skies in the early 1900s, discovered about 2,400 variable stars. In 1939, Helen Sawyer Hogg (1905–1993) created the first complete listing of the known 1,116 variable stars in the Milky Way galaxy. In 1955, she updated this catalogue, adding 329 new variables, one-third of which she discovered herself.
Cepheids: Pulsating yellow supergiant stars that can be used to measure distance in space.
Eclipsing binaries: Double star system in which the orbital plane is nearly edgewise to a viewpoint on Earth, meaning that each star is eclipsed (partially or totally hidden) by the other as they revolve around a common point of gravity.
Globular clusters: Tight grouping of stars found near the edges of the Milky Way.
Red giants: Stage in which an average-sized star (like the Sun) spends the final 10 percent of its lifetime; its surface temperature drops and its diameter expands to 10 to 1,000 times that of the Sun.
RR Lyrae: A class of giant pulsating stars that have light variation periods of about a day.
Supergiant: Largest and brightest type of star, which has more than fifteen times the mass of the Sun and shines over one million times more brightly.