variable star

Star whose brightness changes, either regularly or irregularly, over a period ranging from a few hours to months or years. The Cepheid variables regularly expand and contract in size every few days or weeks.

Stars that change in size and brightness at less precise intervals include long-period variables, such as the red giant Mira in the constellation Cetus (period about 331 days), and irregular variables, such as some red supergiants. Eruptive variables emit sudden outbursts of light. Some suffer flares on their surfaces, while others, such as a nova, result from transfer of gas between a close pair of stars. A supernova is the explosive death of a star. In an eclipsing binary, the variation is due not to any change in the star itself, but to the periodic eclipse of a star by a close companion. The different types of variability are closely related to different stages of stellar evolution.

History Apart from a few novae, no star was observed to be variable until the end of the 16th century. David Fabricius noticed the variability of Mira in 1596, and English astronomer John Goodricke (1764–1786) discovered the smaller but very regular variations of Delta Cephei in 1784. Fewer than 20 variable stars were known until the introduction of photography into astronomy in the 1840s. Photography facilitates the discovery of variable stars, and more recently photoelectric photometry has enabled their light curves to be plotted very accurately.

Eclipsing binaries Of 19,000 known variable stars, 80% are intrinsic variables and 20% are eclipsing binaries, pairs of stars in orbit around one another, whose combined light drops when one star eclipses the other. Much can be learnt of the structure of the eclipsing system from detailed measurements of the light curve. In cases in which the radial velocities of both components can also be observed, it is possible to deduce the mass, diameter, and temperature of each star. The best known eclipsing binary is Algol; its variability was detected by the Italian astronomer Geminiano Montanari in 1669, and Goodricke in 1782 pointed out its regular periodicity of 2.87 days, and the cause. Some eclipsing binaries consist of pairs of stars orbiting very close to one another or almost in contact, and such stars show severe deformation due to their mutual gravitational attraction. Orbital periods of eclipsing binaries range from about 4 hours to 27 years.

Intrinsic variable stars Intrinsic variables fall into many classes, named after typical members. Regular or irregular pulsations account for the light variations of the majority of known variable stars, with periods ranging from minutes to years. In general the light variation is greater in the stars of longer period. A numerous class of variable stars of short to moderate period consists of Cepheid variables, named after Delta Cephei. This star pulsates in a very regular period of 5.37 days, with a light variation of 0.8 magnitude, just over twice as bright at maximum as at minimum. The periods of Cepheids range from 3 to 50 days, and Henrietta Leavitt showed 1912 that there is a relationship between period and their brightness: the longer the period, the brighter the Cepheid. Once the relationship had been established, the observed period of a very distant Cepheid could be used to determine its intrinsic luminosity, and hence, taking into account its apparent luminosity, to find its distance. Thus Cepheids became important in calculating the distance of remote galaxies.

The revision of this period–luminosity relationship 1952, which doubled the accepted distances of the galaxies, was the direct result of the recognition that W Virginis and RR Lyrae variables did not belong to the Cepheids, as had been assumed. W Virginis stars are sometimes called Type II Cepheids, while RR Lyrae stars are Cepheidlike variables with periods that fall in the range 0.3 and 0.7 days. They are sometimes described as cluster-type variables, as many of them are found in globular clusters.

There are many variable red giant stars, with periods ranging from 30 to 1,000 days. Well known examples are Betelgeuse and Mira. An interesting type of eruptive variable star is named after UV Ceti; such stars undergo small, sudden, irregular outbursts every few hours or days, with simultaneous emission of radio waves.