AAVSO: AAVSO Section

Variable stars are stars that change brightness. The brightness changes of these stars can range from a thousandth of a magnitude to as much as twenty magnitudes over periods of a fraction of a second to years, depending on the type of variable star. Over 30,000 variable stars are known and catalogued, and many thousands more are suspected to be variable. There are a number of reasons why variable stars change their brightness. Pulsating variables, for example, swell and shrink due to internal forces. While an eclipsing binary will dim when it is eclipsed by a faint companion; and then brightens when the occulting star moves out of the way. The different causes for light variation in variable stars provides the impetus for classifying the stars into different categories.

Variable stars are classified as either intrinsic, wherein variability is caused by physical changes such as pulsation or eruption in the star or stellar system (pulsating variables and eruptive variables, or extrinsic, wherein variability is caused by the eclipse of one star by another or by the effects of stellar rotation (eclipsing binaries and rotating variables). More information on these classifications and sub-classes with examples, may be found on our Types of Variable Stars page.

Why Observe Variable Stars

Variable stars need to be systematically observed over decades in order to: determine the long-time behavior of a star, provide professional astronomers with data needed to analyze variable star behavior, to schedule observations of certain stars, to correlate data from satellite and ground-based observations, and to make computerized theoretical models of variable stars.

Research on variable stars is important because it provides information about stellar properties, such as mass, radius, luminosity, temperature, internal and external structure, composition, and evolution. This information can then be used to understand other stars. Professional astronomers have neither the available time nor the unlimited telescope access needed to gather data on the brightness changes of thousands of variable stars. Thus it is amateur astronomers utilizing visual, photographic, photoelectric, and now CCD techniques, who are making a real and highly useful contribution to science by observing variable stars and submitting their observations to the AAVSO International Database.

Celebrated Variables

A number of variable stars have received a lot of attention both inside and outside of the astronomical community. Cepheid Variables have played a pivotal role in determining distances to far away galaxies and the age of the universe. Supernovae captivate the public with their powerful explosions; especially the great supernova of 1987 that went off in the Large Magellenic Cloud. Other famous stars include the Mira variables, or the long period variable stars, and eclipsing binaries such as Algol (the demon star) in Perseus.

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