Variable star

A star whose apparent brightness changes over time, either because its emitted light varies or because intervening events alter the light reaching Earth.

Stellar pulsation: Periodic expansion and contraction of a star’s outer layers that changes its radius, temperature, and luminosity. Pulsation drives the repeating brightness cycles of many intrinsic variables.
Apparent magnitude: A logarithmic measure of how bright an astronomical object appears from Earth. Variable-star light curves track changes in apparent magnitude.
Mira: A red giant in Cetus and the prototype of long-period pulsating variable stars. Its large, regular brightness swings gave an entire variable-star class its name.
Cepheid variable: A pulsating giant or supergiant whose pulsation period is related to its intrinsic luminosity. Calibrated Cepheids provide distances to galaxies beyond the Milky Way.
Standard candle: An astronomical object whose intrinsic luminosity is known or can be inferred from observable properties. Some variables become standard candles because their cycles predict intrinsic luminosity.
Eclipsing binary: A binary star system whose orbital plane places one star in front of the other from an observer’s viewpoint. Its changing overlap dims the system without requiring either star’s luminosity to change.
Luminosity: The total energy a celestial object emits per unit time across all wavelengths. Intrinsic variables change their emitted energy, not merely their apparent brightness.
Algol: A bright multiple-star system in Perseus whose eclipses cause conspicuous periodic dimming. Its predictable dips made eclipsing variability recognizable long before modern astrophysics.
RR Lyrae variable: A pulsating horizontal-branch star used as a standard candle, especially within the Milky Way. Its predictable luminosity helps map globular clusters and nearby galactic structure.
Transit photometry: A technique that detects periodic dimming when an orbiting body passes in front of a star. Planetary transits resemble eclipsing variability but involve a much smaller companion.

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