A variable star is a star whose apparent brightness changes when observed from Earth. Brightness variations may be tiny and gradual, like subtle sunspot cycles on the Sun, or very large and rapid, as in novae and some eclipsing systems. The term covers many physical causes and a wide range of timescales, from fractions of a second to many years.

Classification and causes

Broadly, variable stars are sorted into two groups: intrinsic variables, where the star itself changes luminosity, and extrinsic variables, where apparent brightness changes because of geometric effects or obscuration. Common subtypes include:

  • Pulsating stars (Cepheids, RR Lyrae, Mira): their outer layers rhythmically expand and contract, producing regular light curves and measurable periods.
  • Eruptive variables (flares, novae): sudden energy releases alter a star's output for short or long intervals.
  • Eclipsing binaries: brightness dips when one star passes in front of its companion, as first convincingly explained for Algol-like systems.
  • Rotational variables: changes caused by spots, uneven surface brightness or oblate shapes as the star rotates.

Observation and importance

Astronomers study variable stars by recording light curves with photometry and sometimes spectroscopy. Periods, amplitudes and spectral changes reveal mass, radius, composition and evolutionary state. Certain pulsating stars, notably Cepheids and RR Lyrae, show a relation between period and intrinsic luminosity and therefore serve as distance indicators (standard candles) for measuring galactic and extragalactic scales.

Variable-star research also informs models of stellar interiors, binary evolution and mass transfer. Many discoveries and long-term monitoring projects involve amateur astronomers working with professional observatories and surveys.

History and notable facts

Observers recognized bright, repeating variables centuries ago; Mira (omicron Ceti) was among the earliest recorded. Systematic classification advanced in the 19th and 20th centuries, and work by astronomers such as Henrietta Swan Leavitt established the period–luminosity link that underpins cosmic distance measurements. Catalogues compile thousands of known variables and modern time-domain surveys continue to discover new types and rare events.

For a basic context on what a star is and why brightness matters, see introductory materials on stellar properties and observational techniques. Variable stars remain a vital and active field connecting observation, theory and practical applications in astronomy.