# Variable star

Also called: Cepheid, Mira, eclipsing binary.

A star whose brightness changes — because it pulsates, erupts, or is eclipsed by a companion.

Delta Cephei swells and shrinks every 5.4 days, and its brightness rises and falls with it. Stars like it — Cepheids — pulse at a rate set by their true brightness, which let astronomers measure the distances of galaxies. Mira, in Cetus, fades from naked-eye brightness to invisibility over almost a year; Algol dims for a few hours every 2.9 days when a companion passes in front of it.

**Examples:** [delta-cephei](https://astropilot.space/object/delta-cephei.md)

**Related:** [Magnitude](https://astropilot.space/glossary/magnitude.md), [Double star](https://astropilot.space/glossary/double-star.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/variable-star