Cepheid Variables

First published: 1997

Brief summary

NASA explanation of Cepheid variable stars and period-luminosity cycles.

Article

Cepheid variables are luminous stars whose brightness rises and falls in a regular cycle. Their variability results from repeated expansion and contraction of the star’s outer layers, which changes its radius, temperature and emitted light.

The pulsation is driven by changes in opacity within partially ionised layers of the stellar atmosphere. During part of the cycle, trapped energy increases pressure and expands the star; expansion then lowers temperature and opacity, allowing energy to escape and the layers to contract again.

A Cepheid’s pulsation period is closely related to its intrinsic luminosity. Longer-period Cepheids are more luminous than shorter-period Cepheids, creating a period–luminosity relation that can be calibrated using stars whose distances are independently known.

Astronomers compare a Cepheid’s intrinsic luminosity with its observed brightness to determine distance. Cepheids therefore provide an important step in the astronomical distance scale and have been used to measure distances to nearby galaxies and calibrate other distance indicators.

Source details and credits

• Source / publisher: NASA
• Source type: Educational article
• URL type: WWW
• Credits: NASA
• URL: https://imagine.gsfc.nasa.gov/science/objects/cepheids1.html