Regular, periodic oscillations in the brightness of a star caused by internal waves.Stellar pulsation refers to the regular, periodic expansion and contraction of a star's outer layers, which causes its brightness to vary over time. These oscillations are not random; they are driven by internal physical processes that make the star behave like a giant, ringing bell. The study of stellar pulsation is a powerful tool in astrophysics, as the period and amplitude of these oscillations reveal the star's internal structure, composition, and age—a field known as asteroseismology. It also underpins the cosmic distance ladder, as certain pulsating stars, like Cepheid variables, have a well-defined relationship between their pulsation period and intrinsic luminosity.
The first recognized pulsating star was Delta Cephei, discovered by John Goodricke in 1784, but the physical explanation came much later. The period-luminosity relation for Cepheids was discovered by Henrietta Swan Leavitt in 1908, and the theory of stellar pulsation was developed by Arthur Eddington in the 1910s-20s.
Pulsation occurs when a star's outer layers are not in perfect hydrostatic equilibrium. A perturbation, such as a slight change in opacity, can cause the layer to expand. As it expands, it cools and becomes more opaque, trapping radiation and causing it to heat up and expand further. Eventually, the expansion overshoots, and gravity pulls the layer back inward, compressing and heating it. The cycle repeats, with the star oscillating at a natural frequency determined by its size, mass, and internal structure. The driving mechanism often involves a layer of partially ionized helium that acts as a valve, alternately storing and releasing energy.
Stellar pulsation is used to measure distances to galaxies via Cepheid variables, and in asteroseismology to probe the internal structure of stars, including our Sun (helioseismology).
The Sun itself pulsates with a period of about 5 minutes, with millions of resonant modes, making it ring like a bell.





