Astronomers know that pulsating variable stars are actually expanding and contracting in diameter with the help of the Doppler effects of light waves.
What is the Doppler effect?
The Doppler effect is a phenomenon that occurs when two objects move closer or farther apart, increasing (or decreasing) the frequency of sound, light, or other waves.
The Doppler effect is applicable to sound waves as well as light waves,
Thus, the Doppler effect on sound may be compared to the color of light waves, which helps astronomers to know about pulsating variable stars actually expanding and contracting in diameter.
Learn more about the Doppler effect from here,
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