Some stars vary in brightness because they pulsate in and out.
In the outer layers of pulsating stars, the inward pull of gravity and the outward push of pressure are out of balance. When outward pressure overwhelms inward gravity, the star expands. When inward gravity surpasses outward pressure, the star contracts. These pulsations can be regular or irregular, and the pulsation periods range from more than a year to only a few hours. To see a brief animation of pulsating stars, click here .