Why Stars Twinkle
Stars appear to twinkle mainly because of Earth’s atmosphere. Turbulent air layers at different temperatures have slightly different optical properties and continually change how starlight is refracted and redirected. As a result, a star’s apparent brightness and position fluctuate rapidly. This is called atmospheric scintillation.
Outside the atmosphere, this atmospheric twinkling disappears, so starlight would look much steadier. Some stars do vary in their intrinsic brightness, however, so “stars do not twinkle” refers to the familiar atmospheric effect seen from Earth’s surface, not every possible change in stellar light.