Twinkling, also called scintillation, is a generic term for variations in apparent brightness, colour, or position of a distant luminous object viewed through a medium.
[1] If the object lies outside the Earth's atmosphere, as in the case of stars and planets, the phenomenon is termed astronomical scintillation; for objects within the atmosphere, the phenomenon is termed terrestrial scintillation.
[2] As one of the three principal factors governing astronomical seeing (the others being light pollution and cloud cover), atmospheric scintillation is defined as variations in illuminance only.
In simple terms, twinkling of stars is caused by the passing of light through different layers of a turbulent atmosphere.
[7][8] Many modern large telescopes also use adaptive optical systems which precisely deform the figure of a mirror in order to compensate for scintillation.