Edited sentence: As it expands, it becomes more transparent, allowing the radiation to escape.At the dimmest part of the cycle, the ionized gas in the outer layers of the star is opaque, and so is heated by the star's radiation, and due to the increased temperature, begins to expand.The accepted explanation for the pulsation of Cepheids is called the "kappa mechanism", where the Greek letter kappa denotes gas opacity, is as follows:.The process then repeats.Then it cools, and contracts due to the star's gravitational attraction