They are stable in size. At a star's surface the energy is released to form the spectrum of the star. Only convection and radiation transfer are important in stars and the opacity determines which method is used. When the temperature is high and all the atoms are stripped of their electrons, the opacity is low and radiation transfer is dominant. When the temperature drops, such as in the outer layers of a stars interior, the protons and electrons recombine to form atoms and the opacity goes up.
High opacity slows the transfer of energy by radiation, so bubbles form.
These bubbles are hot and low in density, thus starting a convective flow. Whether convection or radiation transport is used depends on the temperature make-up of the stellar interior. When the changes in temperature are sharp, convection is used. Think of the photons as grains of sand on a pile. If the pile is low, radiation is used.