Star types. Low-mass stars, less than half a solar mass, are fully convective from core to the surface and thus of uniform composition. This is because their low temperatures make their opacity to photons high. In intermediate-mass stars such as the sun, radiation transport works well enough in the core that convection does not take place; a stagnant core develops, surmounted by a convective region. These stars' surfaces do not include fusion products from the core, but retain the same composition as the cloud from which they formed. At 1.5 solar masses, the convective layer has almost disappeared and the star is essentially fully radiative. Above 1.5 solar masses, the CNO cycle contributes the majority of the energy production. Because this cycle is much more sensitive to temperature, the star's energy production is more tightly concentrated in the center, and this causes the development of a convective core.