Humans are aware of heat and light being produced together - fire, the Sun, and elements heated until they glow. Humans are also aware of light being produced without heat, as with fireflies or minerals that are phosphorescent. And of course, there is heat without light, close at hand as in 98.6 degrees. The 12th Planet has both heat and light, generated from within its core. Life on the 12th Planet, which is inhabited by a large hominoid indistinguishable from humans other than by its size, experiences continuous day. Life that has evolved there does not sleep, but rests. The light is diffused in the atmosphere, and returns to the land surface, but emerges from the core to interact with the atmosphere only via the surface of the deep oceans, which cover the majority of the planet's surface. You may equate this to volcanic activity, where the Earth has numerous places both above ground and under the oceans that ooze molten lava. Just so the 12th Planet has places where the molten and churning substance in its core escapes to the surface.
Fires such as go on in the center of suns do not only proceed full bore. Why would they? Do you not have a fire within you that is maintained at a steady temperature? Do you not find that the fire in your fireplace can be slowed by adjusting the damper? Humans do not understand what is occurring within the Sun, a combustion of sorts that ignited because of the pressure of elements following the big bang, during the congealing period. The Sun was not born, lit. It lit as compression continued to the point where a product of subatomic particle collision did not dissipate, but accumulated, and the degradation of this substance is what you are viewing and feeling in your sunlight. This is a simplistic explanation for a complicated process. The 12th Planet has a similar process ongoing within its core, but being composed of heavier substances than your Sun, this process is slowed. You can equate this to mixing medicine into food, rather than taking it directly. This is tolerated because the incident or rate of medicine reacting with a taste bud is reduced, or slowed.
On the 12th Planet, the heat released diffuses in all directions, so the surface of the planet is warm but not hot. Your feet would not blister, but they may feel cool in places where the rock strata do not provide a good passage for heat, should you visit that planet. Light only escapes the core where what is essentially volcanic activity under the water occurs. Of course, this would occur if there was volcanic activity on the land surface of the planet, but there is little land surface, and this long ago hardened. Volcanic activity would not be on the surface of the Earth, either, should her proportion of water to land be increased. Water is pooling in the low spots, and the low spots have less hardened surface mantle as a barrier. Thin places break, and give release to the pressure from the churning molten core. The oceans of the 12th Planet, therefore, have places where diffused light is rising, and other places where the oceans would appear to you much as your own do here on Earth.