Stars Are Nuclear Furnaces. In their super-hot cores, which are at many millions of degrees Kelvin, stars generate their own energy by nuclear reactions. This newly released energy flows from the stars' hot interiors to the cooler surface layers, where the energy is radiated into space. We see that radiation and say that stars shine.

The Sun Gets its Energy from the Fusion of Hydrogen. Let us again take the Sun as an example. The temperature at its center is around 15 million degrees Kelvin. That's hot enough for hydrogen to fuse into helium with the release of large amounts of energy. It is the same process that powers a hydrogen bomb explosion, except that in the sun hydrogen fusion is not explosive. Instead, it goes on at a steady pace for billions of years.

The Fusion of Helium and Heavier Elements: Most of the stars that you see in the night sky get their energy from the fusion of hydrogen into helium at their centers, just like the Sun. But not all.

In due time, stars convert all of the hydrogen at their centers into helium and acquire a helium core. In stars with masses comparable to and greater than that of the sun, the helium in the core ignites and fuses into carbon and oxygen with the simultaneous release of energy. In very massive stars, those elements eventually ignite and fuse into still heavier elements, again with the release of energy.