When a high-mass main sequence star runs out of both hydrogen and helium in its core, the core begins to fuse
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Low mass stars end their lives here, by expelling their outer layers due to thermal pulses in a planetary nebula phase, but high mass stars have so much mass that they can survive this phase. In our earlier analogy of a pressure cooker, high-mass stars have a heavy "lid," so they keep on cooking. We will treat that in a moment, but first let's look at how high mass stars differ from low mass stars in nuclear burning. In low mass stars, the burning of hydrogen to helium in the Main Sequence stage happens in the slow, proton-proton reactions that we discussed earlier. For high mass stars, there is a faster mechanism to convert hydrogen to helium, called the CNO cycle, but it requires a higher core temperature than occurs in a star like the Sun. The CNO cycle uses carbon as a catalyst, in which carbon is used in the reaction, but in the end the carbon is returned to be used again. Here is the reaction, step by step: