Internal Structure The internal structure of a main sequence star depends on its mass. The more massive main sequence stars have the highest central temperatures-as great as 40 million K. This is hot enough that massive main sequence stars can use the carbon cycle to fuse hydrogen into helium. Because the rate at which fusion reactions occur is very sensitive to temperature, the massive main sequence stars consume their hydrogen at rates much faster than the Sun, which has a core temperature of only about 15 million K. The rapid consumption of hydrogen in massive main sequence stars is also the source of their tremendous luminosities. Main sequence stars of low mass, on the other hand, have core temperatures below 10 million K, so nuclear fusion goes on very slowly in their cores and produces only a relatively feeble output of energy.