Main Sequence Evolution of Massive Stars

What happens to stars that are more massive The central temperature is much higher and additional nuclear reactions become possible. One example of such reaction is the CNO cycle. The main reactions are listed below. 

In principle, this reaction is the most important one in stars with more than 1.5 solar mass. There is a further difference between low massive and massive stars concerning their central core. In high massive stars there is a large temperature gradient because of the greater temperature dependence of the CNO cycle, the core becomes 

As a star undergoes post-main sequence evolution, it may make several passes through the instability strip in the H-R diagram. There the star becomes variable, i.e. a Cepheid variable which alternately blows up and shrinks again causing an observable brightness variation. 

The different burning stages depend on the mass of the stars. In Table 8.2 some examples are given. It is seen that oxygen burning lasts only several months. 

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