Evolution of Massive Stars

·  Stars more massive than 8 times the Sun’s mass, evolve differently than stars of lower initial mass.

·  Like lower-mass stars, high-mass stars fuse hydrogen in their cores during their main sequence lifetime. Massive stars also spend 90% of the total lifetimes as stars located on the main sequence.

·  The main sequence is also a time sequence – more massive stars complete their evolution in progressively shorter times. O-stars live 3-10 million years, an A-star, like Sirius or Vega, last about 500 million years.

·  All evolutionary phases occur faster for more massive stars. The larger mass produces greater inward pressure, forcing the stars to burn fuel furiously, in order to remain stable.

·  When hydrogen is consumed in the core, the core contracts and heats up. The outer layers of the star expand and cool, and this star evolves first to a blue, then a red supergiant star. Hydrogen burns in a shell around the inert helium core. Betelgeuse is an example of a red supergiant.

·  Massive stars evolve to the right within the HR diagram.

·  Helium fusion, the triple-alpha process, begins while the star appears to us as a red supergiant.

·  Unlike low-mass stars, high-mass stars continue to fuse light elements into heavier elements because their mass is sufficient to compress the core to high enough temperatures.

·  Carbon fusion requires temperatures greater than 600 million K. Only high-mass stars can achieve this core temperature.

·  First carbon, then oxygen, then neon fuse, eventually ending with a core composed largely of the element iron. During this phase, the core of the star assumes an “onion skin” shape: with multiple shells of matter, each burning progressively heavier nuclei up to iron.

·  The last stages of nuclear burning occur very rapidly, within a few years.

·  Iron cannot be fused and release energy; iron fusion requires energy.

·  When the iron core forms, the star is doomed. The core will quickly (days) consume the last fusable elements, and fusion reactions will terminate. The entire star will implode, then explode in an enormous explosion - a supernova.