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(Note that in this HR diagram we have chosen the spectral class for the horizontal axis, instead of the temperature or color index as in many previous HR diagrams.) Roughly, we may expect the following fates for three different mass ranges once the star leaves the main sequence.
| 1. The lowest mass stars, which form the red dwarf region on the HR diagram, will evolve directly to the white dwarf stage when their fuel is consumed. Because these low mass stars burn their fuel so slowly, none of them have evolved to white dwarfs yet because the Universe is not old enough to have given them enough time to consume their core fuel. |
| 2. Intermediate mass stars (like the Sun) will be able to initiate a series of additional fusion burning stages in their cores and in concentric shells around their cores. This will cause them to become giant stars, eventually shed their envelopes as planetary nebula, and also end up as white dwarfs. |
| 3. The most massive stars will initiate a series of additional fusion burning stages too, and grow to giant and supergiant size. But in this case the center of the star will eventually collapse, triggering a supernova explosion that blasts the outer layers of the star into space and crushes the center of the star into either a neutron star or a black hole. |
We shall divide our discussion primarily into two logical parts: the fate of intermediate mass stars and the fate of very massive stars. There is not much to say about the least massive ones, because they are all still on the main sequence and will be for tens of billions of years. That is, we can discuss the least massive stars theoretically, but we have little data by which to test the theories for the evolution of these stars.