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The structure of a star is maintained in equilibrium via the balance of the gravitational attraction with a tendency to contract and the thermal pressure with a tendency to expand. When the star has exhausted its hydrogen fuel, it cools off and collapses until the pressure has risen sufficiently to ignite helium and other types of nuclear burning. This process of re-igniting fuel burning with different nuclear species is represented by the zigzag paths in Figure 08-05a. The variation of stellar radius can be traced with the curves crisscrossing the loci of constant radius. It shows that the maximum extent can be 100 Rsun or more and hence the names of giant, and supergiant. These stars have evolved to the terminal phase as shown in Figure 08-05a and 08-05b. Eventually, all the available fuels are consumed, there is no more source to supply the thermal pressure necessary to stop further collapsing. However, for star with mass smaller than 5 Msun the degeneracy pressure of the electrons lends its support to stop complete collapse and it forms |