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Stellar Evolution

HR Post-Main-Sequence 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

Figure 08-05a Post-Main-Sequence [view large image]

lends its support to stop complete collapse and it forms a white dwarf with remnant less than 1.4 Msun. For star with mass in between 5 and 15 Msun the protons combine with electrons to form neutrons under the tremendous pressure. Then the degeneracy pressure
HR Post-Main-Sequence HR Evolution, 3 Sun of the neutrons can provide support up to 3 Msun (of the remnant) and it becomes a neutron star (or pulsar - spinning neutron star). For star with mass greater than 15 Msun, no amount of support is sufficient to stop the collapse to a black hole. Figure 08-05b portrays the post-main-sequence evolutionary track for the Sun in details. While Figure 08-05c shows the post main-

Figure 08-05b HR Post-Main-Sequence [view large image]

Figure 08-05c HR Post-Main-Sequence, 3 Msun [view large image]

sequence evolution of a three solar mass star with metallic abundance Z = 0.02.

Starlife Figure 08-05a depicts the evolutionary tracks for stars with mass ranging from 0.3 - 30 Msun. The atomic element at each turning point indicates the beginning of nuclear burning for that particular species. Less massive stars don't have enough pressure to convert the elements all the way to irons. Only for stars with a mass of 30 Msun or more are able to complete the process of transforming silicons to irons (see top curve in Figure 08-05a). Figure 08-05d illustrates four different paths of stellar evolution depending on the stellar mass.

Figure 08-05d Stellar Evolution

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