Comparison with Observations

The most important direct support for the theoretical evolutionary models is obtained from the properties of observed HR diagrams. If the theoretical models are correct, the observed number of stars should reflect the duration of the various evolutionary phases. These are given for stars of different masses in Table 11.1. The

^Fig. 11.11a-h. Evolution of a massive binary. It has been assumed that the supernova explosion of a 5 Mq helium star leaves a 2 Mq compact remnant (neutron star or black hole). (a) Main sequence phase; (b) beginning of the first mass transfer phase; (c) end of the first mass transfer phase; the first Wolf-Rayet phase begins; (d) the helium star (Wolf-Rayet star) has exploded as a supernova; (e) the 23 Mq component becomes a supergiant; the compact component is a strong X-ray source; (f) beginning of the second mass transfer phase; the X-ray source is throttled and large-scale mass loss begins; (g) second Wolf-Rayet phase; (h) the 6 Mq helium star has exploded as a supernova; the binary may or may not be disrupted, depending on the remaining mass stars are most numerous along the main sequence. Giants are also common and, in addition to these, there are white dwarfs, subgiants, etc. The sparsely populated region to the right of the main sequence, the Hertzsprung gap, is explained by the rapid transition from the main sequence to the giant phase.

The cepheids provide an important test for the evolutionary models. The pulsations and the relation between period and luminosity for the cepheids can be understood on the basis of theoretical stellar models.

The evolutionary models can also explain the HR diagrams of star clusters. Let us assume that all the stars in a cluster were formed at the same time. In the youngest systems, the associations, the stars will mainly be found on the upper main sequence, since the most massive stars evolve most rapidly. To the right of the main sequence, there will be less massive T Tauri stars, which are still contracting. In intermediate age open clusters, the main sequence will be well developed and its upper end should bend to the right, since the most massive stars will already have begun to evolve off the main sequence. In the old globular clusters, the giant branch should increase in importance in the older clusters. These predictions are confirmed by the observations, which will be further discussed in Chap. 16 on star clusters.

Of course, the most detailed observations can be made of the Sun, which is therefore a crucial point of comparison for the theoretical models. If a star of one solar mass with an initial composition of 71% hydrogen, 27% helium and 2% heavier elements is allowed to evolve for 5000 million years, it will be very similar to our present Sun. In particular, it will have the same radius, surface temperature and luminosity. According to calculations, about half of the Sun's supply

11.8 The Origin of the Elements

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