10 20 30 40 50 60 70 80 90 100 Atomic Number (Z)

Fig 9.8

Cosmic abundances of the elements as a function of atomic number Z.

section that tunneling by those nuclei with higher than average energy leads to a strong temperature dependence on the reaction rates. For example, the rates of some important reactions depend on temperature as T7 or even higher powers.

We say that a collapsing object makes a transition from protostar to star when its primary source of energy generation is fusion via the p-p chain, rather than gravitational collapse. The changeover is not a sudden one, as the material closer to the center heats faster. Eventually, enough energy is generated internally for the collapse to halt. When this happens, the star reaches a stable condition. It is on the main sequence.

As nuclear reactions take place in the star, the composition of the star is actually changing. This change could affect the spectral type and luminosity of the star, while the basic structure doesn't change very much. These changes result in a main sequence that is a band on the HR diagram rather than a thin line. However, there is a line that we can identify as connecting the points on the HR diagram where stars of each spectral type first appear on the main sequence. We call this line the zero-age main sequence, or ZAMS. This line is shown relative to the main sequence band in Fig. 9.9.

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