Origin of the galaxy and galactic chemical evolution

Once the primary isotopes had built up in abundance, then the full range of reactions discussed above became available and nuclides such as C, N, O, O, F, Ne, Ne, 25Mg, 26Mg, and so on, were produced by reactions between primary isotopes and protons and neutrons. Isotopes that require the presence of metals in the initial composition of the star in order to be efficiently synthesized are secondary isotopes. Abundances of primary isotopes built up rapidly in the early universe via synthesis in massive stars. Secondary isotopes initially built up more slowly, but their rate of synthesis increased as metallicity increased. 

After 2 billion years, when stars of 8 M began to reach the end of their lifetimes, type la supemovae began to contribute to the elemental budget of the galaxy. Because a type la supernova completely dismpts the star, it efficiently ejects iron-group elements. The iron group elements thus built up more slowly than the elements from carbon to titanium. 

Recent modeling based on the lifetimes of stars, their IMF, the star formation rate as a function of time, and nucleosynthesis processes have succeeded in matching reasonably well the abundances of the elements in the solar system and in the galaxy as a whole (e.g. Timmes et al., 1995). These models are still very primitive and do not include nucleosynthesis in low and intermediate-mass stars. But the general agreement between model predictions and observations indicates that we understand the basic principles of galactic chemical evolution. 

In this chapter, we reviewed the broad outlines of the Big Bang model for the origin of the universe and discussed some of the supporting observations. We showed that the Big Bang gave rise to hydrogen, helium, and some lithium, beryllium, and boron, but that other elements were produced primarily in stars. The rest of the elements were synthesized in stars via the nuclear reactions that cause the stars to shine. To understand stellar nucleosynthesis, it is necessary to understand the characteristics of stars. Astronomers use 

In the final section of this chapter, we discussed the formation of galaxies and the formation and chemical evolution of the Milky Way. This sequence of events set the stage for the formation of the solar system. In Chapter 4, we will look at the resulting abundances of the elements and isotopes, both in the solar system and in the galaxy. The solar system abundances of the elements are a fundamental constraint for understanding the Sun, the planets, and the smaller bodies in the solar system. 

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