Syntheses, in stars

Short-lived radionuclides are those with half-lives sufficiently short that any atoms present in the early solar system would have completely decayed away. In 1960, John Reynolds found the first clear evidence that short-lived nuclides were present in the form of large excesses of Xe, the decay product of short-lived I, in chondritic meteorites. This discovery showed that elements had been synthesized in stars shortly before the formation of the solar system. A more important short-lived radionuclide, 26A1, was demonstrated to have been present in meteorites by Typhoon Lee and coworkers in... 

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... 

Fowler further concludes "My major thente has been that till of the heavy elements from carbon to uranium have been synthesized in stars. Our bodies consist lor the most part of these heavy elements. Apart from hydrogen, wc are 65 percent oxygen and 18 percenl carbon, with smaller percentages of nitrogen, sodium, magnesium, phosphorus, sulfur, chlorine, potassium, and traces of still heavier elements. Thus, it Is possible to say that each one of us and all of us are truly and literally a little bit of stardust."... 

Even after several decades of research [BUR57] into the mechanisms by which the elements are synthesized in stars, it is still often true that the degree to which an astrophysical environment can be understood is limited by the degree to which the underlying microscopic input nuclear physics data have been measured and understood. As new and more exotic high-temperature astronomical environments have been discovered and modeled (and as observations and models for more familiar objects have been refined) the needs for more and better data for nuclei away from stability have increased. In this brief overview, we discuss a few of the explosive astrophysical environments which are currently of interest and some of their required input nuclear data. 

Bob, says Miss Muxdroozol, I ve been thinking about how heavy elements are synthesized in stars. But couldn t some of these heavy elements have been made in the intense heat and pressure of the Big Bang Maybe if the Big Bang made carbon, we wouldn t need stars for complex materials. ... 

From the isotopic decomposition of normal oxygen one finds that the mass-17 isotope, 170, is 0.038% ofallO on Earth. It is the least abundant, by a wide margin, of the three stable O isotopes, 2630 times less abundant than the most abundant of the O isotopes, namely l60. This much smaller abundance reflects the history of the Galaxy in an important way namely, it shows that17O cannot be synthesized in stars directly from H and He, as 16 O can be. Using the total abundance of elemental O = 15.2 million per million silicon atoms in solar-system matter, this isotope has... 

K is the 56th most abundant nucleus within the universe. Of the lighter stable isotopes that are synthesized in stars, only 40Ar and 3 S are less abundant On the other hand, 41K is more abundant than any stable isotope heavier than zinc. In that sense then, 41K may be thought of as among those few dividing the very abundant isotopes from the rare ones. 

Zn is synthesized in stars by the s process and, to some poorly known degree, by the quasiequilibrium alpha-rich freezeout. About half of 7Zn from the s process is synthesized direcdy in massive stars in their He-burning shell. The AGB stars produce only 2% of67Zn, although they produce the main s process at larger atomic weights. 

Some may challenge me on the matter of mineral constituents of the human body. How about the calcium for bones and teeth How about the sodium chloride we eat How about the potassium, the phosphorus, and the iron in all of us All these elements were synthesized, too. They were synthesized in stars like our sun, long ago. Everything has been synthesized from the primordial hydrogen by nuclear reactions fer off and long ago. When I explained this concept to freshmen in Chemistry I in connection with teaching them some elementary nuclear chemistry, one shiny-eyed Radcliffe girl came up afterward and said, Am I really made of stardust I said, Yes, and it actually shows ... 

Element abundance ratios are another important piece of information, since the various elements are synthesized in stars with different masses and lifetimes. The a-element/Fe ratio, in particular is a good diagnostic, because the a-capture elements are produced... 

Figure I. Time scale for the various element syntheses in stars. The highly schematic curve gives the central temperature as a function of time fora star of about one. solar mass Degens, I9H9).

The mechanisms by which magnesium is synthesized in stars depend on stellar properties such as mass, temperature, and density. In stars massive enough that the core reaches a temperature where carbon burning (the fusion of two carbon atoms) can occur, two different magnesium isotopes, 23Mg and 24Mg, can result from the following reactions ... 

The number of nuclei comprising calciums core is in a multiple of four, which makes it an alpha-process element. This means that calcium is generally synthesized in stars by the collisional capture of a helium nucleus (a) onto an argon 36 atom, as represented in the following ... 

Some short-lived radionuclides were sufficiently abundant at the start of the solar system to produce variations in the abundance of their daughter isotopes in early-formed objects (Table 10.2). The half-lives of these nuclides are between about 0.1 and 100 Ma (Table 10.2). Hence, the parent isotopes are no longer present today, but they were synthesized in stars shortly before solar system formation and therefore they were present in the early solar nebula. The isotopic record of these nuclides provides information about stellar nucleosynthetic sites active shortly before the birth of the solar system and the time scales over which the early solar system formed and first differentiated. Depending on half-life and chemical affinities of parent and daughter isotopes, extinct radionuclide systems can be used to date processes as diverse as the formation of CAIs and chondrules, volatile element depletion and planetary difierentiation (e.g., core segregation and differentiation of early silicate reservoirs). In particular, they are powerful tools to study the Earth s accretion and core formation [90-92],... 

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