Química

Nuclear Chemistry-State

of the Rrt for Teachers

Stellar Alchemy: The Origin of the Chemical Elements
Eric B. Norman Nuclear Science Division, Lawrence Berkeley Laboratory, Berkeley, CA94720 By examining the light that comes to us from the stars, we can deduce a great deal about the nature of the universe. Remarkably. it seems that the same hasic laws of physics and chemistry applyeverywhere we look. Furthermore, all ohsewable astronomical obiects seem to be made up ofthe same 92 chemical elementsfound ouEarth. From such observations, we now know that approximately 73% of the mass of thevisible universe is in the form of hydrogen, and helium makes up about 25%. Everything else represents only 2% of the mass of the universe. Although the abundance of these "heavy" (A > 4)elementsseems quite low, most of the atoms in our bodies and in the Earth are a part of this small portion. I t is generally believed that the hydrogen and helium were produced in the hot, dense conditions prevailing a t the birth of our universe known as the hie bane. As discussed below, the heavy elements are the products of nuclear reactions in stars. Several excellent books have been and I written onthis aspect of nuclear astrophysics (131, have relied heavily on them in preparing this article. muth that has no stable isotopes. I n fact, the isotope ob"c sewed in stars ( T ) has a half-life of only 2 x lo5 years. Although this may seem long by human standards, i t is very short on astronomical timescales. The only plausible explanation for the presence of such "short-lived" material in a staris its recent synthesis within that star. Stellar Classes Another important piece of astronomical data is the observed relationship hetween the surface temDeratures and luminosities of stars. Figure 1 shows a HertzsPrungRussell (HR) diagram. About 80% of all the observed stars, including our Sun, fall on a roughly diagonal band known as the main sequence. There are also two other important classesof stars. I n the upper right-hand comer of the HR diagram, there is a group of cool but luminous stars known a s red giants. In the lower left-hand comer, there is a population of hot hut dim stars known as white dwarfs. The significance and origin of these stellar classes will be discussed later in terms of stellar evolution. Abundnnces Finally, a large amount of information can be obtained frommore detailed analysis of the elemental and isotopic composition of matter. Shown in Figure 2 are the observed ahundances of the material in our solar system. As discussed previously, hydrogen and helium are by far the most abundant species. The next heaviest group of elements-lithium, beryllium, and horon-are by comparison exceedingly rare. Above this group, the abundances start out higher, butgradually decrease a s one moves up to heavier elements. Several important features about this pattern should he pointed out. There is a large abundance peak near mass 60 that is associated with the elements around iron. Above this noint. there is another eeneral decrease in abundance with increasing mass number that is interrupted by two
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The Origin of Stellar Energy and theElements
Clues Found in Nature

Nuclear fusion reactions are now generally accepted a s the source of stellar energies. However, until the last ten vears or so. this conclusion was based almost entirely on circumstantial evidence. The reason is quite simple.- he light we observe from stars is emitted from the surface; we cannot look inside to determine what is actually going on. We must rely onmore indirect means or use sensors for other types 0%radiation to extend our "vision". Obseruation of Technetium Spectral Lines One of the early pieces of evidence that nuclear reactions do occur in stars was the ohsewation of spectral lines of the element technetium on the surfaces of certain old stars. Technetium is one of only two elements below his-

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