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a Abundances from meteorites; adapted from N. Grevesse and E. Anders, in Cosmic Abun-

dances of Matter, ed. C. J. Waddington, AIP, New York, 1989, p. 1. The hydrogen abundance is set to 1.00. The elements presented are those with abundances greater than 3.6 x 10-6.

dances of Matter, ed. C. J. Waddington, AIP, New York, 1989, p. 1. The hydrogen abundance is set to 1.00. The elements presented are those with abundances greater than 3.6 x 10-6.

Cosmic abundances

The net effect of all the atoms in the interstellar medium must be taken into account if the true transparency or attenuation in the Galaxy is to be obtained; different elements dominate the absorption at different frequencies. The photoelectric cross sections for hydrogen and other elements that make up the interstellar medium are well known from earth-bound laboratory experiments and theoretical considerations.

Abundances by number The relative abundances of the most abundant elements in the solar system are presented in Table 2. They come from studies of meteorites, stony solar-system objects that have fallen to the earth's surface from their circumsolar orbits. Studies of the spectra of the sun's photosphere yield abundances that are in good agreement.

Since the solar system formed from the debris of previous episodes of star formation and evolution (e.g., stellar winds and supernova remnants), these relative abundances may be taken as characteristic of the interstellar medium elsewhere in the Galaxy at the time the solar system was formed, hence the terms cosmic abundances and universal abundances are sometimes used. Of course fluctuations from place to place in the Galaxy are expected.

Table 2 gives the abundances in terms of the relative numbers of atoms. The ratio of the numbers of helium atoms to hydrogen atoms in Table 2 is about one in ten. The other elements are much less abundant, one in 2500 or less.

Mass fractions

In contrast to relative number abundances, one can describe the relative abundances in terms of their contributions to the mass of the interstellar medium. The fraction of the presumed interstellar mass that resides in a given element can be characterized as a mass fraction. The several elements are often grouped to yield the mass fractions X, Y, and Z, for hydrogen, helium and all heavier elements respectively. Since these fractions presumably include all the mass (excluding dark matter), the sum of the fractions must be unity,

The values of these fractions may be calculated from the values in Table 2, giving each element a weighting proportional to its abundance and atomic weight. Let MA be the total mass of atoms of atomic weight A in some volume of interstellar gas. Then, from the data of Table 2 (including omitted elements),

X = H = 0.71 ± 0.02 (Hydrogen mass fraction) (10.40)

E Ma

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