ABSTRACT
Because the Sun is relatively close to the Earth, readily observed, and apparently fairly typical of stars within our Galaxy, astronomers tend to use it as a standard object. In particular, the relative abundances of elements in the Sun may be measured and are frequently assumed to be typical of the Galaxy. Whether or not this is really true is still unclear. However, it is true that the relative abundances found in the Sun are not typical of interstellar clouds. By the means we discussed in Chapter 2, abundances of some elements are determined and turn out to be very greatly reduced, in comparison to the Sun. We show in figure 4.1 results of observations in the direction of the star ζ Ophiuchi. For example, titanium is seen to be depleted, that is, underabundant relative to the Sun by more than a factor 1000 in the direction towards this star; and oxygen is depleted by a factor of about 3. The depletion data are displayed as a function of condensation temperature, i.e. the temperature below which we expect solids of that material to be stable.
