We have already seen that the mass of the Sun is 2.0 X 1033 g and that its radius is 7 X 1010 cm (7 X 105 km). Its average density is
For comparison, the density of water is 1 g/cm3. The Sun is composed mostly of hydrogen (94% by the number of atoms), with some helium (6% by number of atoms), and only 0.1% other elements. The abundances of the elements are given in Appendix G. Our best measurement of the effective temperature of the Sun is 5762 K. The solar luminosity is 3.8 X 1033 erg/s. (The effective temperature is the temperature that we use in the Stefan-Boltzmann law to give the solar luminosity.)
When we look at the Sun, we see only the outermost layers. We have to deduce the internal structure from theories of stellar structure (which we will discuss in Chapter 9). The basic structure of the Sun is shown in Fig. 6.2. The center is the core. It is the source of the Sun's energy. Its radius is about 10% of the full solar radius. The outermost layers form the atmosphere. We divide the atmosphere into three parts. Most of the light we see comes from the photosphere, the bottom layer of the atmosphere. Above the photosphere is the chromosphere, named because it is the source of red light seen briefly during total eclipses of the Sun. The chromosphere is about 104 km thick. The outermost layer is the corona, which extends far into space. It is very faint, and, for most of us, can only be seen during total solar eclipses. Beyond the corona, we have the solar wind, not strictly part of the Sun, but a stream of particles from the Sun into interplanetary space.
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