This is only a very brief account of the Sun, and it is biased towards topics of importance for the Solar System as a whole. Fuller accounts of the Sun are in books listed in Further Reading.

1.1.1 The Solar Photosphere

The bright surface of the Sun is called the photosphere (Plate 1). Its radius is 6.96 x 105km, about 100 times the radius of the Earth. It is rather like the 'surface' of a bank of cloud, in that the light reaching us from the photosphere comes from a range of depths, though the range covers only about one-thousandth of the solar radius, and so we are not seeing very deep into the Sun. It is important to realise that whereas a bank of cloud scatters light from another source, the photosphere is emitting light. It is also emitting electromagnetic radiation at other wavelengths, as the solar spectrum in Figure 1.1 demonstrates. The total power radiated is the area under the solar spectrum, and is 3.85 x 1026 watts (W). This is the solar luminosity. The photosphere, for all its brilliance, is a tenuous gas, with a density of order 10-3kgm-3, about 1000 times less than that of the air at the Earth's surface.

The spectrum in Figure 1.1 enables us to estimate the mean photospheric temperature. This is done by comparing the spectrum with that of an ideal thermal source, sometimes called a black body. The exact nature of such a source need not concern us. The important point is that its spectrum is uniquely determined by its temperature. Turning this around, if we can fit an ideal thermal source spectrum reasonably well to the spectrum of any other body, then we can estimate the other body's temperature. Figure 1.1 shows a good match between the solar spectrum and the spectrum of an ideal thermal source at a temperature of 5770 K. Also shown is the poor match with an ideal thermal source at 4000 K, where the peak of the spectrum is

Discovering the Solar System, Second Edition Barrie W. Jones © 2007 John Wiley & Sons, Ltd u>

Ultraviolet Visible Infrared

Ultraviolet Visible Infrared

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