The Radio Sun
Our Sun is an ordinary star, and the nearest one to Earth. Its distance from us defines a convenient unit of measure in our Solar System, the astronomical unit, or AU. One AU is approximately 150 million km, which is the mean distance from Earth to the Sun. The solar diameter is 696,000 km, or approximately 0.7 million km, presenting an angular diameter of about 0.5° as seen from Earth’s surface. Its spectral class is G2, and it lies centrally placed on the main sequence of the Hertzsprung–Russell diagram in Fig. 1.1.