The Sun is a main-sequence star, one of over 100 billion stars in the Milky Way Galaxy. It takes the Sun over 200 million years to complete one orbit of the galaxy. It is located at present close to the Sagittarius-Carina spiral arm, in what is called the Orion spur.

    The orbital trajectory is not planar, but undulating, the Sun passing through the principal plane of the galaxy once every 30 million years. The planets of the Solar System revolve about the Sun in a plane normal to the Sun's circum-galactic trajectory. Thermonuclear reactions in the Sun's core convert hydrogen into helium, so that the helium fraction is progressing increasing through time. The reactions produce the energy to make the Sun a yellow-orange main-sequence star of spectral type dG2 (d indicates dwarf). The central temperature is about 15 million K; the surface temperature is about 6000 K.

    The Sun is a nearly-perfect sphere of gas with a diameter of 1,400,000 km, held together by its own gravity. Its dimensions are at present not static and recent findings have shown that the Sun's diameter is at the present time decreasing about one meter every hour. This decrease may be a long-term oscillation which may be one effect of the long-term stabilization of the Sun's output of energy. It has also been found that every 2h 40min the Sun's surface pulses at a speed of 6 kmh^-1. Thus its surface moves in and out to change the diameter by nearly 10 km.

    The Sun contains 99.86% of all matter in the solar system. It is the source of light and heat for all planets and the support of life on Earth. A number of properties of the Sun is listed in Table 1-1 (cp. also Allen 1973; Lang 1980).