Our Sun Introduction
The Sun is a typical star, but is unique as far as we are concerned because of its proximity to Earth; it is about 150 million km away. A summary of basic solar properties is given in table 8.1. Solar energy provides virtually all the heat and light which our planet receives, and it therefore sustains every living entity. The character of our own environment is strongly influenced by solar radiation which has been a major factor in determining the course of natural evolution on Earth. The Sun is important to astronomers and physicists because it enables them to investigate physical conditions, which are typical of most stars, in detail. Historically, the study of the Sun led to significant advances in atomic physics, nuclear physics, magnetohydrodynamics, and plasma physics. An understanding of solar processes is therefore of biological and physical interest, and several fundamental areas of scientific research are furthered by observing this local astrophysical laboratory. Satellites have enabled space scientists to probe more closely the interaction between the Sun and the Earth, especially the influence of the Sun on the magnetic field of the Earth.
Before describing the Sun in detail we must issue an important warning about observing the Sun: intense solar radiation permanently damages the tissue of the human eye. The Sun must never be viewed directly with a telescope or binoculars, which would have the effect of concentrating a massive dose of radiation on to the delicate tissue. The solar filters sold with many cheap telescopes are not an adequate safeguard because they may admit a dangerous dose of invisible ultraviolet light or may shatter unexpectedly. They should be destroyed in order to remove the temptation to use them. With a little experimentation, good images of the Sun can be produced by projecting through an eye¬piece on to a piece of stiff card. This arrangement is adequate for viewing sunspots or the progress of an eclipse. An eclipse may also be observed by the unaided eye by looking at the Sun through a really dark filter or looking at the reflection in a dark container of still water, but even these methods require caution. We repeat: to look directly at the Sun through binoculars or a telescope is a dangerous act that can cause permanent blindness.