THE SUN AND THE SOLAR WIND

The Sun is a typical star of intermediate size and luminosity. Sunlight and other radiation are produced by the conversion of hydrogen into helium in the Sun’s hot, dense interior (see Nuclear Energy). Although this nuclear fusion is transforming 600 million metric tons of hydrogen each second, the Sun is so massive (2 × 10³⁰ kg, or 4.4 × 10³⁰ lb) that it can continue to shine at its present brightness for 6 billion years. This stability has allowed life to develop and survive on Earth.

For all the Sun’s steadiness, it is an extremely active star. On its surface, dark sunspots bounded by intense magnetic fields come and go in 11-year cycles and sudden bursts of charged particles from solar flares can cause auroras and disturb radio signals on Earth. A continuous stream of protons, electrons, and ions also leaves the Sun and moves out through the solar system. This solar wind shapes the ion tails of comets and leaves its traces in the lunar soil, samples of which were brought back from the Moon’s surface by piloted United States Apollo spacecraft (see Space Exploration; Apollo program).

The Sun’s activity also influences the heliopause, a region of space that astronomers believe marks the boundary between the solar system and interstellar space. The heliopause is a dynamic region that expands and contracts due to the constantly changing speed and pressure of the solar wind. In November 2003 a team of astronomers reported that the Voyager 1 spacecraft appeared to have encountered the outskirts of the heliopause at about 86 AU from the Sun. They based their report on data that indicated the solar wind had slowed from 1.1 million km/h (700,000 mph) to 160,000 km/h (100,000 mph). This finding is consistent with the theory that when the solar wind meets interstellar space at a turbulent zone known as the termination shock boundary, it will slow abruptly. However, another team of astronomers disputed the finding, saying that the spacecraft had neared but had not yet reached the heliopause.