measurements of the variability of the sun's output and of how other stars like the sun vary. Different perspectives come from observing other planets and their satellites with robotic spacecraft sent on voyages of discovery, and from telescopes on, or in orbit around, the earth. Observations of the atmospheres of the other planets made possible by infrared and radio telescopes have revealed the constituents of those atmospheres, as well as variations in their temperature and density with height. Careful examination of the dimming of the light from a star as it passed behind the planet Pluto was used to probe the atmosphere of this distant planet. Future observations will probe the climatology and meteorology of the planets and their satellites to give a comparative basis for understanding our own environment.

The Voyager spacecraft found sulfur-spewing volcanoes on Jupiter's satellite Io that appear to be driven by the grinding tides raised by the giant planet. These volcanoes are now monitored by earth-bound telescopes using cameras sensitive to the heat, or infrared radiation, from the volcanoes (Plate 2.2). The sizes, shapes, and compositions of asteroids are revealed by infrared observations and by bouncing strong radar signals off these objects (Plate 2.3).


The Sun

In his masterwork the Principia (1687), Newton wrote that “those who consider the sun one of the fixed stars” may estimate the distance from the earth to a star by comparing its apparent brightness with that of the sun—in the manner that the distance to a candle may be judged by comparing its brightness with that of an identical candle nearby. Newton then calculated that the closest stars are about a million times farther away than the sun, in good agreement with later measurements. The sun is the closest star, and its careful study has important implications for our knowledge of all stars.

The sun is a great ball of hot gas, about a million miles in diameter. According to modern theory, its central density is about 100 times that of water, and the temperature is about 15 million degrees celsius. Such high temperatures are needed to smash subatomic particles together violently enough to fuse them and release nuclear energy. The liberated energy does two things. It maintains the heat within the sun, providing sufficient pressure to resist the inward pull of gravity. The liberated energy also turns into radiation, which slowly makes its way to the solar surface, finally creating the light we see. Some of the sun 's energy goes into churning up its surface and producing extremely energetic particles, magnetic fields, solar flares, and a tenuous atmosphere of higher temperature called the corona.

Many mysteries remain. How do the magnetic fields get their energy? What is the role of magnetic fields in the curious 11-year cycle of activity

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