Studying the Sun

To many ancient astronomers, the sun was a ball of pure fire that revolved around the earth. This general belief was held until the early 1600's, when telescopes first came into use and the sun was recognized to be at the center of the solar system.

Galileo and other 17th-century astronomers saw numerous sunspots through their telescopes, but had no idea what they were seeing. In 1785, the British astronomer William Herschel determined that the sun was a star within the Milky Way galaxy. Herschel also suggested, incorrectly, that sunspots revealed a cool, dark crust beneath the fiery visible surface. He further proposed that this relatively cool interior might contain intelligent life. Speculation such as Herschel's continued for many years as astronomers struggled with inadequate tools to learn more about the sun.

With the development in the mid-1800's of such important astronomical aids as photography and spectroscopy, the true nature of the sun began to be discovered. More than 60 chemical elements were soon found to exist on the sun's surface, including one that was unknown on earth. The new element, named helium after the Greek word for sun, helios, was eventually found to occur on earth as well.

The mystery of the sun's energy source was not solved until extensive nuclear research began in the 1930's. Using the newly discovered principles of nuclear physics, the German-American physicist Hans Bethe and others were able to explain the processes by which the sun and other stars obtain their energy.

Much of the equipment and many of the techniques used by solar astronomers are similar to those used by other astronomers. Because of the size and brilliance of the sun's image, however, large-diameter telescopes are not needed. Some solar telescopes resemble small conventional refractor or reflector telescopes. Others, such as the McMath solar telescope on Kitt Peak in Arizona, use a series of large mirrors to reflect the sun's image and relay it to an observing room for spectroscopic study.

An important tool used with some solar telescopes is the coronagraph, an instrument that artificially eclipses the sun so that the corona and chromosphere may be observed. The spectroheliograph produces a photograph of the sun by recording light of a single wavelength—such as that emitted by atoms of hot hydrogen gases in the chromosphere—to reveal otherwise invisible details in the sun's atmosphere.

Intensive studies are also made of the sun's invisible radiation, particularly X rays, infrared light, and radio waves. Since X rays and some infrared light do not penetrate the earth's atmosphere, their observation is usually done with the help of artificial satellites or high-altitude balloons. Detailed investigations of the entire solar spectrum have been made with the aid of such satellites as those of the Orbiting Solar Observatory (OSO) series. Because there are millions of stars similar to the sun in the Milky Way galaxy alone, astronomers frequently use information from solar observation as an aid in understanding more about other stars.