Chapter 11: Our Sun: The Nearest Star

The Sun featuring several large sunspots

In ancient times, astronomers in China and India noticed and recorded dark spots on the Sun, called sunspots. In the 1600s, when the first telescopes were pointed at the Sun, astronomers were able to closely track the sunspots. Galileo saw these blemishes as evidence that the Sun was not a smooth and perfect sphere. He used this evidence to argue against the ideas of Aristotle, who had thought that the celestial objects were perfect and unchanging. Although it seems like a simple insight, Galileo's observation marked a decisive break with the ancient Greek conception of the universe. Unfortunately, direct observation of the Sun is extremely dangerous. Galileo spent his last years almost totally blind from his years of observation of the Sun.

A schematic of the different layers of the Sun.

What do sunspots reveal about the Sun? For nearly 400 years, astronomers have counted sunspots to measure solar activity and have tracked them to measure solar rotation. Today we know that sunspots are magnetic disturbances on the Sun's surface. The Sun is close enough that we can watch storms develop on its surface. We know that the Sun rotates at its equator once every 25.4 days relative to the stars. It rotates every 27.3 days relative to the Earth; since the Earth's orbital motion is in the same direction as the solar rotation and must be added in. Like Jupiter, the Sun rotates differentially. Its equatorial region rotates faster (25 days) than the polar regions (33 days) — proof that the Sun has a gaseous, not solid, surface.

Solar spectrum showing the dark absorption lines.

Ever since Newton showed that sunlight is a mixture of all colors, scientists have studied the Sun's spectrum. In 1817, German physicist Joseph Fraunhofer found that certain wavelengths were missing from the Sun's spectrum; the spectrum appeared to be crossed by narrow, dark absorption lines. What were these lines? When a gaseous element is heated, it emits radiation of certain wavelengths and no others. These very narrow wavelength intervals, unique to each element and as unmistakable as a set of fingerprints, are called emission lines. Researchers soon found that the emission lines of hydrogen exactly matched the position of some of Fraunhofer's solar absorption lines. The two sets of lines were caused by the same element. The Sun is primarily made of hydrogen. In 1868, French astronomer Pierre Janssen was observing a solar eclipse in India when he noticed unfamiliar spectral features. This second most abundant ingredient in our star was named helium, after the Greek word for the Sun. It took almost thirty years for this "alien" element, known only out in space, to be identified on Earth!