No space vehicle has ever probed the Sun's atmosphere, as we have begun to do for the planets. To study the Sun or stars astronomers must for the present rely on indirect evidence: interpreting their light gathered by telescopes. Imagine you had to deduce as much as you could about a person from the contents of their house or apartment. Their clothes, their books and music, and the food in their kitchen would give you valuable clues. You could learn a lot without ever meeting the person. Deduction from indirect evidence is one of the standard methods of astronomy. Most of that indirect evidence is electromagnetic radiation from various regions of the Sun. We interpret that evidence in terms of the laws of physics as they apply to extremely hot gas. Although our evidence is indirect, powerful techniques of analysis help us create models that fill in gaps in our observations.
The Sun featuring several large sunspots. Click here for original source URL.
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.
Schematic of the parts of the Sun. Click here for original source URL.
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 of 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. Click here for original source URL.
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, and the Greek work for the Sun. It took almost thirty years for this "alien" element, known only out in space, to be identified on the Earth!