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The Sun is composed mostly of hydrogen. When the Sun first formed, there was enough hydrogen in the estimated size of its core for this reaction to keep the Sun shining for approximately ten billion years. Since the Sun is about 4.5 billion years old, it is only about halfway through generating energy in its core by hydrogen fusion.

In 1960, Robert Leighton of the California Institute of Technology discovered that small areas of the Sun’s surface oscillated up and down with a period of about five minutes. He observed that the spectra of various points on the Sun’s surface were Doppler-shifted alternately toward shorter and longer wavelengths. The Doppler effect shifts electromagnetic radiation to shorter wavelengths (termed “blueshifts”) when the source moves toward the observer and to longer wavelengths (termed “redshifts”) when the source moves away from the observer. However, he was not able to tell whether the oscillations occurred only in small regions or over the entire Sun.

The Sun is a ball of gas and, as such, has no solid surface. The Sun’s photosphere approximates its surface, marking the region between the solar interior and the chromosphere (the region that approximates the solar “atmosphere”). Because the photosphere is relatively opaque, it blocks any view of the solar interior. As a result, most images of the solar disk show only the photosphere. The photosphere is also the coolest layer of the Sun, with a temperature of about 6,600 kelvins near its bottom and a temperature of about 4,400 kelvins near the chromosphere at the top; in the 400 kilometers between, the photosphere’s temperature gradually decreases with altitude above the solar interior. Astronomers know that the photosphere is relatively cool because it displays absorption lines that are darker than the spectrum of the hot solar interior. The temperature of the overlying chromosphere is likewise hotter, climbing to about 30,000 kelvins as the distance from the photosphere increases. Where the chromosphere ends is the corona, and in the approximately 100-kilometer-thick transition between the chromosphere and corona, the temperature rapidly increases from about 6,000 kelvins to several hundred thousand kelvins. In the corona itself, the temperature can be millions of kelvins, but the gas is very thin.

The Sun is a ball of gas without a solid surface. The Sun’s photosphere, however, is usually considered to mark the surface of the Sun. Relatively opaque, the photosphere blocks our view of the solar interior. It is also the coolest layer of the Sun. When astronomers observe the solar spectrum, they see dark absorption lines from the photosphere because the photosphere is cooler than the hot solar interior.

The Sun is composed primarily of hydrogen and helium. Though these substances are gaseous on Earth, the conditions in the Sun are such that they do not behave inside the Sun the way that they do on Earth. The Sun’s gravity compresses and heats these gases in the Sun’s interior. About one-fifth of the way below the surface of the Sun, the gases are so hot that they are ionized—that is, they have been stripped of their electrons. The closer to the center of the Sun these gases are, the more compressed and heated they are. Almost 94 percent of the Sun’s mass is contained within the innermost half of the Sun’s radius.