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11.5 The Sun's Composition

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Joseph Fraunhofer. Click here for original source URL.

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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. These lines matched lines seen in chemistry labs when a gaseous element was heated. It was discovered that every compound emits and absorbs 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 spectral emission and absorption 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.

The Sun is about 76% hydrogen and 22% helium by mass — roughly the same H/He proportions found in the giant planets’ atmospheres. Contrast this with the Earth. The heavy elements that are common in the Earth, like iron, nickel, silicon, and carbon, make up only 2% of the Sun by mass. The most abundant elements reflect the overall material from which the Solar System formed. 

In practice, identifying elements using spectroscopy can be like solving a puzzle. Each element produces its own characteristic set of spectral lines. However, the sets of lines of many elements are present, and the resulting pattern can be very complex. We can collect so much radiation from the Sun that we can detect the presence of elements even in very low concentrations. For example, even though fewer than one in a million atoms in the Sun is an iron atom, we can easily detect spectral lines of iron in the solar spectrum. Remote sensing by spectroscopy is a very sensitive technique. Some elements have been detected in space at abundances a trillion (1012) times less than hydrogen, the most abundant element. Here are the most common ten elements in the outer layers of the Sun, in terms of percentage of atoms and percentage of mass. Almost 70 elements have been detected in the Sun, most in trace amounts.

Hydrogen - 91.2% of atoms, 71.0% of mass

Helium - 8.7% of atoms, 27.1% of mass

Oxygen - 0.078% of atoms, 0.97% of mass

Carbon - 0.043% of atoms, 0.40% of mass

Nitrogen - 0.0088% of atoms, 0.096% of mass

Silicon - 0.0045% of atoms, 0.099% of mass

Magnesium - 0.0038% of atoms, 0.076% of mass

Neon - 0.0035% of atoms, 0.058% of mass

Iron - 0.0030% of atoms, 0.14% of mass

Sulfur - 0.0015% of atoms, 0.040% of mass 

Spectroscopy is a great example of the reach of the scientific method. Forensic chemists use spectroscopy to detect traces of particular elements in samples of human skin or hair. The same techniques are used to detect trace elements in giant gas balls trillions of miles away! In 1868, the French astronomer Pierre Janssen and the English astronomer Norman Lockyer independently found spectral lines in the Sun that corresponded to an unknown element. This element, named helium (from the Greek helios, "sun"), was the first to be discovered in space instead of on the Earth.

The discovery of helium raised an uncomfortable question about the scientific method as applied to astronomy. If the Sun contains strange materials that are unknown on Earth, how well can we ever understand the universe? Helium is exceedingly rare on Earth, but was eventually detected by Lockyer in 1895. As became clear from our understanding of planetary atmospheres, it escaped into space early in the Earth's history. The earlier concern has been put to rest after another century of research; no element has been discovered in the Sun or any other star that cannot be measured in the laboratories of the Earth.