A great mystery of our Sun still remains unsolved even after a 100 years

The Sun's missing colours reveal its composition, atmospheric structure, gas motion, temperature and changing brightness through spectroscopy.

At first glance, the Sun appears to shine with plain white or yellow light. However, when its light passes through a prism—a transparent optical device that bends light and separates it into its component colours—it spreads into a brilliant spectrum, the complete range of visible colours arranged by wavelength. Surprisingly, this colourful band is interrupted by countless dark lines, where certain colours are missing. These mysterious features, known as absorption lines or Fraunhofer lines, have fascinated scientists for centuries and remain one of the most powerful tools for understanding the Sun.

Although the Sun emits light across nearly the entire visible spectrum, it appears brightest in the yellow-green region. Yet not every colour reaches us equally. The dark patches in the solar spectrum occur because gases at or above the Sun's surface absorb light at specific wavelengths—the distances between successive peaks of light waves that determine colour—before it escapes into space. Since every chemical element absorbs a unique set of wavelengths, these missing colours form distinctive patterns that allow scientists to identify the gases present in the Sun.

One of the most remarkable discoveries made using this technique was helium, which was identified in the Sun's spectrum in 1868 before it was ever found on Earth. Today, the majority of these absorption lines have been identified, although not all of them are yet fully understood.

The dark lines seen in the solar spectrum are collectively known as the Fraunhofer lines, named after the German physicist Joseph von Fraunhofer, who catalogued the principal features using the letters A through K, arranged from longer (redder) to shorter (bluer) wavelengths. For example, the D line is associated with sodium, while the H and K lines are produced by calcium. Some Fraunhofer lines, however, do not originate from the Sun itself but are caused by absorption in Earth's atmosphere.

The Sun is composed primarily of hydrogen, but tiny amounts of other elements—including calcium, sodium, magnesium and iron—act as impurities that absorb light at specific wavelengths. Without these trace elements, the Sun's spectrum would contain no absorption lines. In that case, solar physicists would lose one of their most powerful methods for studying the solar atmosphere, the outer layers of the Sun through which light passes before reaching space. Thanks to these impurities, researchers can investigate the Sun's composition and physical conditions in remarkable detail.

Beyond revealing what the Sun is made of, the Fraunhofer lines also provide valuable information about its atmosphere. The depth of an absorption line reflects temperature conditions, while shifts in its wavelength reveal the motion of solar gas. Because each absorption line forms where the Sun's atmosphere becomes less transparent at a particular wavelength, scientists can observe different atmospheric layers by studying different lines. By combining observations from multiple absorption lines, researchers can reconstruct the three-dimensional structure of the Sun's atmosphere.

The importance of these missing colours extends beyond studying the solar atmosphere. Fraunhofer lines also play a central role in understanding solar brightness variability—changes in the Sun's brightness that occur over timescales ranging from minutes to decades. Identifying the physical processes responsible for these variations helps scientists improve models of the Sun's behaviour and better understand the connections between the Sun, Earth and other stars.

Despite more than a century of research, the Sun's spectrum still contains unanswered questions. While scientists have identified most absorption lines and understand the processes behind many of them, not every missing colour has been explained. These dark lines are far more than gaps in a rainbow—they are a record of the Sun's composition, temperature, motion and changing brightness, offering an indispensable window into the workings of our nearest star.

Source: NASA, ESO, NAOJ

This article was generated with some help from AI and reviewed by an editor. Under Section 107 of the Copyright Act 1976, this material is used for the purpose of news reporting. Fair use is a use permitted by copyright statute that might otherwise be infringing.

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Posted Saturday 18 July 2026 at 6:39 pm AEST (my time).

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