03 October 2013

Every color of the Sun’s rainbow: Why are there so many missing?

What you see here is the entire gamut of our Sun’s visible light output. It clearly shows you how the Sun emits almost every color, but how the output of some colors, such as yellow and green, are brighter than others. Perhaps more interestingly, though, the black lines illustrate the portions of the visible light spectrum that are not emitted by the Sun — and to this day, we still don’t know why some portions of the visible solar spectrum are absent.

The image above is called the absorption spectrum of the Sun, was observed by the Fourier Transform Spectrometer at the National Solar Observatory on Kitt Peak, near Tucson, Arizona. The data, which is in essence gathered by shining sunlight through a very accurate prism, was compiled into a Solar Flux Atlas. The Atlas recorded the entirety of Sun’s emitted light from 296nm to 1300nm, but for the absorption spectrum above, that range was narrowed to the visible light range — 400nm (purple) to 700nm (red). In the image above, each of the 50 rows represents 60 angstroms, or 6nm.

The black lines in the Sun’s spectrum are caused by gases on, or above, the Sun’s surface that absorb some of the emitted light. Every gas (such as helium, hydrogen, oxygen, and so on) has a very specific set of frequencies that it absorbs. If you shine some light through some gas, and then a prism, and record the absorption spectrum, you can say with certainty what that gas is — a valuable tool in chemistry called absorption spectroscopy. NASA’s Curiosity rover uses spectrometers (though not absorption spectrometers) to work out what gases and compounds are present on Mars.

Fraunhofer lines, on the Sun’s absorption spectrum. The letters correspond to various elements (such as helium, sodium) that cause the lines.

For the most part, we know exactly which gases cause each of the black lines — called Fraunhofer lines, after Joseph von Fraunhofer who discovered them in 1814 – in the Sun’s absorption spectrum. Some lines, however, remain mysteriously unidentified. It’s probably not the case that these lines are produced by weird and wonderful elements that don’t exist on Earth, but it’s a possibility.

Now read: NASA confirms that Voyager 1 has finally left the Solar System



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