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light apart to see what it could reveal about the star that was emitting it. The formal name of this process is spectroscopy, and it’s been a staple of physics and astronomy since the 1800s. Its roots go back even further: Chinese scholars suggested as far back as the 1000s that rainbows were created when drops of water in the air split sunlight into a spectrum of colors. Persian and Arab astronomers came to the same idea independently, and in the 1200s the English natural philosopher Roger Bacon experimented with glasses of water and crystals that split light just as a rainbow does. Isaac Newton used prisms to split sunlight as well. William Herschel, who won worldwide fame for his discovery of the planet Uranus in 1781, did his own experiments with light and color that led to his discovery of infrared light in 1800—the first hint that light comes in colors the human eye can’t perceive.
    Physicists now understand that light comes
mostly
in colors we can’t see. If you think of light as a piano keyboard, with each note representing a color, you can think of the visible spectrum as the octave right in the middle. Infrared is lower in pitch than visible light. Microwaves are lower still, and radiowaves are the deepest bass notes. At the other end of the keyboard, ultraviolet light is just a little too high in pitch for us to see. Gamma rays are higher, and X-rays still higher—the shrill, tinkly notes at the far right. (Like most analogies, this one isn’t perfect. There are vastly more colors of light, most of them invisible to us, than there are notes on a piano).
    The Sun shines in virtually all of these colors, at least to some degree. We can see only the colors of the visible-light rainbow because these are the colors that penetrate our atmosphere most easily. We evolved to take maximum advantage of the kind of light that’s most available. If we want to see at night, when there’s no sunlight, we can put on night-vision goggles that are sensitive to infrared light. Living things, including plants and people, glow in the dark, but mostly in the infrared. We’ve also invented technologies for sensing ultraviolet, microwaves, gamma rays, and all the other colors of light. When astronomers began using these, in the 1920s, they began finding all sorts of cosmic phenomena they’d never imagined before—including, in 1965, the light left over from the Big Bang, which is now detectable only as microwaves. Ultraviolet astronomy, X-ray astronomy, radio astronomy, and gamma ray astronomy are now distinct, though obviously related, branches of science.
    In the early 1800s, William Hyde Wollaston, an Englishman, noticed that fine, dark lines interrupted the artificial rainbows he had created with prisms. A German chemist named Josef von Fraunhofer independently discovered the same thing. Within a few decades, chemists understood that the lines were caused by chemical elements in the Sun’s outer layers. Theelements absorbed very specific colors of light on their way from the Sun out into space. The elements are like filters. They catch
very
specific colors, so not just “blue” but “the exact color of blue represented by a particular wavelength of light.” That’s how specific we’re talking about. A given element or compound doesn’t just absorb light at a single wavelength, but at many different wavelengths at once. It’s like a bar code, with a pattern unique to each substance. If light is shining through many different elements or compounds at the same time, the multiple overlapping barcodes have to be untangled before you can figure out what you’re looking at.
    Still, by comparing the lines they saw in the Sun with the ones they were able to create in labs on Earth, scientists were able to figure out what the Sun is made of. (It’s made mostly of the same elements Earth is made of, although in very different proportions.) So that was