Spectra, Star's Chemical Bar Code, spectrograph, spectroscopy Starlight contains plenty of useful information, but astronomers have to decode it. In a technique known as spectroscopy, a telescope equipped with a spectrograph splits starlight into a rainbow spectrum of different wavelengths. The dark lines that appear along the spectrum, known as absorption lines, represent how much starlight was absorbed by different chemical elements in the star's outer atmosphere. The thinner the line, the less abundant the element. Below, arrows indicate absorption lines for iron and other heavy elements in the sun and two stars in dwarf galaxies that orbit the Milky Way. The dwarf galaxy stars have much weaker absorption lines and therefore smaller portions of heavy elements than our relatively young sun, which makes sense because dwarf galaxies formed long before the sun when the heavier elements were still scarce. THIS IS A PREVIEW. or subscribe to access the full article. Already a subscriber or purchased this issue? Sign In SOURCE: ANNA FREBEL four-starry-nights_3.jpg THIS IS A PREVIEW. or subscribe to access the full article. Already a subscriber or purchased this issue? Sign In ADVERTISEMENT Scientific American is a trademark of Scientific American, Inc., used with permission © 2015 Scientific American, a Division of Nature America, Inc. All Rights Reserved.