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To find out what the stars are made of, we study their light. Starlight is studied by using a spectroscope, in which it is passed through a glass prism and broken up into a coloured band like a rainbow. Dark lines across this 'spectrum' are due to substances in the star's outer layers which have absorbed certain colours. By comparing the colours with those produced by known chemical substances in the laboratory, it has been found that stars consist almost entirely of hydrogen, with small amounts of gaseous iron, chromium, nickel, and other metals.
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