The Interstellar Medium Family

  • Steven J. Dick
Part of the Astronomers' Universe book series (ASTRONOM)


With this class of objects, we enter the Family of the interstellar medium. The interstellar medium consists of about 99% gas and 1% dust, and the gas component consists of three main classes of objects: cool atomic clouds composed mainly of neutral hydrogen (H I); hot ionized clouds also composed mainly of hydrogen, known as H II regions (S 24) and observed as emission nebulae; and cold molecular clouds (S 25) composed largely of hydrogen (H2) with a sprinkling of other molecules.

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© Springer Nature Switzerland AG 2019

Authors and Affiliations

  • Steven J. Dick
    • 1
  1. 1.AshburnUSA

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