The House that Jack Built

    Caption: Star formation is a bit like This Is the House That Jack Built:

      This is the house that Jack built.
      This is the malt that lay in the house that Jack built.
      ...
      That tossed the dog that worried the cat
      That killed the rat that ate the malt
      That lay in the house that Jack built.

    To explicate:

      These are the stars whose stellar winds
            and supernova explosions
      Polluted the interstellar medium (ISM) with metals
      That clumped in spiral arms into dark,
            dusty nebulae
      That shielded the interstellar gas from
            ultraviolet light (fiducial range 0.01--0.4 μm)
      That permitted interstellar molecules to form
      That formed molecular clouds
      That radiated electromagnetic radiation (EMR)
      That cooled the interstellar gas
      That lowered the pressure
      That permitted runaway gravitational collapse
            of the interstellar gas to dense cores
            maybe following a triggering event
      That became protostars when they got hot enough to radiate in the infrared
      That followed Hayashi tracks on the Hertzsprung-Russell diagram downward and/or blueward
      That settled onto the zero-age main sequence (ZAMS) when the inner cores
            got hot and dense enough to start hydrogen burning
      That eventually had stellar winds and supernova explosions
      That polluted the interstellar medium (ISM) with ...

    The above is NOT exactly coherent---the relative pronouns, the antecedents---but poetic license ...

    Credit/Permission: Walter Crane (1845--1915), 1886 (uploaded to Wikimedia Commons by User:DcoetzeeBot, 2012) / Public domain.
    Image link: Wikimedia Commons: File:Walter Crane - The House that Jack Built - Google Art Project.jpg.
    Local file: local link: star_formation_jack.html.
    File: Star file: star_formation_jack.html.