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
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 and 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.