 Nebula, and M17. 

The darkness of these 
<a href="http://archive.ncsa.uiuc.edu/Cyberia/Bima/GMC.html"
>molecular clouds</a> results from background starlight 
being absorbed by thick carbon-based smoke-sized 
<a href="http://antwrp.gsfc.nasa.gov/apod/ap990509.html">dust</a>. 

As bright 
<a href="http://antwrp.gsfc.nasa.gov/apod/ap011125.html">massive stars</a> form, 
they produce intense and 
<a href="http://imagers.gsfc.nasa.gov/ems/uv.html"
>energetic light</a> that 
slowly boils away the dark shroud. 