This image from the Hubble Space Telescope reveals a curtain-like sheet that marks the boundary between the hot gas of the Great Nebula and the cool, dark molecular gas and dust behind it. The red areas indicate regions of sulfur emission. This emission is typical of temperaure and density conditions intermediate between those found in the nebula and those in the cold cloud behind it. Astronomers believe the sulfur light, which forms clumps and filaments in the image, comes from regions where the hot stars in the nebula are ΓÇ£boiling offΓÇ¥ the ma- terial in the cold molecular cloud. The smoother green and blue re- gions are from light emitted by hydrogen and oxygen. This emission is typical of the physical conditions within the nebula.
