In this image from NASA's Spitzer Space Telescope, "cometary knots" on the Helix Nebula show blue-green heads caused by excitation of their molecular material from shocks or ultraviolet radiation.

This color-composite image of the Helix Nebula was created from images obtained using the the Wide Field Imager, an astronomical camera attached to the 2.2 meter Max-Planck Society/ESO telescope ...