A computer has removed the stars from this image, leaving only knots and filaments of nebulosity. The red light is from the emission of hydrogen and nitrogen; the blue is oxygen emission (it was green in the optical image); the green is sulfur. The fact that the elements are not mixed tells us a great deal about supernova explosions and the behavior of the late-stage stars that produce them: if the elements are not mixed now, they could not have been mixed inside the progenitor star.
