This image of the northeastern region of the Cygnus Loop supernova remnant
shows a combination of optical and X-ray data. Red indicates filaments
emitting in the light of hydrogen, green shows regions emitting in the light
of twice-ionized oxygen, and the diffuse blue glow shows X-ray emission.
Various combinations of these produce other colors. For instance, green
filaments with surrounding X-ray (blue) emissions produce a turquoise result.
Background stars, which do not emit in X-rays, look yellow (green + red).
The white region at lower right is very bright in all three colors.
HUT scientists observed these different kinds of supernova remnant
emissions to better understand how supernova shock waves interact with
the gas and dust in the interstellar medium.
(PHOTO from J. Hester, J. Raymond, and W. Blair, 1994, Astrophysical
Journal, Vol. 420, pg. 721.)