This two-panel figure shows HUT spectra of two different gaseous
filaments in the Cygnus Loop supernova remnant.
for a picture of a portion of the Cygnus Loop.)
The Cygnus Loop is the leftover, expanding remnant of a stellar
explosion that occurred roughly 20,000 years ago some 2000 light years
away from earth. As the blast wave expands, it encounters "clouds"
of interstellar gas. These clouds get heated by the blast wave, and
then cool down by emitting optical and ultraviolet radiation we can
see with telescopes.
The upward "spikes" in the figures are called emission lines,
and arise from different chemical elements in the gas, such as carbon,
nitrogen, oxygen, and helium. The lines are marked with element
abbreviations. (Roman numerals indicate the "ionization stage" of
the element.) The two spectra from the same object show many
differences that are related to the time since the emitting material was
first struck by the blast wave. (Figure adapted from W. P. Blair et al.
1991, ApJ, 379, L33, and K. S. Long et al. 1992, ApJ, 400, 214.)
[Note: lines marked with an earth symbol (a circle with a "+" sign)
are due to residual atmospheric emissions still present in low earth