In this STIS spectral image of the oxygen gas, the velocities of the knots are determined by comparing the knots of gas in the stationary WFPC2 image to the horizontal location of the knots in the STIS image.

Credit: John Hutchings (Dominion Astrophysical Observatory), Bruce Woodgate (GSFC/NASA/ESA), Mary Beth Kaiser (Johns Hopkins University), Steven Kraemer(Catholic University of America), and the STIS Team