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In the photograph of X-ray jets and point sources in the galaxy Centaurus A, shown in “Shocks jolt jet set galaxy, X rays reveal,” many of the point sources seem to be regularly spaced along arcs. The pattern is reminiscent of an X-ray diffraction pattern from a crystal (a microscopic event mirrored here on a cosmic scale!). This could be an optical illusion–or perhaps the result of some physical phenomenon such as spherical shock waves colliding or cosmic strings. Barton M. Bresnick
Mansfield, Mass.