How about crescent stars? (Letters).
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" (SN: 8/25/01, p. 118), 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