During 10 years, five high-energy cosmic rays (plotted as orange asterisks) came from one small part of the sky near the star Merak in the Big Dipper. Astrophysicists suspect their source is a merging pair of galaxy clusters (blue). Progressively more distant galaxy clusters appear in light blue, green, and red, while the gray shading shows the positional error for each of the five cosmic-ray detections.
G. R. Farrar, A. A. Berlind, D. W. Hogg (New York University)