A decade's worth of telescope observations of the sun has revealed a startling mystery: Gamma rays, the highest frequency waves of light, radiate from our nearest star seven times more abundantly than expected. Stranger still, despite this extreme excess of gamma rays overall, the narrow bandwidth of frequencies is curiously absent. The surplus light, the gap in the spectrum, and other surprises about the solar gamma-ray signal potentially point to unknown features of the sun's magnetic field, or more exotic physics. "It's amazing that we were so spectacularly wrong about something we should understand really well: the sun," said Brian Fields, a particle astrophysicist at the University of Illinois, Urbana-Champaign. The unexpected signal has emerged in data from the Fermi Gamma-ray Space Telescope, a NASA observatory that scans the sky from its outpost in low-Earth orbit. As more Fermi data have accrued, revealing the spectrum of gamma rays coming from the sun in ever-greater detail, the puzzles have only proliferated. "We just kept finding surprising things," said Annika Peter of Ohio State University, a co-author of a recent white paper summarising several years of findings about the solar gamma-ray signal. "It's definitely the most surprising thing I've ever worked on." Not only is the gamma-ray signal far stronger than a decades-old theory predicts; it also extends to much higher frequencies than predicted, and it inexplicably varies across the face of the sun and throughout the 11-year solar cycle. Then there's the gap, which researchers call a "dip" - a lack of gamma rays with frequencies around 10 trillion hertz. "The dip just defies all logic," said Tim Linden, a particle astrophysicist at Ohio State who helped analyze the signal. Fields, who wasn't involved in the work, said, "They've done a great job with the data, and the story it tells is really kind of amazing."