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This illustration shows a star surrounded by a protoplanetary disk. The star's irregular illumination allows astronomers to measure the gap between the disk and the star by using a technique called "photo-reverberation" or "light echoes."  (Illustration: NASA/JPL-Caltech) This illustration shows a star surrounded by a protoplanetary disk. The star's irregular illumination allows astronomers to measure the gap between the disk and the star by using a technique called "photo-reverberation" or "light echoes."