Radial and latitudinal variations of the sound speed in the Sun as derived from MDI measurements. Red = hotter regions than in standard model, blue = cooler regions. Concentric layers in a cutaway image show oddities in the speed of sound in the deep interior of the Sun, as gauged by two instruments. MDI measures vertical motions due to sound waves reverberating through the Sun, at a million points on the visible surface. VIRGO detects the solar oscillations by rhythmic variations in the Sun's brightness, a rapid change in the speed of rotation about the Sun's axis, between the faster-turning outer region and the slower interior.