Radio map of the hydrogen gas in the Andromeda Galaxy (M31), colour coded for velocity. The brightness of the radio emission, at the hydrogen line wavelenth of 21cm, reveals the amount of gas present, while the colour shows its speed towards or away from us: green & blue regions are approaching, yellow & red regions are receding. The galaxy is in effect cartwheeling relative to Earth. M31 is a massive spiral galaxy & the dominant members of the Local Group of galaxies to which our own Milky Way belongs. The image was recorded by the Westerbork Synthesis Radio Telescope in the Netherlands.