Dots show locations of stars in the Keck Observatory spectroscopic survey superimposed on an image of Andromeda. Credit: Claire Dorman/ESA
Dots show locations of stars in the Keck Observatory spectroscopic survey superimposed on an image of Andromeda. The stars are color coded according to their velocity relative to Earth, as measured from the spectra (negative velocity = moving towards Earth). The center of Andromeda is moving towards Earth at a speed of −300 km/s, whereas stars to the northeast (upper left) of the map have less negative velocities, indicating that they are moving away from Earth, relative to Andromeda’s center. Credit: Claire Dorman/ESA