... Credit: Mischa Schirmer Explanation: What's happening to galaxy NGC 474? ... The cause of the shells is currently unknown, but possibly tidal tails related to debris left over from absorbing numerous small galaxies in the past billion years. Alternatively the shells may be like ripples in a pond, where the ongoing collision with the spiral galaxy to the right of NGC 474 is causing density waves to ripple though the galactic giant. ... Publications with keywords: galaxies - interacting galaxies . ...