English: A spectacular trio of merging galaxies in the constellation Boötes takes centre stage in this image from the NASA/ESA Hubble Space Telescope. These three galaxies are set on a collision course and will eventually merge into a single larger galaxy, distorting one another’s spiral structure through mutual gravitational interaction in the process. An unrelated foreground galaxy appears to float serenely alongside the collision, and the smudged shapes of much more distant galaxies are visible in the background.This colliding trio — known to astronomers as SDSSCGB 10189 — is a relatively rare combination of three large star-forming galaxies lying within only 50 000 light-years of one another. While that might sound like a safe distance, for galaxies this makes them extremely close neighbours! Our own galactic neighbours are much further away; Andromeda, the nearest large galaxy to the Milky Way, is more than 2.5 million light-years away from Earth.This observation was designed to help astronomers understand the origin of the largest, most massive galaxies in the universe. These galactic behemoths are called Brightest Cluster Galaxies (BCGs) and — as the name suggests — are defined as the brightest galaxies in any given galaxy cluster. Astronomers suspect that BCGs form through the merger of large, gas-rich galaxies like the ones in this image. They turned to Hubble’s Wide Field Camera 3 and Advanced Camera for Surveys to investigate this galactic trio in painstaking detail, hoping to shed light on the formation of the Universe’s most massive galaxies.[Image description: Three galaxies stand together just right of centre. They are close enough that they appear to be merging into one. Their shapes are distorted, with strands of gas and dust running between them. Each is emitting a lot of light. Further to the left is an unconnected, dimmer spiral galaxy. The background is dark, with a few smaller, dim and faint galaxies and a couple of stars.]