Interstellar car-crashes: When galaxies collide...

To misquote the opening credits of Star Wars, these spectacular images show interstellar car-crashes happening a long time ago, in a place that's far, far away. Taken by the Hubble Space Telescope, they capture various points in the collision process between two galaxies, which can take hundreds of millions of years to go from start to finish.

Galaxy mergers were more common in the early universe than they are now, and are thought to have been one of the principal driving forces for cosmic evolution, from the frenetic births of stars to their explosive deaths. Indeed, our own Milky Way galaxy is currently absorbing the smaller Sagittarius dwarf elliptical galaxy, and will in turn be merged with our bigger neighbour, the Andromeda galaxy.

The interaction of two galaxies is driven by the tidal pull of gravity, which draws the massive objects together (though actual collisions between stars are rare, as most of a galaxy is empty space). The billions of stars in each galaxy move individually, so that the interweaving tidal forces produce intricate and varied effects as the galaxies gradually coalesce.

As the outer reaches of the galaxies begin to intermingle, long streamers of gas and dust – known as tidal tails – stretch out and sweep back to wrap around the galactic cores. Gas and dust within colliding galaxies are buffeted and accelerated by the tidal forces, sending shockwaves through the matter, which becomes so heated that it radiates light at a luminosity several thousand billion times brighter than the Sun.

These images were part of a selection released yesterday by the US National Aeronautics and Space Administration (Nasa) to celebrate the 18th anniversary of the Hubble telescope's launch.