A huge set of new Hubble Space Images show galactic collisions in action and the variety of peculiar forms that merging galaxies can take.
The series of 59 new photographs, released today on the 18th anniversary of the Hubble Space Telescope's launch, are the largest collection of Hubble images ever released together.
Galaxy mergers are now known to be more common than was previously thought. They were even more common in the early universe than they are today. The early universe was smaller, so galaxies were closer together and therefore more prone to smash-ups. Even apparently isolated galaxies can show signs of past mergers in their internal structure.
Our own Milky Way contains the debris of the many smaller galaxies it has brushed against and devoured in the past. And it hasn't stopped munching away at its neighbors: It is currently absorbing the Sagittarius dwarf elliptical galaxy.
The Milky Way isn't the top predator though, as our giant neighbor, the Andromeda galaxy, is expected to devour the Milky Way in about two billion years. The future resulting elliptical galaxy has already been dubbed "Milkomeda."
Though colliding galaxies rush towards each other at hundreds of kilometers per hour, the interactions can take hundreds of millions of years to complete.
This game of celestial bumper cars is driven by the gravitational pulls that galaxies exert on one another. Typically the first sign of a collision is a bridge of matter connecting two galaxies as gravity's first gentle tugs tease out dust and gas. As the outer reaches of the galaxies begin to interact, long streamers of gas and dust, called tidal tails, sweep back to wrap around the galactic cores.
As the cores approach each other, the conflicting pull of matter from all directions can result in shockwaves that ripple through interstellar clouds. Gas and dust are siphoned off to fuel bursts of star formation that appear as blue knots of young stars. Given the vast distances between stars in a galaxy — the nearest star to us is 4.3 light-years away — stars rarely collide when galaxies merge.
The Hubble images capture galaxies in various stages of the collision process and show the variety of new and unusual shapes the mergers can create, including mergers that look like an owl in flight and a toothbrush.