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 18^th 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.

• Hubble
  Images: When Galaxies Collide


• Vote: The Best of the
  Hubble Space Telescope


• The
  Hubble Image Collection