Two spiral galaxies in collision - Hubble/NASA 

Like two spinning saw blades slicing through each other, a pair of spiral galaxies collide in this Hubble image of an object called NGC 6050. The Space Telescope Science Institute in Baltimore has released 59 new images of galaxies in collision, part of a celebration today of the 18th anniverary of the orbiting observatory's launch.

Collectively titled "Galaxies Gone Wild," the photos constitute the largest collection of Hubble images ever released at one time by the Institute.

"Galaxies have a wild side. They have flirtatious close encounters that sometimes end in grand mergers and overflowing 'maternity wards' of new star birth as the colliding galaxies morph into wondrous new shapes," the institute said.

The variety of collisions documented in the images is amazing, as is the detail provided by Hubble's superior optics.

Astronomers estimate that only a couple of galaxies in a thousand are experiencing a collision as we see them in the night sky. But most, if not all of them, have likely undergone collisions or mergers in their past. In fact, scientists believe that's how the big galaxies we see today got started - through the merger of smaller clusters of stars and small galaxies going back to the earliest moments of the evolution of the universe.

The simple elliptical galaxies we see today are believed to be products of the mergers of graceful and complex spiral galaxies millions or billions of years ago.

Our own spiral Milky Way is no exception.  Baltimore astronomer Massimo Stiavelli said recent discoveries suggest that the Milky Way is currently digesting the stars of a small "dwarf" galaxy consumed not too long ago.

It was noticed when astronomers began measuring the movement of stars near the center of the galaxy - in the direction of the constellation Sagitarius. "When you measure very accurately the positions and motions of the stars," Stiavelli said, "there are some stars that move differently than the others, as if they are coming from another object."

The Magellanic Clouds - two small galaxies visible from the Southern Hemisphere - are close companions of the Milky Way. They are believed to be connected to our galaxy by a slender stream of gas, perhaps drawn away under the gravitational influence of the Milky Way. It's not exactly a collision, but it is certainly a gravitational interaction.

And in time, astronomers say, the grand spiral Andromeda Galaxy - another relatively close companion of the Milky Way, is expected to collide and merge with it.

Happily for anyone alive in that time, these collisions are not as violent as it might seem. The stars in a galaxy may be connected by gravity, but they are actually very far apart. In a "collision," virtually all of them will pass by each other without actually crashing.

And the time scales of these events - 10s to 100s of millions of years - are so vast that no individual living thing would see any movement.