Thanks to photographs from space telescopes and computer modeling, astronomers are able to search for and observe the existence of both galactic and stellar collisions. Scientists originally believed these types of space collisions, also known as mergers, to be fairly rare, but research in the early 21st century has found them to be fairly common. As experts understood more about the beginning of the universe and the Big Bang theory, they realized that galactic collisions were even more common in the early stages of time. Because the universe was much smaller, galaxies were huddled closer together, and, shooting out from the origin of the Big Bang, were likely to collide into others during their journey throughout space. Even our own galaxy, the Milky Way, carries with it debris from early collisions with other massive bodies, and astronomers expect the Andromeda galaxy, our nearest large neighbor, to swallow us up sometime in the distant future.
A space collision might sound like perfect material for an expensive Hollywood summer blockbuster, but watching one take place would actually be much less exciting than you'd think. Even though galaxies and stars move toward each other at hundreds of miles per hour, their mergers can take millions of years to form. Instead of exploding like massive bombs, space collisions act like smooth, undefined balls of gas. Once two stellar bodies meet, the massive gravity of each one will distort the shape of the other, usually resulting in a droplet shape. On April 24, 2008, for instance, the Hubble Space Telescope captured images of Arp 148, the aftermath of two galaxies colliding. While one galaxy took on the typical ring shape, its neighboring galaxy was stretched thin like a tail.
One common type of collision is between two neutron stars. Neutron stars are actually corpses of old stars -- when a star reaches the end of its life, it explodes, and a mass equivalent to the amount found in our sun condenses into an area the size of a city. When two are created in close proximity, they form what's called a binary pair and orbit each other, eventually merging after hundreds of millions of years. The combined masses of the dead stars are so heavy the event creates a black hole in space, and split second flashes of light brighter than a billion suns give off huge magnetic fields. Gravitational waves from a near neutron star pair could have the effect of displacing the oceans by about 10 times the diameter of an atomic nucleus -- a seemingly small amount, but quite large if we're talking about all the water in the ocean. Although there are only six known pairs of neutron stars on a path for collision, scientists believe there are many more out in space and that these types of mergers could happen as frequently as once or twice a year.
What about space collisions on a much smaller scale, such as one between an asteroid and the Earth? To read about asteroid impacts and the possibility of life surviving, see the next page.