It has been almost 100 years since Edwin Hubble measured the universe beyond the Milky Way Galaxy. Today, astronomers believe that as many as 100 billion other galaxies are sharing the cosmos. Most of these cosmic islands are classified by shape as either spiral or elliptical, but stargazing scientists have discovered galaxies that don't quite fit these molds.
Common to this "irregular" category are galaxies that interact with other galaxies. These gravitational interactions are often referred to as mergers, and their existence invites the question: Is the Milky Way collision-prone? To evaluate the probability, look to the Andromeda Galaxy. Located more than 2.5 million light-years away, Andromeda appears as a small fuzzy patch in the sky. However, there is nothing miniature about it. Similar to the shape (spiral), size and mass of the Milky Way, Andromeda is home to a trillion other stars.
Astronomers have known for decades that our galactic neighbor is rapidly closing in on us -- at approximately 250,000 miles per hour. They know this because of blueshift, a measured decrease in electromagnetic wavelength caused by the motion of a light-emitting source, in this case Andromeda, as it moves closer to the observer.
Recently, data collected from the Hubble Space Telescope has allowed astronomers to predict a merger with certainty, in 4 billion years. Our sun will still be shining, and Earth will most likely survive the impact. Reason being, galaxies, although single units of stars gravitationally tied together, are mostly gigantic voids. One can compare a galaxy-on-galaxy collision to the pouring of one glass of water into another. The end result is a larger collection of water, or in the case of a cosmic collision, a larger galaxy. Future Earth inhabitants, billions of years from now, could look up and observe only small portions of such an event because it will take 2 billion years for these cosmic islands to become one.