The Andromeda galaxy is the nearest large galaxy to our own Milky Way galaxy. Andromeda is approximately two million light-years away. A light-year is the distance that light travels through space in one year. The speed of light is 186,000 miles per second. So, a light-year is the distance that light travels at this speed in one year.
All galaxies consist of dust, stars, planets, and gas held together by gravity (Brecher, 2005). All galaxies rotate very slowly. Astronomers believe that all or most galaxies have a supermassive black hole at their center. Scientists know that both the Milky Way and Andromeda galaxies have supermassive black holes at their center.
The Andromeda galaxy is a spiral galaxy like the Milky Way. All spiral galaxies have a flattened galactic disk in the center with spiral arms that extend outward.
The top image of the Andromeda galaxy is an ultraviolet composite image in false color taken by the Galaxy Evolution Explorer and the Spitzer Space Telescope. The blue color in this image represents young stars. Shown in green are older stars and the yellow center represents a dense population of old stars. The red regions represent areas where new stars are forming. Also see the true color image of the Andromeda galaxy on the right.
Although the Andromeda galaxy is about 2 million light years away, we can still see it during the fall from the northern hemisphere of Earth with the naked eye. It contains hundreds of billions of stars and appears as a blurry white area in the Andromeda constellation.
Andromeda has a diameter of 260,000 light-years, whereas the Milky Way's diameter is only 100,000 light-years. Astronomers believe that Andromeda has about twice the mass that the Milky Way has.
Galaxies not only rotate, but they also move through space. Most galaxies move away from one another, however, sometimes galaxies move toward each other. Sometimes they come so close to one another that they collide. The two spiral galaxies colliding in the image below are located in the Hercules galaxy cluster in the Hercules constellation.
What's the outcome of two galaxies colliding? Sometimes the two galaxies pass right through each other without anything changing much, but this only occurs if the galaxies are both fast-moving. If two slow-moving galaxies collide they will combine to form one very large galaxy (Brecher, 2005).
In fact, the Andromeda galaxy is moving toward the Milky Way. The two galaxies are expected to collide in few billion years or so. Because Andromeda is located slightly more than 2 million light-years away, and the galaxy is traveling much slower than the speed of light, it will take a few billion years before Andromeda reaches the Milky Way.
The result of this collision will be that the two galaxies will combine into one larger galaxy. This merger will take over a million years.
What does this mean for the inhabitants of Earth? First of all, astronomers aren't certain that a collision will take place. It is possible the Andromeda galaxy may miss the Milky Way entirely.
If a collision does take place, given how spread out everything is in a galaxy, it is possible that the stars will not hit us, although I would expect that there would be some changes in store for us.
Scientists believe that the Andromeda galaxy may have already merged with another galaxy in the past.
Brecher, Kenneth. "Galaxy." World Book Online Reference Center. 2005. World Book, Inc. http://www.worldbookonline.com/wb/Article?id=ar215080.
Image of true color Andromeda galaxy: NASA Marshall Space Flight Center (NASA-MSFC).
Image of colliding galaxies and composite color image of Andromeda galaxy: Courtesy of NASA/JPL-Caltech.