July 13, 2012

For nearly 100 years, scientists have known that the Andromeda Galaxy, our nearest galactic neighbor, was heading our way. But they didn’t know whether an encounter would be a glancing blow or a head-on collision or if Andromeda would miss our Milky Way altogether. Now, thanks to measurements made using the Hubble Space Telescope, scientists with the Space Telescope Science Institute in Baltimore have calculated that Andromeda, which is hurtling toward us at a speed of about 250,000 miles (400,000 kilometers) per hour, will plow directly into the Milky Way in about 4 billion years. The encounter will produce some dramatic changes, the scientists said. But the destruction of the sun and the solar system will not be one of them.

The Milky Way, our "home" galaxy, is a spiral galaxy. Astronomers believe that after neighboring galaxy Andromeda--also a spiral galaxy--smashes into the Milky Way, the two will form an elliptical galaxy. (Artwork © Jon Lomberg and the National Air and Space Museum)

As Andromeda gets closer, it will fill more and more of our night sky. Eventually, the two spiral galaxies will begin to merge. A blaze of new stars will appear in the sky as clouds of dust and gas are compressed by the gravitational forces tearing at the galaxies. Stars within galaxies are so far apart that the sun or planets will not collide with other space bodies. But scientists think the solar system will be flung into a different part of the Milky Way, probably even farther from the galactic core than it is today. Over the next 2 billion years, the two spiral galaxies will combine to form an elliptical galaxy that some scientists are calling “Milkomeda.”

Earth and the sun will probably not be around to witness the final product, however. Scientists have estimated that about 5 billion years from now, the sun will have used up its hydrogen fuel. Eventually, it will expand enormously, probably nearly to the current orbit of Mercury, and swallow Earth.

Additional World Book articles:

•  Galaxy
• The Formation of Galaxies and Other Structures  (a Special Report)