The event “will indeed be unprecedented” in the history of the two galaxies, notes Rosemary Wyse, a researcher who specializes in galaxy evolution at the Johns Hopkins University in Baltimore, who was not a member of the research team.
Andromeda, also known by its astronomical designation M31, is a Milky Way-size galaxy some 2.5 million light-years away. On a clear night under dark skies, its nucleus is visible to the naked eye as a small, glowing fuzz ball.
Using the Hubble Space Telescope, the team observed the galaxy's movement, using a technique that allowed stargazers in ancient civilizations to distinguish planets from stars in our own solar system. They measured the relative motion of objects in the foreground when seen against apparently fixed objects farther away.
During two observing campaigns in 2007 and 2009, the team gathered high-resolution images of stars in Andromeda's halo and looked for changes in their positions compared with galaxies so far away they they displayed no motion from one period to the next. The two galaxies are approaching each other at about 250,000 miles an hour. The team's measurements indicated that Andromeda was sidling sideways at between one-quarter to one-third of the two galaxies' closing speed, explains Dr. van der Marel.
The kicker: The closing speed increases with time because each galaxy's gravitational tug on the other grows stronger as they two draw closer. By the time of the first encounter about 4 billion years from now, the two will have reached a combined collision speed of roughly 1.25 million miles an hour. This means that an already small sideways motion, which doesn't change change with time, becomes increasingly insignificant compared with the rising collision speed.