Galactic slam: Milky Way to hit nearby galaxy – in 4 billion years (+video)
The collision between the Milky Way and the nearby Andromeda galaxy will result in a single, enormous new elliptical galaxy, but not for another 6 billion years, researchers say.
It's the biggest thing to hit the Milky Way. Ever.Skip to next paragraph
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In four billion years, the Andromeda Galaxy will plow through the Milky Way head on in a collision of cosmic proportions – not once, not twice, but three times over about a 2 billion year span.
The edge-on view of the Milky Way now visible on a clear night under dark skies would first yield to a dazzling array of large, bright regions of star formation all across the sky. Ultimately, the sky would be dominated by a giant brightly glowing ellipse of stars – the core of the new galaxy that our solar system would inhabit at a much greater distance from the center than it now does.
Those are the implications of study that for the first time firmly answers a question that has vexed astrophysicists for a century. Ever since the first measurements of the motions of galaxies were taken, the data showed that unlike virtually every other galaxy observed, Andromeda is heading toward the Milky Way, not away from it, as the universe expands. That led to the inevitable questions: Will Andromeda hit the Milky Way? And what will happen if it does?
Observations of other galaxy collisions over the past 100 years have allowed researchers to answer the second question before they had a clear answer to the first one. Now, a team led by Roeland van der Marel at the Space Telescope Science Institute in Baltimore reports that they've made the key, missing measurement – Andromeda's sideways motion – with enough confidence to call the head-on collision 4 billion years in advance.
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.
An old solution to a new problem
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.