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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.

The transit of Venus will be transfixing sky-watchers on June 5 and 6 but astronomers at NASA are now looking ahead to a cosmic event of far greater significance. They say they are certain that the Milky Way and Andromeda Galaxies are headed for a cosmic collision, in about four billion years.
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It's the biggest thing to hit the Milky Way. Ever.

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.

When cosmic do-si-do ends, the result will be an enormous elliptical galaxy that will alter a starry night sky in ways that would make Vincent van Gogh drop his brushes in awe (see video).

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.

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