Our Galaxy to Collide With Andromeda in 4 Billion Years

Astronomers, using data from NASA’s Hubble Space Telescope, have predicted with certainty the titanic collision of our Milky Way galaxy with the neighboring Andromeda.

This illustration shows a stage in the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy. In this image, representing Earth's night sky in 3.75 billion years, Andromeda, left, fills the field of view and begins to distort the Milky Way with tidal pull (NASA / ESA / Z. Levay / R. van der Marel / STScI / T. Hallas / A. Mellinger)

The Milky Way is destined to get a major makeover during the encounter, which is predicted to happen 4 billion years from now. It is likely the Sun will be flung into a new region of our galaxy, but our Earth and solar system are in no danger of being destroyed.

“Our findings are statistically consistent with a head-on collision between the Andromeda galaxy and our Milky Way galaxy,” said Dr Roeland van der Marel of the Space Telescope Science Institute in Baltimore, co-author of three papers that will appear in an upcoming issue of the Astrophysical Journal (paper1, paper2 and paper3).

The solution came through Hubble measurements of the motion of Andromeda galaxy, known as M31. The galaxy is now 2.5 million light-years away, but it is inexorably falling toward the Milky Way under the mutual pull of gravity between the two galaxies and the invisible dark matter that surrounds them both.

“After nearly a century of speculation about the future destiny of Andromeda and our Milky Way, we at last have a clear picture of how events will unfold over the coming billions of years,” said Sangmo Tony Sohn of STScI.

The scenario is like a baseball batter watching an oncoming fastball. Although Andromeda is approaching us more than two thousand times faster, it will take 4 billion years before the strike.

Computer simulations derived from Hubble’s data show that it will take an additional two billion years after the encounter for the interacting galaxies to completely merge under the tug of gravity and reshape into a single elliptical galaxy similar to the kind commonly seen in the local universe.

Although the galaxies will plow into each other, stars inside each galaxy are so far apart that they will not collide with other stars during the encounter. However, the stars will be thrown into different orbits around the new galactic center. Simulations show that our Solar system will probably be tossed much farther from the galactic core than it is today.

The findings also show that M31′s small companion, the Triangulum galaxy, M33, will join in the collision and perhaps later merge with the M31/Milky Way pair. There is a small chance that M33 will hit the Milky Way first.

“In the worst-case-scenario simulation, M31 slams into the Milky Way head-on and the stars are all scattered into different orbits,” said Dr Gurtina Besla of Columbia University in New York. “The stellar populations of both galaxies are jostled, and the Milky Way loses its flattened pancake shape with most of the stars on nearly circular orbits. The galaxies’ cores merge, and the stars settle into randomized orbits to create an elliptical-shaped galaxy.”
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Bibliographic information:

Sangmo Tony Sohn, Jay Anderson, Roeland P. van der Marel. 2012. The M31 Velocity Vector. I. Hubble Space Telescope Proper Motion Measurements. To be published in ApJ. arXiv:1205.6863

Roeland P. van der Marel, Mark Fardal, Gurtina Besla, Rachael L. Beaton, Sangmo Tony Sohn, Jay Anderson, Tom Brown, Puragra Guhathakurta. 2012. The M31 Velocity Vector. II. Radial Orbit Towards the Milky Way and Implied Local Group Mass. To be published in ApJ. ArXiv:1205.6864

Roeland P. van der Marel, Gurtina Besla, T.J. Cox, Sangmo Tony Sohn, Jay Anderson. 2012. The M31 Velocity Vector. III. Future Milky Way-M31-M33 Orbital Evolution, Merging, and Fate of the Sun. To be published in ApJ. ArXiv:1205.6865