Contact: Janet Wong
University of Toronto

The gigantic clouds of gas and matter that pelted the Milky Way in its infancy are mere fender-benders compared to the catastrophic collision set to occur with the Andromeda galaxy in several billion years – and one University of Toronto astrophysicist has mapped the fallout.

“We’re on a collision course right now,” says John Dubinski, professor of astronomy at U of T and the Canadian Institute for Theoretical Astrophysics, who led the project with co-author Lars Hernquist of the Harvard-Smithsonian Center for Astrophysics. “Within three billion years, the Milky Way will be swallowed up and merged with the Andromeda galaxy.”

The 2.2-million-light-year gap between the Milky Way and Andromeda is closing at about 500,000 kilometres an hour, he explains. That pace will quicken as the two galaxies near each other.

According to Dubinski, merging galaxies are not uncommon. In fact, this type of interaction plays a key role in helping build larger galaxies and structures in the universe. While mergers of galaxies are less frequent now than in the early days of the universe, it is still an ongoing process, and one in which our own Milky Way and its big sister, the Andromeda galaxy, are active participants, he notes.

Dubinski simulated this Milky Way-Andromeda interaction by following the motion of more than 100 million stars and dark matter particles as the gravitational forces of the two galaxies force them to collide. The simulation was a feat of parallel computing that took four days to complete on the San Diego Supercomputing Center’s 1152-processor IBM SP3 “Blue Horizon” – one of a new class of supercomputers that can perform more than one trillion arithmetic operations per second. In the end, the simulation required the equivalent of three years of continuous operation on a single workstation. The result is a high-resolution computer animation of the collision and merger of the two galaxies from start to finish and some very detailed snapshots of the structure and dynamics of a galaxy merger.

“We just used the Milky Way and Andromeda galaxies as a test case,” says Dubinski. “It’s the first time we’ve been able to develop a full picture of tens of millions of stars in two separate galaxies merging and interacting. The power of these new machines will allow us to improve the dynamic range and reliability of our simulations of galaxies and large-scale structures in the universe.”

Perhaps even more intriguing is the fact that life on Earth – whatever it may be – will probably live through and witness the entire merger over the billion-year dance of the two galaxies, he says. The reason is that the expected lifetime of our sun is projected to last another five billion years. Plus, the likelihood of stars and planets slamming into each other is very low because the distance between them is so vast. The interaction will be “collision-less,” with the most significant effect involving huge gravitational distortions of the systems as they coalesce.

At some point three billion years hence, the night sky will be completely filled by the approaching Andromeda galaxy and when the two galaxies intersect there will be two bands of light arching overhead – looking like two Milky Ways, says Dubinski. With the merger, two possible fates await the sun and Earth – we could be flung into the depths of intergalactic space and escape the galaxy forever or hurled into the centre of the merging pair where new stars will be formed.

And for those on Earth, it will be a spectacular display of galactic fireworks, he says. Massive stars near the sun will be exploding as supernovae at such a rate that the night sky will be bright enough to read a newspaper.


For MPEG movie of the Andromeda-Milky Way encounter: (6.2 MB file)

Prof. John Dubinski
U of T Department of Astronomy
(416) 978-8494

Janet Wong
U of T Public Affairs
(416) 978-6974