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.
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.
Simulations the team conducted show the two galaxies passing thorough one another and continuing their travel, but only for awhile. Gravity brings them back together for a second collision about 1.3 billion years after the first. They pass through each other again, but only cover a relatively short distance before gravity brings them back together in their final merger some 100 million years after the second encounter (see video).
Remaking the stellar neighborhood
When Andromeda hits home, the likelihood of any two stars from each galaxy colliding are vanishingly small, van der Marel says. The distances between stars are just too great and stars are so small.
The sun, for instance, is about 863,000 miles across. The distance between the sun and its nearest neighbor, Proxima Centauri, is 4.2 light-years away – about 25 trillion miles, or 29 million suns, away, give or take a few burning balls of gas.
But the gravitational interactions between the two galaxies will lead to some major renovations.
For instance, simulations suggest our solar system would remain intact, but it most likely would orbit the new galactic center at a much greater distance that it does today. Today, the sun orbits some 27,000 light-years from the Milky Way's core, notes Gutina Besla, a researcher at Columbia University in New York who ran the computer simulations of the collision's effects as a member of the research team. After the big crunch, the sun's orbit would expand to 160,000 light-years, about the distance to the Large Magellanic Cloud, a satellite galaxy to the Milky Way.
Both galaxies also harbor supermassive black holes at their centers, though neither is "active" – spewing high-energy electrons, positrons, and protons from its poles as it ingests large amounts of gas. That suggests neither galaxy has enough gas at its center to feed the black holes to keep them persistently energized. When the two galaxies merge, the black holes would be expected to merge as well, with some fireworks associated with the merger. But the resulting supermassive black hole still would lack enough nearby gas to significantly ramp up the radiation seen from galactic cores where a supermassive black hole is gorging on gas, the team estimates.
Still, the galaxies' collision would trigger bursts of star formation as otherwise amorphous clouds of interstellar gas collide and form clumps of gas dense enough to begin to collapse under their own gravity. Clumping also would occur from the gravitational disturbances each galaxy triggers within the other as they interact.
As if to add insult to injury, Andromeda is tugging its own satellite galaxy along with it: The Triangulum galaxy. It has about 10 percent of Andromeda's mass, and the team gives it a 9 percent chance that it may reach us before Andromeda does.
"It won't have as much of an impact," van der Marel says. "But we may actually get a one-two punch."
Moreover, the simulations suggest a 20 percent chance that within the next 10 billion years, what's left of our sun – which will have gone through its own death throes by then – will travel for part of its orbit through the Triangulum galaxy, or what's left of it.
Minor mergers between large galaxies and their satellites "are quite common," says Johns Hopkins's Dr. Wyse. "The major mergers – those between galaxies of approximately equal mass – are much more profound" for each system. These new results give "unique insights into how typical large galaxies evolve."
"We've known for awhile that disk galaxies evolve by snacking on small satellite galaxies," Wyse says "The appetite for both the Milky Way and M31 has not been satisfied and in fact they're going to end up devouring each other."