Astronomers have long known that the Andromeda Galaxy is headed our way. Now they've concluded that it most likely will collide with the Milky Way head-on — with dramatic consequences.
For rock-'em, sock'em cosmic action, you can't beat the collision of two galaxies. It's a scene that has played out countless times since the beginning of the universe. Our own Milky Way, for example, isn't the serene, stately pinwheel it appears to be. Over the eons it has gobbled up many close associates. In fact, right now the Milky Way is devouring a small system called the Sagittarius Dwarf Elliptical Galaxy, or SagDEG for short.
But it turns out that these skirmishes are merely warm-ups for the upcoming main event: a head-on collision of the Milky Way with the equally massive Andromeda Galaxy (Messier 31) in about 4 billion years.
Astronomers have long known that M31 is relatively nearby, just 2½ million light-years distant, and moving toward us at about 250,000 miles (400,000 km) per hour. That's because both of these massive spiral systems are members of the Local Group, a gravitationally bound collection of three biggies (these two and M33), along with about 50 much smaller dwarf galaxies that include the two Magellanic Clouds. I liken the large trio as "anchor stores" in a medium-size intergalactic mall.
So is the Andromeda Galaxy destined to collide with the Milky Way? No one could say for sure because the answer depended on side-to-side movement across the sky, what astronomers term proper motion. A lateral shift of 60 miles (100 km) per second one way or the other makes all the difference between a near miss, glancing blow, or head-on collision. Yet this amount of proper motion corresponds to a positional change of just 27 millionths of an arcsecond per year — far beyond the ability of ground-based telescope's to measure, even over decades-long baselines.
Difficult — but not impossible — for the Hubble Space Telescope and the unprecedented positional precision made possible by its latest-generation instruments.
A series of three papers, submitted to the Astrophysical Journal last week, reveals how astronomers finally puzzled it all out. First, a trio led by Sangmo Tony Sohn (Space Telescope Science Institute) used dedicated HST observations over several years to measure proper motions in three locations within M31. The average of these turns out to be about 125 miles (200 km) per second. In the second paper, a team led by Roeland van der Marel (also at STScI) modeled the total mass of the Local Group, to see what other gravitational influences might be in play.
Finally, van der Marel and four colleagues simulated the possible outcomes when these titans of the Local Group finally meet. "Most likely, the Milky Way and M31 will merge first," they write, with a 41% chance that they'll strike head-on (their centers no farther than 80,000 light-years apart). In that case, M33 will settle into an orbit around them. However, they add that there's a small chance (9%) that M33 will strike the Milky Way first — or it might get flung from the Local Group altogether (7%).
The first round of this intergalactic smashup won't happen for another 4 billion years, and another 2 billion years will pass before the two (or three) systems merge into a single giant elliptical galaxy. Although our Sun will still be around then, still a billion years shy of its red-giant phase, life on Earth (as we know it) will not.
An STScI press release notes that the hundreds of billions of stars inside each galaxy will be jostled into new orbits during the encounter but won't collide with each other — there's just too much empty space between them. The simulations also suggest that our solar system will likely end up much farther from the galactic core than it is today.