Smithsonian.com

One hundred years ago, in 1912, astronomer Vesto Slipher of the Lowell Observatory, in Flagstaff, Arizona, attempted to figure out the speed of Andromeda, the closest spiral galaxy to our own Milky Way. As he examined the shift in wavelengths that indicate Andromeda’s motion relative to us, he was surprised by what he found. Unlike nearly every other galaxy, which (we would soon learn) is moving away from us, indicating that the universe is expanding, Andromeda was doing something quite unusual: heading straight for us at a speed of 250,000 miles per hour.

Scientists were unsure what this would mean for our galaxy in the long-term. Would we collide directly with Andromeda, a galaxy roughly the same size as our Milky Way? Or would we slide past it, like two ships passing in the night? Now, as indicated in a paper published last week in the Astrophysical Journal, we know the Milky Way’s ultimate fate: a galactic collision.

“We’ve known for 100 years that Andromeda is getting closer to us, but to really know the trajectory it’s going to take, you need to know its sideways motion,” said Roeland van der Marel, an astronomer at the Space Telescope Science Institute and an author of the study. If Andromeda were moving enough on a sideways trajectory, it could have meant that it would fail to collide with the Milky Way, instead moving laterally past our galaxy.

“What we’ve done now is, for the first time, actually obtained a measurement for the sideways motion, using the Hubble space telescope,” he said, “and it turns out that the Andromeda galaxy is heading straight for us. Previously, this was a well-reasoned conjecture, and now it’s really a demonstrated fact.”

Scientists are now fairly certain that the Milky Way and Andromeda will collide and merge in roughly 4 billion years, as depicted above. Illustration by NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

As shown in the NASA computer animation above, the Milky Way and Andromeda will be slowly drawn together due to their mutual gravitational pull, colliding roughly 4 billion years from now. Subsequently, the two galaxies will orbit around each other before merging in one big galactic pile-up. “On the first passage, they may either hit each other directly, smack on the face, or they may sort of just graze each other,” van der Marel said. “But either way, after that first passage, they get slowed down a lot, and that slowing down leads them to fall back together and merge as one.”

The resulting supergalaxy will be different from either of the current ones: Instead of the elegant, flat, spiral-shaped disc we know and love, the new galaxy will be a three-dimensional ball of stars.

The research team used images captured by the Hubble Space Telescope to determine the exact degree of sideways motion of Andromeda relative to our galaxy. ”To measure the sideways motion, you basically take an image, you wait a couple of years, and then you do it again and look if things have shifted,” van der Marel said. The researchers looked at images of three specific sections of Andromeda, taken either five or seven years apart, and used distant galaxies in the background—from our vantage point, behind Andromeda—as a stationary reference to compare them with.

“In the end, we found that there is a shift, and it was about one hundredth of a pixel on the cameras on Hubble,” he said. In other words, not enough to avert a collision in our distant future.

Astoundingly, this massive crash won’t have an enormous impact on earth, or the solar system as a whole. “Galaxies are mostly empty space, so there are lot of stars in them, but when the galaxies collide, individual stars don’t actually collide like billiard balls,” said van der Marel. “No star from Andromeda will actually directly hit our sun, and in fact, no star from Andromeda will even come close enough to the sun to perturb the orbit of the earth.” Instead, gravitational forces will fling the solar system as a whole outward within the new galaxy, so our night sky will change as we explore a different portion of the universe.

Regardless, the much bigger issue for life on earth is an unrelated long-term problem: the sun will gradually increase in temperature and then run out of nuclear fuel around the time the galaxies finish merging, 6 billion years from now, making the existence of life on this planet virtually impossible.

So, since this galactic collision isn’t something we need to fear, maybe we ought to welcome it. Perhaps we can begin by figuring out a name for our new supergalaxy. The Milkydromeda? The Andro Way? Clearly, suggestions are welcome. We’ve got roughly 4 billion years to figure something out.