By Dennis Overbye
The apocalypse has been postponed. Astronomers have long known that the Andromeda galaxy, aka Messier 31, a swirling city-state of a trillion stars — plus all the accouterments of gas, dust, dark matter and black holes — is rumbling through the cosmos right toward us at 68 miles per second.
Five years ago astronomers using the Hubble Space Telescope calculated that M31 would hit our Milky Way galaxy head on 3.9 billion years from now.
That collision, they said, would initiate a series of do-si-do encounters that would splash streamers of stars and gas across space, and end with the two galaxies merged into a single, supergiant globe of stars.
New data have now revised this forecast. It turns out that the Andromeda galaxy is also going about 20 miles per second sideways. As a result, it will take a more winding path toward us, won’t arrive for another 4.5 billion years and won’t hit so hard, at least not at first, according to a paper in the Astrophysical Journal.
“The earlier results suggested a more head-on collision, and the new results suggest a more glancing blow,” wrote Roland van der Marel, of the Space Telescope Science Institute, and lead author of the paper, in an email.
But the ending will be the same, he said: the merger of both galaxies into a cosmic monstrosity.
So enjoy the extra half-billion years here in the tranquil suburbs of the Milky Way.
This reprieve, if it can be called one, is the latest tidbit in a cornucopia of data from Gaia, a European spacecraft tasked with measuring the precise positions, velocities and other attributes of more than a billion stars in the Milky Way and other nearby galaxies.
The data have provided new insight into the history, dynamics and future of the Local Group, the small cluster of galaxies to which the Milky Way belongs. Joining us is the Andromeda galaxy, a slightly larger twin of the Milky Way, about 2.5 million light years distant, and, slightly farther away, a smaller spiral in the Triangulum constellation called M33. Other members of the group include a few dozen dwarf galaxies such as the Large and Small Magellanic clouds — puffs of light visible in the Southern hemisphere.
By precisely measuring the motions of stars in M31 and M33, van der Marel and his colleagues were able to measure the sidelong trajectories of those two galaxies for the first time, and determine that Andromeda is not coming straight at us. Instead it will sideswipe our galaxy, like an out-of-control driver, 4.5 billion years from now.
That event will be less dramatic than it sounds, however. Because galaxies are mostly empty space, they will pass through each other like ghosts. The chances of stars or planets actually colliding are the inverse of astronomical, astronomers say. But gravity will disrupt the stars and strew them across space in vast, spectacular ribbons. Eventually the stars will collect themselves into a giant elliptical galaxy.
The supermassive black holes that anchor the core of each galaxy will find each other and slowly circle inward. In the end they will collide, producing one of those space-quivering explosions of gravitational waves detected by the LIGO antennas.
The data also allowed the astronomers to refine their knowledge of the motions of M33. That galaxy, they concluded, is still on its first trip into the center of the Local Group from farther out in space. Eventually it will enter a wide orbit around the merged galaxies, until, slowed by friction, it spirals into the center and joins the crowd.
“But this will take a long time after the elliptical galaxy has formed,” van der Marel said. “Billions of years.”
You might ask what the view from Earth will be like by then. If our world exists at all in that far future, it will be a cinder: Long before the galaxies collide, the dying sun will billow into a red giant and roast it.