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| © ESO / S. Brunier The Milky Way above a space observatory in Chile |
Ah, the Milky Way, our glittering home in the cosmos. Seen in an unencumbered night sky, far from the glare of city lights, it seems magnificent and eternal in its enormity. Nothing could shift this ancient web of stars, nothing could disturb its transcendent stoicism.
Except, that is, another galaxy. Galaxies orbit millions of light-years apart, but gravity, the immutable magnet of the cosmos, can pull them together, producing spectacular collisions that reshuffle stars millions of years. According to the leading theory, the Milky Way will collide with one of its closest neighbours, Andromeda, sometime between 6 billion and 8 billion years from now.
But the Milky Way may face another galactic threat before that, from a different neighbour. A new study predicts our galaxy will collide with a galaxy called the Large Magellanic Cloud between 1 billion and 4 billion years from now.
This is a rather surprising change in schedule, considering that the Large Magellanic Cloud, which is close enough to be seen with the naked eye, is currently moving away from the Milky Way. What gives?
Marius Cautun, an astrophysicist at Durham University’s Institute for Computational Cosmology, says that recent observations of the Large Magellanic Cloud have revealed that the galaxy has more mass than previously thought. Cautun and his fellow researchers decided to run computer simulations that took this new factor into account and fast-forwarded the conditions of our cosmic neighbourhood. They tested multiple scenarios, making adjustments in mass, velocity, and other measures. In the end, the simulations predicted that in several hundred million years, the Large Magellanic Cloud will turn around and head straight for the centre of the Milky Way.
“The collision between our galaxy and the [Large Magellanic Cloud] takes place in the majority of cases—over 93 percent,” Cautun says.
The collision would be a slow showdown, unfolding over the course of billions of years. Stars from the Large Magellanic Cloud would ricochet like pinballs, dislodging some of the Milky Way’s stars from their orbits. Our galaxy as a whole would survive, but some stars may be flung right out of the Milky Way, Cautun says.
Meanwhile, the sleeping, supermassive black hole at the centre of the Milky Way would wake up. Like volcanoes, black holes alternate between peaceful dormancy and ferocious activity, depending on the surrounding conditions. Ours is in a quiet period. But the chaos of the merger would send cosmic gas swirling toward it, and cosmic gas is dinner to black holes. The resulting feast is a spectacular show. A disk of luminous, hot cosmic material swirls around the black hole at great speed, and bursts of high-energy radiation erupt from its centre. Cautun says one serving of a Large Magellanic Cloud could lead our black hole to gobble up enough material to grow 10 times its current size.
And what would happen to us, if there is any kind of “us”—life in some form—on Earth when this all goes down?
It is possible that our sun could be among the small fraction of stars that gets lobbed from the galaxy. The jostling would disturb the orbits of our solar system’s planets, which could be perilous for any inhabitants. Even a small change in the relationship between the sun and the Earth could knock it out of the region where liquid water (and, therefore, life) can exist.
If life on Earth survived, though, it would take ages for anyone to realize the planet’s position in the cosmos has shifted. Like the merger, the solar system’s ejection would occur over such a large timescale that it’d be almost meaningless to humans. “Only at the end of the collision could our descendants tell if we have been kicked out of our galaxy,” Cautun says.
The change in scenery would be remarkable. In this scenario, “our descendants will see a very different night sky, much darker than currently, with only a modest bright patch that will correspond to the Milky Way galaxy,” Cautun says. “It will be tremendously more difficult for our descendants to travel to other stars—if they haven’t yet done so by that time.”
If this imagined future scares you, consider that a collision with Andromeda would be much worse. The Milky Way would easily devour the smaller Large Magellanic Cloud and maintain its signature spiral shape, even if its insides will be all jumbled. Andromeda, on the other hand, is about the same size as the Milky Way. Astronomers expect that mashup to be destructive, and the Milky Way as we know it—the neat, shimmering band of stars—is unlikely to survive.
Cautun says that a collision between the Milky Way and the Large Magellanic Cloud would shift our galaxy’s position in space. Even still, Andromeda will still come for it, however many billions of years later.
“Ultimately, there is no escape,” he says.
