Mark your calendars for a rendezvous with the Large Magellanic Cloud.
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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.
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