There's a 50/50 Chance the Milky Way and Andromeda Galaxy Will Merge
Though about 2.5 million light-years currently lie between the Milky Way and Andromeda, the two galaxies are creeping closer to each other. In 1913, astronomer Vesto Melvin Slipher noticed via Arizona's Lowell Observatory that Andromeda (then known as the Andromeda Nebula) was approaching the Milky Way at 186 miles per second. Since then, researchers have not only verified Slipher's math but also found via multiple simulations that Andromeda will someday combine with the Milky Way. One paper from 2021 even proposes that the two galaxies will meet 4.3 billion years from now, with a complete merger taking another 6 billion years after that.
But these simulations failed to account for one small yet mighty factor: the Large Magellanic Cloud. Roughly 160,000 light-years from our Milky Way, this dwarf galaxy has long been considered an insignificant part of the so-called Local Group. But in 2015, the beginning of the Survey of the MAgellanic Stellar History, or SMASH, found that the Large Magellanic Cloud was larger and more complex than initially thought. Astronomers have spent the years since sifting through SMASH data for dwarf galaxy secrets.
Illustration of a hypothetical merger between the Milky Way and Andromeda Galaxy. Credit: NASA, ESA, STScI, DSS, Till Sawala (University of Helsinki); Image Processing: Joseph DePasquale (STScI)
It's for this reason that the latest Milky Way-Andromeda merger simulation actually includes the Large Magellanic Cloud. To cover for every possible uncertainty, an international team of astronomers ran their simulation nearly 100,000 times and found that just under 50% of the time, the Milky Way and Andromeda collided and merged. Alternately dropping different nearby galaxies showed that Messier 33 (the third largest galaxy in the Local Group) made a merger more likely, while the Large Magellanic Cloud reduced the odds of a convergence. That's because the Large Magellanic Cloud pulls the Milky Way out of Andromeda's path, as a comic book hero would pull a civilian off some train tracks.
The Large Magellanic Cloud might only get to bask in its glory for a few hundred million years, however. The researchers' simulation showed that the Milky Way will almost certainly collide with the Large Magellanic Cloud in 2 billion years, disappearing the latter galaxy.
As observatories gather more data about the universe—and scientists' models inevitably become more advanced—we'll find out whether the Large Magellanic Cloud really will swoop in to save the day. Of course, we won't see the benefit either way. But it will be nice to know whether our galactic home will continue to exist after we're gone.
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