For years, astronomers have predicted a dramatic fate for our galaxy: a head-on collision with Andromeda, our nearest large galactic neighbor.
The idea that the Milky Way and Andromeda are on a collision course goes back more than a century.
Earlier studies often assumed Andromeda’s transverse motion was small, making a future head-on collision seem almost certain.
M33 nudges Andromeda slightly toward the Milky Way, increasing the chance of a merger.
Meanwhile, the Large Magellanic Cloud shifts the Milky Way’s motion away from Andromeda, reducing the likelihood of a collision.
Astronomers have long foreseen a catastrophic outcome for our galaxy: a direct collision with Andromeda, our closest large galactic neighbor. This merger, which is anticipated to occur in roughly five billion years, is now frequently featured in popular science writing, astronomy documentaries, and textbooks.
But according to our recent research, which was led by Till Sawala of the University of Helsinki and published in Nature Astronomy, the future of the Milky Way may not be as clear-cut as previously thought.
We determined that the likelihood of the Milky Way and Andromeda merging within the next 10 billion years is only roughly 50% after carefully adjusting for uncertainties in current measurements and taking into account the gravitational pull of other nearby galaxies.
We believed a collision was inevitable, but why?
For over a century, people have believed that Andromeda and the Milky Way are headed for collision. By utilizing the Doppler shift, a slight change in the color of Andromeda’s light, to measure its radial velocity—its motion along our line of sight—astronomers were able to determine that the planet is approaching us.
Proper motion, also referred to as transverse velocity, is the movement that galaxies undergo as they move sideways across the sky. It is extremely challenging to detect this sideways motion, particularly for galaxies that are millions of light years away.
A future head-on collision appeared all but certain because earlier research frequently assumed Andromeda’s transverse motion was minimal.
What makes this study unique?
There was no new information in our study. Instead, we reexamined previously published observations from the Gaia mission and the Hubble Space Telescope.
In contrast to previous studies, our work does not assume the most likely values of these measurements, but instead takes into account their uncertainty.
We simulated thousands of potential Andromeda and Milky Way trajectories, each with a slightly different set of assumed initial conditions, such as the two galaxies’ positions and speeds.
The same results were obtained when we began with the same presumptions as the previous studies. But we were also able to investigate a wider variety of options.
Additionally, we included two other galaxies that affect the Milky Way and Andromeda’s future paths: M33, also called the Triangulum galaxy, which orbits Andromeda, and the Large Magellanic Cloud, a massive satellite galaxy that is currently falling into the Milky Way.
The motions of their hosts are altered by the gravitational pull of these companion galaxies.
The likelihood of a merger is increased when M33 moves Andromeda closer to the Milky Way. The likelihood of a collision is decreased as the Milky Way’s motion is shifted away from Andromeda by the Large Magellanic Cloud.
In light of all of this, we discovered that the Milky Way and Andromeda do not merge at all during the next 10 billion years in roughly half of the simulated scenarios.
If they collide, or do not, what will happen?
It is unlikely that a merger will have disastrous effects on Earth, even if it does occur. Direct collisions are uncommon in galaxies because of the vast distances between their stars.
But eventually, the galaxies would come together due to gravity to form a single, bigger galaxy, most likely an elliptical one instead of the spirals we see today.
As close companions that never quite collide, the galaxies might settle into a long, slow orbit around one another if they don’t merge. Even though the result is kinder, it nevertheless changes how we perceive the Milky Way’s far-off future.
What follows?
The transverse velocity of Andromeda is the largest unknown that still exists. A merger or a near-miss can depend on even minor adjustments to this sideways motion. We’ll get closer to a more precise answer with the help of future measurements that help refine this value.
We still don’t know for sure what the future holds for our own galaxy. However, investigating these options demonstrates how much, even locally, we still don’t know about the cosmos.
Under a Creative Commons license, this article has been republished from The Conversation. Go ahead and read the original article.
