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Cosmic Collision: The Future of Our Galaxy

An artistic representation illustrates a future scenario in which the Milky Way galaxy merges with the neighboring Andromeda galaxy. In approximately 3.75 billion years, this event may be visible in Earth’s night sky. — Reuters/File

The Milky Way and the Andromeda galaxy are barreling toward one another at a staggering speed of about 250,000 miles per hour (400,000 kilometers per hour), raising the possibility of a galactic collision that could drastically alter both galaxies.

But how likely is such a cosmic crash? While earlier studies estimated a collision could happen in roughly 4 to 4.5 billion years, new research incorporating recent observations and additional variables suggests that the merger is anything but guaranteed.

This study places the chances of a collision in the next 5 billion years at less than 2%, and about 50% in the next 10 billion years.

Unlike the chaotic wreckage of a demolition derby, galactic mergers involve a more complex merging process on a monumental scale. "If a collision occurs, it would spell the end for both the Milky Way and Andromeda," explained Till Sawala, an astrophysicist at the University of Helsinki and lead author of the study published in Nature Astronomy on Monday. The merger would disrupt their structures and result in the formation of a new, elliptical galaxy.

"If a merger happens, it is more likely to occur 7 to 8 billion years into the future. However, our current data do not allow us to predict the timing of such an event, if it occurs at all," added Sawala.

Right now, the two galaxies are about 2.5 billion light-years apart. A light-year is the distance light travels in one year, which is about 5.9 trillion miles (9.5 trillion kilometers).

The anticipated collision is so distant that Earth by that time is expected to be a dramatically different environment. Scientists predict our planet will become uninhabitable in about a billion years, as the sun heats up enough to boil away our oceans.

The sun is just one of the billions of stars in the Milky Way. The total mass of our galaxy—including its stars, interstellar gas, and dark matter (an invisible substance revealed through its gravitational effects)—is estimated to be roughly one trillion times that of the sun.

Andromeda’s mass and shape mirror those of the Milky Way.

To simulate the Milky Way’s trajectory over the next 10 billion years, researchers used updated data from the Gaia and Hubble space telescopes, along with various ground-based telescopes and revised mass estimates.

Nearby galaxies will also influence the probability of a collision. Previous studies accounted for the gravitational pull of the Triangulum galaxy, also known as Messier 33 or M33. However, this latest research also included the Large Magellanic Cloud, a smaller satellite galaxy of the Milky Way. Sawala noted, "We found that while adding M33 increases the chances of a Milky Way-Andromeda merger, including the Large Magellanic Cloud actually decreases those chances."

Interestingly, researchers concluded that a merger between the Milky Way and the Large Magellanic Cloud is nearly certain to occur within the next 2 billion years, well before any potential collision with Andromeda.

One significant distinction between the two galaxies is the mass of their central supermassive black holes. The Milky Way’s Sagittarius A* weighs about 4 million times that of the sun, while Andromeda’s supermassive black hole is about 100 million times the sun’s mass.

"While direct collisions between stars are exceedingly rare, the two supermassive black holes would likely drift toward the center of the newly formed galaxy and eventually merge," remarked Sawala.

Galactic mergers have been part of cosmic history since the universe’s early days and are especially common in regions where galaxies are clustered together. Sawala stated, "In the early universe, mergers were much more frequent, occurring shortly after the formation of the first galaxies. Minor mergers—those involving smaller galaxies—are also occurring more regularly. In fact, the Milky Way is currently merging with several dwarf galaxies."