You might have heard that the Andromeda galaxy is heading toward our Milky Way galaxy, set to collide in a few billion years. But here’s the exciting part: this galactic collision has already started! For the first time, astronomers have found that the two galaxies are already exchanging stars, with Andromeda’s halo overlapping with the Milky Way’s.
Why is Andromeda Moving Toward Us?
Despite the universe expanding, which makes most galaxies move away from each other, Andromeda is moving toward the Milky Way. This is due to the strong gravitational pull between the two galaxies, which is more powerful than the universe’s expansion effect over such short distances.
Slow Motion Collision
When we think of collisions, we imagine sudden impacts. However, galactic collisions are different. They unfold over hundreds of millions of years. For instance, the Antennae Galaxies, located about 70 million light-years away in the constellation Corvus, have been merging for at least 100 million years, creating spectacular star tails due to tidal forces at play during their interaction.
Evidence of the Merger
Hypervelocity Stars: The Cosmic Sprinters
One of the first pieces of evidence that Andromeda and the Milky Way have started colliding comes from hypervelocity stars. These stars are like cosmic sprinters, moving so fast that they can escape the gravitational pull of their home galaxies. Imagine throwing a ball so hard that it flies off into the sky, never to return. Hypervelocity stars do something similar, zooming away at speeds of around 1,000 kilometers per second.
These stars achieve such high speeds mainly by interacting with supermassive black holes, which can sling them out of the galaxy at incredible velocities. Some hypervelocity stars in the Milky Way might be from Andromeda. To understand their origin, astronomers ran simulations and found that a small percentage of these stars could indeed reach the Milky Way, hinting that the star exchange between the galaxies had begun.
Galactic Halos: Touching Across Space
Another significant piece of evidence comes from the galactic halos. A galactic halo is a vast region of gas and dark matter surrounding a galaxy. Andromeda’s halo already overlaps with the Milky Way’s. By observing light from distant quasars behind Andromeda, astronomers mapped this halo and found it extends 1.3 million light-years towards us. This indicates that the halos of the two galaxies are interacting, signaling the beginning of their merger.
What Happens Next?
The collision between Andromeda and the Milky Way will be a long, drawn-out process, starting with initial encounters and eventually leading to a full merger:
- First Encounter: In about 3.75 billion years, Andromeda and the Milky Way will begin their initial collision. This close encounter will trigger bursts of star formation and significantly disrupt both galaxies’ structures. However, they will move apart after this first encounter.
- Second Encounter: About 5 billion years from now, the galaxies will come back together for a more dramatic interaction, leading to the merging of their central supermassive black holes. This will result in even more star formation and possibly the ejection of stars from the system.
- Final Merger: Over the next billion years, the galaxies will continue to pass through each other, gradually losing energy through dynamic friction until they finally merge into a single, massive elliptical galaxy. This final merger will mark the end of the Milky Way as we know it. By this time, the solar system may have been flung into a distant part of the new galaxy or even ejected into intergalactic space.
This grand cosmic event will reshape our galaxy’s structure and create a spectacular new galaxy. While it may sound alarming, this process is so slow that it won’t affect our solar system for billions of years. So, no need to pack your bags for another galaxy just yet!
