The Romans knew our Galaxy as the Via Lactea or “road of milk”. From its gauzy white haze in the sky, it does certainly seem like a fresh, bright line of milk spilled across the speckled dark. Greek myths told the story of Hera pushing away an infant Heracles while she was breastfeeding, spilling drops of her milk that became the array of galaxies we see today. This image of our galaxy (indeed even the word “gala” in Greek means milk) has persisted since the existence of mankind. Whether it be the luminous stripe across the sky or whether it be the flat spiral we visualize looming in deep space, there is a certain image of our home galaxy that we have known for hundreds of thousands of years. And yet, in the universe, nothing is forever.
On moonless nights the Andromeda galaxy is bright enough to see with the naked eye, a sister galaxy to the Milky Way with a familiar spiral structure and over a trillion stars — twice that of our own galaxy. It was thought to be a nebula when it was first spotted. In the 1700’s it was described as an “island universe” with a pool of stars separate from our own. We now know it’s actually the largest galaxy of our local group, followed by the Milky Way and the Triangulum Galaxy. It is these three bodies which are set to collide in the next few billion years, forever erasing the image of the Milky Way we’ve come to know and creating a truly mesmerizing spectacle for anyone lucky enough to see it.
Although Andromeda is 2.5 million light years away, it’s heading towards us at a rate of 250,000 miles (402,000 km) per hour. It’s the proximity of these two galaxies which caused them to attract while all other galaxies in our local group speed away, receding from view as the universe continues to expand. This phenomenon was first noticed in the 1900’s when a scientist studied light waves from Andromeda arriving here on our planet. When objects move away, their light waves become stretched and red-shifted. But light waves from Andromeda were blue-shifted, meaning that they were compressing as the galaxy came closer.
And though we’ve known for over a century that Andromeda was coming towards us, it hasn’t always been clear whether or not a collision would occur. This is because rather than moving in a straight line, Andromeda has an amount of sideways velocity as well, meaning that it could veer off to one side of the Milky Way and never collide with us at all. Sideways motion of faraway objects is incredibly difficult to discern, sometimes taking as many as hundreds of years to calculate. Astronomers had to use very detailed telescopes that could help map Andromeda’s position relative to the background stars. This is such a subtle, such a delicate observation to capture. In the end they concluded that Andromeda’s sideways velocity was moving 10.5 miles per second, not enough to sidestep the Milky Way.
So then, while it’s currently only a white smudge to our eyes, Andromeda will continue to grow and grow in size as it approaches, eventually filling half the night sky after 3.75 billion years. It’ll then crash right into our Milky Way 4 billion years from now, filling the dark space around the galaxies with tendrils of light and gas — known as tidal tails — and bursting into a colorful light show visible from here on Earth.
The Triangulum Galaxy, though much smaller than the other two, will join in the collision as well. It might even crash into the Milky Way before Andromeda does. Whether or not it will fully merge with us, however, is still unknown.
Does this mean the end of our galaxy?
Well, in a way, yes. The collision between the three galaxies will permanently disrupt the flat, spiral shapes of Andromeda and the Milky Way. When they merge in 6 billion years, they’ll instead create an elliptical shaped galaxy with brand new constellations and a reddish color. A lot of stars will have larger orbits or might be thrown out of the new galaxy altogether. This also goes for our sun which might find itself slingshotted into intergalactic space after interacting with the black hole at Andromeda’s core. However, it’s more likely that the sun with survive and that our solar system will, in fact, outlive our galaxy.
Because galaxies are mostly made of empty space, collisions between stars are unlikely. The average distance between stars is immense and is always much bigger than the size of the star itself. There is a chance that our solar system will be disrupted by a star entering Neptune’s orbit but the chances of that are very small — only 1 in 10 million.
The black holes at the center of both the Milky Way and Andromeda galaxies will merge and might briefly create a quasar — luminous, powerful objects driven by black holes — depending on how much of their gas reserves both galaxies have used.
The following gallery depicts how Andromeda will change the night sky as it approaches the Milky Way, starting with how it currently appears to us here on Earth:
I should note that by this time, 5 billion years from now, the sun will have evolved into a red giant and consumed the Earth. Seems a bit of a funny fate. Our star, which played a large part in the formation of life, will also be an obstacle in its survival. Even as little as 1 billion years from now the sun will have grown hot enough to boil our oceans and render our planet uninhabitable, meaning that whoever is watching this galactic show won’t be doing so from the then empty, arid Earth.